Table of Contents
Java Language Specification Java SE 17 Edition Chapter 5 - Conversions and Contexts
Return to Java 17 Language Spec Table of Contents, Java 17 Language Spec, Java Language Specifications, Language Specifications, Java bibliography, Java DevOps, Java, Java topics, Awesome Java, Java development tools
“ (JSR-392 JvLngSpc17 2021)
5
C H A P T E R
Conversions and Contexts
EVERY expression written in the Java programming language either produces no
result (§15.1) or has a type that can be deduced at compile time (§15.3). When an expression appears in most contexts, it must be compatible with a type expected in that context; this type is called the target type. For convenience, compatibility of an expression with its surrounding]] context is facilitated in two ways:
- First, for some expressions, termed poly expressions (§15.2), the deduced type can be influenced by the target type. The same expression can have different types in different contexts.
- Second, after the type of the expression has been deduced, an implicit [[conversion from the type of the expression to the target type can sometimes be performed.
If neither strategy is able to produce the appropriate type, a compile-time error occurs.
The rules determining whether an expression is a poly expression, and if so, its type and compatibility in a particular context, vary depending on the kind of context and the form of the expression. In addition to influencing the type of the expression, the target type may in some cases influence the run time behavior of the expression in order to produce a value of the appropriate type.
Similarly, the rules determining whether a target type allows an implicit [[conversion vary depending on the kind of context, the type of the expression, and, in one special case, the value of a constant expression (§15.29). A conversion from type S to type T allows an expression of type S to be treated at compile time as if it had type T
instead. In some cases this will require a corresponding action at run time to check the validity of the conversion or to translate the run-time value of the expression into a form appropriate for the new type T.
107
Example 5.0-1. Conversions at Compile Time and Run Time
- A conversion from type double to type long requires a non-trivial translation from a 64-bit floating-point value to the 64-bit integer representation. Depending on the actual run-time value, information may be lost.
The conversions possible in the Java programming language are grouped into several broad categories:
- Unboxing conversions
There are six kinds of conversion contexts in which poly expressions may be influenced by context or implicit [[conversions may occur. Each kind of context has different rules for poly expression typing and allows conversions in some of the categories above but not others. The contexts are:
- Assignment contexts (§5.2, §15.26), in which an expression's value is bound to a named variable. Primitive and reference types are subject to widening, values
may be boxed or unboxed, and some primitive constant expressions may be subject to narrowing]]. An unchecked conversion may also occur.
108
- Strict invocation contexts (§5.3, §15.9, §15.12), in which an argument is bound to a formal parameter of a constructor or method. Widening primitive, widening
reference, and unchecked conversions may occur.
- Loose invocation contexts (§5.3, §15.9, §15.12), in which, like strict invocation contexts, an argument is bound to a formal parameter. Method or constructor invocations may provide this context if no applicable declaration can be found
using only strict invocation contexts. In addition to widening and unchecked conversions, this context allows boxing and unboxing conversions to occur.
- Casting contexts (§5.5), in which an expression's value is converted to a type explicitly specified by a cast operator (§15.16). Casting contexts are more inclusive than assignment or loose invocation contexts, allowing any specific conversion other than a string conversion, but certain casts to a reference type are checked for correctness at run time.
- Numeric contexts (§5.6), in which the operands of a numeric operator or some other expressions that operate on numbers may be widened to a common type.
The term ”conversion“ is also used to describe, without being specific, any conversions allowed in a particular context. For example, we say that an expression that is the initializer of a local variable is subject to ”assignment conversion“, meaning that a specific conversion will be implicitly chosen for that expression according to the rules for the assignment context. As another example, we say that an expression undergoes ”casting conversion“ to mean that the expression's type will be converted as permitted in a casting context.
Example 5.0-2. Conversions In Various Contexts
public static void main(String[] args) {
// Casting conversion (5.5) of a float literal to
// type int. Without the cast operator, this would
// be a compile-time error, because this is a
// narrowing]] conversion (5.1.3):
// String conversion (5.4) of i's int value:
System.out]].println(”(int)12.5f==“ + i);
// Assignment conversion (5.2) of i's value to type
// float. This is a widening conversion (5.1.2):
float f = i;
// String conversion of f's float value:
109
5.1
Kinds of Conversion
System.out]].println(“after float widening: ” + f);
// Numeric promotion (5.6) of i's value to type
// float. This is a binary numeric promotion.
// After promotion, the operation is float
f = f * i;
// Two string conversions of i and f:
- ” + i + ”==“ + f);
// Invocation conversion (5.3) of f's value
// to type double, needed because the method Math.sin
// accepts only a double argument:
// Two string conversions of f and d:
System.out]].println(”Math.sin(“ + f + ”)==“ + d);
}
}
This program produces the output:
(int)12.5f==12
12.0*12==144.0
Math.sin(144.0)==-0.49102159389846934
5.1 Kinds of Conversion
Specific type [[conversions in the Java programming language are divided into 12
kinds.
5.1.1
A conversion from a type to that same type is permitted for any type.
This may seem trivial, but it has two practical consequences. First, it is always permitted for an expression to have the desired type to begin with, thus allowing the simply stated rule that every expression is subject to conversion, if only a trivial id[[entity conversion. Second, it implies that it is permitted for a program to include redundant cast operators for the sake of clarity.
110
Kinds of Conversion
5.1
5.1.2
19 specific conversions on primitive types are called the widening primitive conversions:
A widening primitive conversion does not lose information about the overall magnitude of a numeric value in the following cases, where the numeric value is
- from an integral type to another integral type
A widening primitive conversion from int to float, or from long to float, or
from long to double, may result in loss of precision, that is, the result may lose some of the least significant bits of the value. In this case, the resulting floating-point value will be a correctly rounded version of the integer value, using the round
to nearest rounding]] policy (§15.4).
A widening conversion of a signed integer value to an integral type T simply sign-extends the two's-complement representation of the integer value to fill the wider format.
A widening conversion of a char to an integral type T zero-extends the representation of the char value to fill the wider format.
A widening conversion from int to float, or from long to float, or from int to
double, or from long to double occurs as determined by the rules of IEEE 754 for converting from an integer format to a binary floating-point format.
A widening conversion from float to double occurs as determined by the rules of
IEEE 754 for converting between binary floating-point formats.
111
5.1
Kinds of Conversion
Despite the fact that loss of precision may occur, a widening primitive conversion
never results in a run-time exception (§11.1.1).
Example 5.1.2-1. Widening Primitive Conversion
public static void main(String[] args) {
System.out]].println(big - (int)approx);
}
}
-46
thus indicating that information was lost during the conversion from type int to type float because values of type float are not precise to nine significant digits.
5.1.3
Narrowing]] Primitive Conversion
22 specific conversions on primitive types are called the narrowing]] primitive conversions:
A narrowing]] primitive conversion may lose information about the overall magnitude of a numeric value, and may also lose precision and range.
A narrowing]] conversion of a signed integer to an integral type T simply discards all but the n lowest order bits, where n is the number of bits used to represent type T. In addition to a possible loss of information about the magnitude of the numeric value, this may cause the sign of the resulting value to differ from the sign of the input value.
A narrowing]] conversion of a char to an integral type T likewise simply discards all but the n lowest order bits, where n is the number of bits used to represent type T. In addition to a possible loss of information about the magnitude of the numeric 112
Kinds of Conversion
5.1
value, this may cause the resulting value to be a negative number]], even though chars represent 16-bit unsigned integer values.
A narrowing]] conversion of a floating-point number to an integral type T takes two steps:
1. In the first step, the floating-point number is converted either to a long, if T is long, or to an int, if T is byte, short, char, or int, as follows:
- If the floating-point number is NaN (§4.2.3), the result of the first step of the conversion is an int or long 0.
a.
If T is long, and this integer value can be represented as a long, then the result of the first step is the long value V.
b. Otherwise, if this integer value can be represented as an int, then the
result of the first step is the int value V.
a.
The value must be too small (a negative value of large magnitude
or negative infinity), and the result of the first step is the smallest representable value of type int or long.
b. The value must be too large (a positive value of large magnitude
or positive infinity), and the result of the first step is the largest representable value of type int or long.
- If T is byte, char, or short, the result of the conversion is the result of a narrowing]] conversion to type T (§5.1.3) of the result of the first step.
A narrowing]] conversion from double to float occurs as determined by the rules
of IEEE 754 for converting between binary floating-point formats, using the round to nearest rounding]] policy (§15.4). This conversion can lose precision, but also lose range, resulting in a float zero from a nonzero double and a float infinity from a finite double. A double NaN is converted to a float NaN and a double infinity
is converted to the same-signed float infinity.
Despite the fact that overflow, underflow, or other loss of information may occur, a narrowing]] primitive conversion never results in a run-time exception (§11.1.1).
113
5.1
Kinds of Conversion
Example 5.1.3-1. Narrowing]] Primitive Conversion
public static void main(String[] args) {
float fmin = Float.NEGATIVE_INFINITY;
float fmax = Float.POSITIVE_INFINITY;
System.out]].println(”long: “ + (long)fmin +
”..“ + (long)fmax);
System.out]].println(”int: “ + (int)fmin +
”..“ + (int)fmax);
System.out]].println(”short: “ + (short)fmin +
”..“ + (short)fmax);
System.out]].println(”char: “ + (int)(char)fmin +
System.out]].println(”byte: “ + (byte)fmin +
”..“ + (byte)fmax);
}
}
This program produces the output:
long: -9223372036854775808..9223372036854775807
int: -2147483648..2147483647
short: 0..-1
char: 0..65535
byte: 0..-1
The results for char, int, and long are unsurprising, producing the minimum and maximum representable values of the type.
The results for byte and short lose information about the sign and magnitude of the numeric values and also lose precision. The results can be understood by examining the low order bits of the minimum and maximum int. The minimum int is, in hexadecimal, 0x80000000, and the maximum int is 0x7fffffff. This explains the short results, which are the low 16 bits of these values, namely, 0x0000 and 0xffff; it explains the char results, which also are the low 16 bits of these values, namely, '\u0000' and '\uffff'; and it explains the byte results, which are the low 8 bits of these values, namely, 0x00 and 0xff.
Example 5.1.3-2. Narrowing]] Primitive Conversions that lose information
public static void main(String[] args) {
// A narrowing]] of int to short loses high bits:
System.out]].println(”(short)0x12345678==0x“ +
// An int value too big for byte changes sign and magnitude:
System.out]].println(”(byte)255==“ + (byte)255);
// A float value too big to fit gives largest int value:
System.out]].println(”(int)1e20f==“ + (int)1e20f);
// A NaN converted to int yields zero:
System.out]].println(”(int)NaN==“ + (int)Float.NaN);
// A double value too large for float yields infinity:
114
Kinds of Conversion
5.1
System.out]].println(”(float)-1e100==“ + (float)-1e100);
// A double value too small for float underflows to zero:
System.out]].println(”(float)1e-50==“ + (float)1e-50);
}
}
This program produces the output:
(short)0x12345678==0x5678
(byte)255==-1
(int)1e20f==2147483647
(int)NaN==0
(float)1e-50==0.0
5.1.4
Widening and Narrowing]] Primitive Conversion
The following conversion combines both widening and narrowing]] primitive
First, the byte is converted to an int via widening primitive conversion (§5.1.2), and then the resulting int is converted to a char by narrowing]] primitive conversion
(§5.1.3).
5.1.5
A widening reference conversion exists from any reference type S to any reference type T, provided S is a subtype of T (§4.10).
Widening reference conversions never require a special action at run time and therefore never throw an exception at run time. They consist simply in regarding a reference as having some other type in a manner that can be proved correct at
The null type is not a reference type (§4.1), and so a widening reference conversion does not exist from the null type to a reference type. However, many conversion contexts explicitly allow the null type to be converted to a reference type.
5.1.6
Narrowing]] Reference Conversion
A narrowing]] reference conversion treats expressions of a reference type S as expressions of a different reference type T, where S is not a subtype of T.
The supported pairs of types are defined in §5.1.6.1. Unlike widening reference conversion, the types need not be directly related. However, there are re[[striction]]s 115
5.1
Kinds of Conversion
that prohibit conversion between certain pairs of types when it can be statically proven that no value can be of both types.
A narrowing]] reference conversion may require a test at run time to validate that a value of type S is a legitimate value of type T. However, due to the lack of parameterized type information at run time, some conversions cannot be
fully validated by a run time test; they are flagged at compile time (§5.1.6.2).
For conversions that can be fully validated by a run time test, and for certain conversions that involve parameterized type information but can still be partially validated at run time, a ClassCastException is thrown if the test fails (§5.1.6.3).
5.1.6.1
Allowed Narrowing]] Reference Conversion
A narrowing]] reference conversion exists from reference type S to reference type T
if all of the following are true:
- S is not a subtype of T (§4.10)
- If there exists a parameterized type X that is a supertype]] of T, and a parameterized type Y that is a supertype]] of S, such that the erasures of X and Y are the same, then X and Y are not provably distinct (§4.5).
Using types from the java.[[util package as an example, no narrowing]] reference conversion exists from ArrayList<String> to ArrayList<Object>, or vice versa, because the type arguments String and Object are provably distinct. For the same reason, no narrowing]] reference conversion exists from ArrayList<String> to List<Object>, or vice versa. The rejection of provably distinct types is a simple static gate to prevent ”stupid“ narrowing]] reference conversions.
116
Kinds of Conversion
5.1
1. S is a class or interface type, and T is a class or interface type, and S names a class or interface that is not disjoint from the class or interface named by
T. (”disjoint“ is defined below.)
2. S is the class type Object or the interface type java.io.Serializable or Cloneable (the only interfaces implemented by arrays (§10.8)), and T is an array type.
3. S is an array type SC[], that is, an array of components of type SC; T is an array type TC[], that is, an array of components of type TC; and a narrowing]] reference conversion exists from SC to TC.
4. S is a type variable, and a narrowing]] reference conversion exists from the upper bound of S to T.
5. T is a type variable, and either a widening reference conversion or a narrowing]] reference conversion exists from S to the upper bound of T.
6. S is an intersection type S1 & … & Sn, and for all i (1 ≤ i ≤ n), either a widening reference conversion or a narrowing]] reference conversion exists from Si to T.
7. T is an intersection type T1 & … & Tn, and for all i (1 ≤ i ≤ n), either a widening reference conversion or a narrowing]] reference conversion exists from S to Ti.
A class or interface is disjoint from another class or interface if it can be determined statically that they have no instances in common (other than the null value). The rules for disjointess are as follows:
– C is sealed, and all of the permitted direct subclasses of C are disjoint from I.
– C is freely extensible (§8.1.1.2), and I is sealed, and C is disjoint from all of the permitted direct subclasses and subinterfaces of I.
117
5.1
Kinds of Conversion
– I is sealed, and all of the permitted direct subclasses and subinterfaces of I are disjoint from J.
– J is sealed, and I is disjoint from all the permitted direct subclasses and subinterfaces of J.
Whether a class is final has the most bearing on whether the class is disjoint from interfaces. Consider the following declarations:
interface I {}
As class C is final and does not implement I, there can be no instances of C that are also an instance of I, so C and I are disjoint. Therefore, there is no narrowing]] reference conversion from C to I.
In contrast, consider the following declarations:
interface J {}
class D {}
Even though class D does not implement J, it is still possible for an instance of D to be an instance of J, for example, if the following declaration occurs:
class E extends D implements J {}
For this reason, D is not disjoint from J, and there is a narrowing]] reference conversion from D to J.
The final clause above implies that two freely extensible interfaces (§9.1.1.4) are not disjoint.
5.1.6.2
Checked and Unchecked Narrowing]] Reference Conversions
A narrowing]] reference conversion is either checked or unchecked. These terms refer to the ability of the Java Virtual Machine to validate, or not, the type correctness of the conversion.
If a narrowing]] reference conversion is unchecked, then the Java Virtual Machine will not be able to fully validate its type correctness, possibly leading to heap pollution (§4.12.2). To flag this to the programmer, an unchecked narrowing]] reference conversion causes a compile-time unchecked warning, unless suppressed by @SuppressWarnings (§9.6.4.5). Conversely, if a narrowing]] 118
Kinds of Conversion
5.1
reference conversion is not unchecked, then it is checked; the Java Virtual Machine will be able to fully validate its type correctness, so no warning is given at compile time.
The unchecked narrowing]] reference conversions are as follows:
- A narrowing]] reference conversion from a type S to a parameterized class or interface type T is unchecked, unless at least one of the following is true:
– All of the type arguments of T are unbounded wildcards.
– T <: S, and S has no subtype X other than T where the type arguments of X are not contained in the type arguments of T.
5.1.6.3
Narrowing]] Reference Conversions at Run Time
All checked narrowing]] reference conversions require a validity check at run time. Primarily, these conversions are to class and interface types that are not parameterized.
Some unchecked narrowing]] reference conversions require a validity check at run time. This depends on whether the unchecked narrowing]] reference conversion is
completely unchecked or partially unchecked. A partially unchecked narrowing]] reference conversion requires a validity check at run time, while a completely unchecked narrowing]] reference conversion does not.
These terms refer to the compatibility of the types involved in the conversion when viewed as raw types. If the conversion is conceptually an “upcast”, then the conversion is completely unchecked; no run time test is needed because the conversion is legal in the non-generic type system of the Java Virtual Machine. Conversely, if the conversion is conceptually a ”downcast“, then the conversion is partially unchecked; even in the non-generic type system of the Java Virtual Machine, a run time check is needed to test the compatibility of the (raw) types involved in the conversion.
Using types from the java.[[util package as an example, a conversion from ArrayList<String> to Collection<T> is completely unchecked, because the (raw) type ArrayList is a subtype of the (raw) type Collection in the Java Virtual Machine.
Conversely, a conversion from Collection<T> to ArrayList<String> is partially unchecked, because the (raw) type Collection is not a subtype of the (raw) type ArrayList in the Java Virtual Machine.
The categorization of an unchecked narrowing]] reference conversion is as follows: 119
5.1
Kinds of Conversion
Otherwise, it is partially unchecked.
& … & Tn is completely unchecked if, for all i (1 ≤ i ≤ n), either S <: Ti or a narrowing]] reference conversion from S to Ti is completely unchecked.
Otherwise, it is partially unchecked.
The run time validity check for a checked or partially unchecked narrowing]] reference conversion is as follows:
120
Kinds of Conversion
5.1
– If R is an ordinary class (not an array class):
› If T is a class type, then R must be either the same class as T (§4.3.4) or a subclass of T, or a ClassCastException is thrown.
› If T is an interface type, then R must implement interface T (§8.1.5), or a ClassCastException is thrown.
› If T is an array type, then a ClassCastException is thrown.
– If R is an interface:
Note that R cannot be an interface when these rules are first applied for any given conversion, but R may be an interface if the rules are applied recursively because the run-time reference value may refer to an array whose element type is an interface type.
› If T is a class type, then T must be Object (§4.3.2), or a ClassCastException is thrown.
› If T is an interface type, then R must be either the same interface as T or a subinterface of T, or a ClassCastException is thrown.
› If T is an array type, then a ClassCastException is thrown.
– If R is a class representing an array type RC[], that is, an array of components of type RC:
› If T is a class type, then T must be Object (§4.3.2), or a ClassCastException is thrown.
› If T is an interface type, then T must be the type java.io.Serializable or Cloneable (the only interfaces implemented by arrays), or a
› If T is an array type TC[], that is, an array of components of type TC, then a ClassCastException is thrown unless either TC and RC are the same primitive type, or TC and RC are reference types and are allowed by a recursive application of these run-time rules.
If the conversion is to an intersection type T1 & … & Tn, then for all i (1 ≤ i ≤ n), any run-time check required for a conversion from S to Ti is also required for the conversion to the intersection type.
121
5.1
Kinds of Conversion
5.1.7
Boxing conversion treats expressions of a primitive type as expressions of a corresponding reference type. Specifically, the following nine conversions are called the boxing conversions:
This rule is necessary because the conditional operator (§15.25) applies boxing conversion to the types of its operands, and uses the result in further calculations.
At run time, boxing conversion proceeds as follows:
122
Kinds of Conversion
5.1
– If p is not NaN, then boxing conversion converts p into a reference r of class and type Float, such that r.floatValue() evaluates to p
– Otherwise, boxing conversion converts p into a reference r of class and type Float such that r.isNaN() evaluates to true
– If p is not NaN, boxing conversion converts p into a reference r of class and type Double, such that r.doubleValue() evaluates to p
– Otherwise, boxing conversion converts p into a reference r of class and type Double such that r.isNaN() evaluates to true
conversion (§5.1.1).
If the value p being boxed is the result of evaluating a constant expression (§15.29)
of type boolean, byte, char, short, int, or long, and the result is true, false, a character in the range '\u0000' to '\u007f' inclusive, or an integer in the range
-128 to 127 inclusive, then let a and b be the results of any two boxing conversions of p. It is always the case that a == b.
Ideally, boxing a primitive value would always yield an identical reference. In practice, this may not be feasible using existing implementation techniques. The rule above is a pragmatic compromise, requiring that certain common values always be boxed into indistinguishable objects. The implementation may cache these, lazily or eagerly. For other values, the rule disallows any assumptions about the id[[entity of the boxed values on the programmer's part.
This allows (but does not require) sharing of some or all of these references.
This ensures that in most common cases, the behavior will be the desired one, without imposing an undue performance penalty, especially on small devices. Less memory-limited implementations might, for example, cache all char and short values, as well as int and long values in the range of -32K to +32K.
A boxing conversion may result in an OutOfMemoryError if a new instance of one
of the wrapper classes (Boolean, Byte, Character, Short, Integer, Long, Float,
or Double) needs to be allocated and insufficient storage is available.
5.1.8
Unboxing conversion treats expressions of a reference type as expressions of a corresponding primitive type. Specifically, the following eight conversions are called the unboxing conversions:
123
5.1
Kinds of Conversion
At run time, unboxing conversion proceeds as follows:
A type is said to be convertible to a numeric type if it is a numeric type (§4.2), or it is a reference type that may be converted to a numeric type by unboxing conversion.
A type is said to be convertible to an integral type if it is an integral type, or it is a reference type that may be converted to an integral type by unboxing conversion.
124
Kinds of Conversion
5.1
5.1.9
Let G name a generic type declaration with n type parameters.
There is an unchecked conversion from the raw class or interface type (§4.8) G to any parameterized type of the form G< T1,…, Tn>.
There is an unchecked conversion from the raw array type
k
G[] to any array type of
the form
k
k
G< T1,…, Tn>[] . (The notation [] indicates an array type of k dimensions.) Use of an unchecked conversion causes a compile-time unchecked warning unless all type arguments Ti (1 ≤ i ≤ n) are unbounded wildcards (§4.5.1), or the warning is suppressed by @SuppressWarnings (§9.6.4.5).
Unchecked conversion is used to enable a smooth interoperation of legacy code, written before the introduction of generic types, with libraries that have undergone a conversion to use genericity (a process we call generification). In such circumstances (most notably, clients of the Collections Framework in java.[[util), legacy code uses raw types (e.g.
Collection instead of Collection<String>). Expressions of raw types are passed as arguments to library methods that use parameterized versions of those same types as the types of their corresponding formal parameters.
Such calls cannot be shown to be statically safe under the type system using generics.
Rejecting such calls would invalidate large bodies of existing code, and prevent them from using newer versions of the libraries. This in turn, would discourage library vendors from taking advantage of genericity. To prevent such an unwelcome turn of events, a raw type may be converted to an arbitrary invocation of the generic type declaration to which the raw type refers. While the conversion is unsound, it is tolerated as a concession to practicality.
An unchecked warning is issued in such cases.
5.1.10
Let G name a generic type declaration (§8.1.2, §9.1.2) with n type parameters A1,…, An with corresponding bounds U1,…, Un.
There exists a capture conversion from a parameterized type G< T1,…, Tn> (§4.5) to a parameterized type G< S1,…, Sn>, where, for 1 ≤ i ≤ n :
125
5.1
Kinds of Conversion
glb( V1,…, Vm) is defined as V1 & … & Vm.
It is a compile-time error if, for any two classes (not interfaces) Vi and Vj, Vi is not a subclass of Vj or vice versa.
- Otherwise, Si = Ti.
Capture conversion on any type other than a parameterized type (§4.5) acts as an id[[entity conversion (§5.1.1).
Capture conversion is not applied recursively.
Capture conversion never requires a special action at run time and therefore never throws an exception at run time.
Capture conversion is designed to make wildcards more useful. To understand the motivation, let's begin by looking at the method java.[[util.Collections.reverse(): public static void reverse(List<?> list);
The method reverses the list provided as a parameter. It works for any type of list, and so the use of the wildcard type List<?> as the type of the formal parameter is entirely appropriate.
Now consider how one would implement reverse():
public static void reverse(List<?> list) { rev(list); }
private static <T> void rev(List<T> list) {
List<T> tmp = new ArrayList<T>(list);
for (int i = 0; i < list.size(); i++) {
list.set(i, tmp.get(list.size() - i - 1));
}
}
The implementation needs to copy the list, extract elements from the copy, and insert them into the original. To do this in a type-safe manner, we need to give a name, T, to the element type of the incoming list. We do this in the private service method rev(). This requires us to pass the incoming argument [[list, of type List<?>, as an argument to rev(). In general, List<?> is a list of unknown type. It is not a subtype of List<T>, for any type T. Allowing such a subtype relation would be unsound. Given the method:
public static <T> void fill(List<T> l, T obj)
126
Kinds of Conversion
5.1
the following code would undermine the type system:
List<String> ls = new ArrayList<String>();
Collections.fill(l, new Object()); // not legal - but assume it was!
String s = ls.get(0); // ClassCastException - ls contains
So, without some special dispensation, we can see that the call from reverse() to rev() would be disallowed. If this were the case, the author of reverse() would be forced]] to write its signature as:
public static <T> void reverse(List<T> list)
This is undesirable, as it exposes implementation information to the caller. Worse, the designer of an API might reason that the signature using a wildcard is what the callers of the API require, and only later realize that a type safe implementation was precluded.
The call from reverse() to rev() is in fact harmless, but it cannot be justified on the basis of a general subtyping relation between List<?> and List<T>. The call is harmless, because the incoming argument is doubtless a list of some type (albeit an unknown one). If we can capture this unknown type in a type variable X, we can infer T to be X. That is the essence of capture conversion. The specification of course must cope with complications, like non-trivial (and possibly recursively defined) upper or lower bounds, the presence of multiple arguments etc.
Mathematically sophisticated readers will want to relate capture conversion to established type theory. Readers unfamiliar with type theory can skip this discussion - or else study a suitable text, such as Types and Programming Languages by Benjamin Pierce, and then revisit this section.
Here then is a brief summary of the relationship of capture conversion to established type theoretical notions. Wildcard types are a re[[stricted]] form of existential types. Capture conversion corresponds loosely to an opening of a value of existential type. A capture conversion of an expression e can be thought of as an open of e in a scoped that comprises the top level expression that encloses e.
The classical open operation on existentials requires that the captured type variable must not escape the opened expression. The open that corresponds to capture conversion is always on a scoped sufficiently large that the captured type variable can never be visible outside that scoped. The advantage of this scheme is that there is no need for a close operation, as defined in the paper On Variance-Based Subtyping for Parametric Types by Atsushi Igarashi and Mirko Viroli, in the proceedings of the 16th European]] Conference on Object-Oriented Programming]] (ECOOP 2002). For a formal account of wildcards, see Wild FJ by Mads Torgersen, Erik Ernst and Christian Plesner Hansen, in the 12th workshop on Foundations of Object-Oriented Programming]] (FOOL 2005).
5.1.11
Any type may be converted to type String by string conversion.
127
5.2
A value x of primitive type T is first converted to a reference value as if by giving
it as an argument to an appropriate class instance creation expression (§15.9):
This reference value is then converted to type String by string conversion.
Now only reference values need to be considered:
method of the referenced object with no arguments; but if the result of invoking the to[[String method is null, then the string ”null“ is used instead.
The to[[String method is defined by the primordial class Object (§4.3.2). Many classes override it, notably Boolean, Character, Integer, Long, Float, Double, and String.
5.1.12
Forbidden Conversions
Any conversion that is not explicitly allowed is forbidden.
5.2 Assignment Contexts
Assignment contexts allow the value of an expression to be assigned (§15.26) to a variable; the type of the expression must be converted to the type of the variable.
Assignment contexts allow the use of one of the following:
- an id[[entity conversion (§5.1.1)
128
5.2
- a widening reference conversion followed by an unboxing conversion, then followed by a widening primitive conversion
- a boxing conversion (§5.1.7)
- an unboxing conversion (§5.1.8)
If, after the conversions listed above have been applied, the resulting type is a raw
type (§4.8), an unchecked conversion (§5.1.9) may then be applied.
In addition, if the expression is a constant expression (§15.29) of type byte, short, char, or int:
short, or char, and the value of the constant expression is representable in the type of the variable.
if the variable is of type Byte, Short, or Character, and the value of the constant expression is representable in the type byte, short, or char respectively.
The compile-time narrowing]] of constant expressions means that code such as:
is allowed. Without the narrowing]], the fact that the integer literal 42 has type int would mean that a cast to byte would be required:
byte theAnswer = (byte)42; // cast is permitted but not required
Finally, a value of the null type (the null reference is the only such value) may be assigned to any reference type, resulting in a null reference of that type.
It is a compile-time error if the chain of conversions contains two parameterized
types that are not in the subtype relation (§4.10).
An example of such an illegal chain would be:
Integer, Comparable<Integer>, Comparable, Comparable<String>
The first three elements of the chain are related by widening reference conversion, while the last entry is derived from its predecessor by unchecked conversion. However, this is not a valid assignment conversion, because the chain contains two parameterized types, Comparable<Integer> and Comparable<String>, that are not subtypes.
129
5.2
If the type of an expression can be converted to the type of a variable by assignment conversion, we say the expression (or its value) is assignable to the variable or, equivalently, that the type of the expression is assignment compatible with the type of the variable.
The only exceptions that may arise from conversions in an assignment context are:
resulting value is an object which is not an instance of a subclass or subinterface
of the erasure (§4.6) of the type of the variable.
This circumstance can only arise as a result of heap pollution (§4.12.2). In practice, implementations need only perform casts when accessing a field or method of an object of parameterized type when the erased type of the field, or the erased return type of the method, differ from its unerased type.
Example 5.2-1. Assignment for Primitive Types
public static void main(String[] args) {
short s = 12; // narrow 12 to short
float f = s; // widen short to float
System.out]].println(“f=” + f);
char c = '\u0123';
long l = c; // widen char to long
System.out]].println(“l=0x” + Long.to[[String(l,16));
f = 1.23f;
double d = f; // widen float to double
System.out]].println(“d=” + d);
}
}
This program produces the output:
f=12.0
l=0x123
d=1.2300000190734863
The following program, however, produces compile-time errors:
public static void main(String[] args) {
130
5.2
short s = 123;
char c = s; // error: would require cast
s = c; // error: would require cast
}
}
because not all short values are char values, and neither are all char values short values.
Example 5.2-2. Assignment for Reference Types
class Point3D extends Point { int z; }
interface Colorable { void setColor(int color); }
class ColoredPoint extends Point implements Colorable {
public void setColor(int color) { this.color = color; }
}
public static void main(String[] args) {
// Assignments to variables of class type:
// OK because Point3D is a subclass of Point
// Error: will require a cast because a Point
// might not be a Point3D (even though it is,
// dynamically, in this example.)
// Assignments to variables of type Object:
Object o = p; // OK: any object to Object
Object o2 = a; // OK: an array to Object
// Assignments to variables of interface type:
ColoredPoint cp = new ColoredPoint();
// OK: ColoredPoint implements Colorable
// Assignments to variables of array type:
a = b;
// Error: these are not arrays of the same primitive type
Point3D[] p3da = new Point3D[3];
Point[] pa = p3da;
// OK: since we can assign a Point3D to a Point
p3da = pa;
// Error: (cast needed) since a Point
// can't be assigned to a Point3D
}
}
131
5.2
The following test program illustrates assignment conversions on reference values, but fails to compile, as described in its comments. This example should be compared to the preceding one.
interface Colorable { void setColor(int color); }
class ColoredPoint extends Point implements Colorable {
public void setColor(int color) { this.color = color; }
}
public static void main(String[] args) {
ColoredPoint cp = new ColoredPoint();
// Okay because ColoredPoint is a subclass of Point:
p = cp;
// Okay because ColoredPoint implements Colorable:
// The following cause compile-time errors because
// we cannot be sure they will succeed, depending on
// the run-time type of p; a run-time check will be
// necessary for the needed narrowing]] conversion and
// must be indicated by including a cast:
cp = p; // p might be neither a ColoredPoint
// nor a subclass of ColoredPoint
c = p; // p might not implement Colorable
}
}
Example 5.2-3. Assignment for Array Types
class ColoredPoint extends Point { int color; }
public static void main(String[] args) {
long[] veclong = new long[100];
Object o = veclong; // okay
Long l = veclong; // compile-time error
short[] vecshort = veclong; // compile-time error
Point[] pvec = new Point[100];
ColoredPoint[] cpvec = new ColoredPoint[100];
pvec = cpvec; // okay
pvec[0] = new Point(); // okay at compile time,
// but would throw an
cpvec = pvec; // compile-time error
}
}
In this example:
132
5.3
- The value of veclong cannot be assigned to vecshort, because they are arrays of primitive type, and short and long are not the same primitive type.
- The value of cpvec can be assigned to pvec, because any reference that could be the value of an expression of type ColoredPoint can be the value of a variable of type Point. The subsequent assignment of the new Point to a component of pvec then would throw an ArrayStoreException (if the program were otherwise corrected so
that it could be compiled), because a ColoredPoint array cannot have an instance of Point as the value of a component.
- The value of pvec cannot be assigned to cpvec, because not every reference that could be the value of an expression of type Point can correctly be the value of a variable of type ColoredPoint. If the value of pvec at run time were a reference to an instance of Point[], and the assignment to cpvec were allowed, a simple reference to a component of cpvec, say, cpvec[0], could return a Point, and a Point is not a ColoredPoint.
Thus to allow such an assignment would allow a violation of the type system. A cast
may be used (§5.5, §15.16) to ensure that pvec references a ColoredPoint[]: cpvec = (ColoredPoint[])pvec; // OK, but may throw an
5.3 Invocation Contexts
Invocation contexts allow an argument value in a method or constructor invocation
(§8.8.7.1, §15.9, §15.12) to be assigned to a corresponding formal parameter.
Strict invocation contexts allow the use of one of the following:
- an id[[entity conversion (§5.1.1)
Loose invocation contexts allow a more permissive set of conversions, because they are only used for a particular invocation if no applicable declaration can be found using strict invocation contexts. Loose invocation contexts allow the use of one of the following:
- an id[[entity conversion (§5.1.1)
133
5.3
- a widening reference conversion followed by an unboxing conversion, then followed by a widening primitive conversion
- a boxing conversion (§5.1.7)
- an unboxing conversion (§5.1.8)
If, after the conversions listed for an invocation context have been applied, the resulting type is a raw type (§4.8), an unchecked conversion (§5.1.9) may then be applied.
A value of the null type (the null reference is the only such value) may be assigned to any reference type.
It is a compile-time error if the chain of conversions contains two parameterized
types that are not in the subtype relation (§4.10).
The only exceptions that may arise in an invocation context are:
the resulting value is an object which is not an instance of a subclass or
subinterface of the erasure (§4.6) of the corresponding formal parameter type.
Neither strict nor loose invocation contexts include the implicit narrowing]] of integer constant expressions which is allowed in assignment contexts. The designers of the Java programming language felt that including these implicit narrowing]] conversions would add additional complexity to the rules of overload [[resolution (§15.12.2).
Thus, the program:
static int byte a, int b) { return a+b; }
static int short a, short b) { return a-b; }
public static void main(String[] args) {
System.out]].println(m(12, 2)); // compile-time error
}
}
causes a compile-time error because the integer literals 12 and 2 have type int, so neither method m matches under the rules of overload [[resolution. A language that included implicit narrowing]] of integer constant expressions would need additional rules to resolve cases like this example.
134
5.4
String contexts apply only to an operand of the binary + operator which is not a String when the other operand is a String.
The target type in these contexts is always String, and a string conversion
(§5.1.11) of the non-String operand always occurs. Evaluation of the + operator then proceeds as specified in §15.18.1.
Casting contexts allow the operand of a cast expression (§15.16) to be converted to the type explicitly named by the cast operator. Compared to assignment contexts
and invocation contexts, casting contexts allow the use of more of the conversions defined in §5.1, and allow more combinations of those conversions.
If the expression is of a primitive type, then a casting context allows the use of one of the following:
- an id[[entity conversion (§5.1.1)
- a boxing conversion (§5.1.7)
- a boxing conversion followed by a widening reference conversion (§5.1.5) If the expression is of a reference type, then a casting context allows the use of one of the following:
- an id[[entity conversion (§5.1.1)
- a widening reference conversion followed by an unboxing conversion, then followed by a widening primitive conversion
135
5.5
- an unboxing conversion (§5.1.8)
If the expression has the null type, then the expression may be cast to any reference type.
If a casting context makes use of a narrowing]] reference conversion that is checked or partially unchecked (§5.1.6.2, §5.1.6.3), then a run time check will be performed on the class of the expression's value, possibly causing a ClassCastException.
Otherwise, no run time check is performed.
If an expression can be converted to a reference type by a casting conversion other than a narrowing]] reference conversion which is unchecked, we say the expression (or its value) is downcast compatible with the reference type.
The following tables enumerate which conversions are used in certain casting contexts. Each conversion is signified by a symbol:
In the tables, a comma between symbols indicates that a casting context uses one conversion followed by another. The type Object means any reference type other
than the eight wrapper classes Boolean, Byte, Short, Character, Integer, Long,
136
5.5
Table 5.5-A. Casting to primitive types
To →
From ↓
≈
ω
ωη
ω
ω
ω
ω
-
η
≈
η
ω
ω
ω
ω
-
η
η
≈
ω
ω
ω
ω
-
η
η
η
≈
ω
ω
ω
-
η
η
η
η
≈
ω
ω
-
η
η
η
η
η
≈
ω
-
η
η
η
η
η
η
≈
-
-
-
-
-
-
-
-
≈
⊗
⊗,ω
-
⊗,ω
⊗,ω
⊗,ω
⊗,ω
-
-
⊗
-
⊗,ω
⊗,ω
⊗,ω
⊗,ω
-
-
-
⊗
⊗,ω
⊗,ω
⊗,ω
⊗,ω
-
-
-
-
⊗
⊗,ω
⊗,ω
⊗,ω
-
-
-
-
-
⊗
⊗,ω
⊗,ω
-
-
-
-
-
-
⊗
⊗,ω
-
-
-
-
-
-
-
⊗
-
-
-
-
-
-
-
-
⊗
⇓,⊗
⇓,⊗
⇓,⊗
⇓,⊗
⇓,⊗
⇓,⊗
⇓,⊗
⇓,⊗
137
5.5
Table 5.5-B. Casting to reference types
To →
Byte Short Character Integer Long Float Double Boolean Object
From ↓
⊕
-
-
-
-
-
-
-
⊕,⇑
-
⊕
-
-
-
-
-
-
⊕,⇑
-
-
⊕
-
-
-
-
-
⊕,⇑
-
-
-
⊕
-
-
-
-
⊕,⇑
-
-
-
-
⊕
-
-
-
⊕,⇑
-
-
-
-
-
⊕
-
-
⊕,⇑
-
-
-
-
-
-
⊕
-
⊕,⇑
-
-
-
-
-
-
-
⊕
⊕,⇑
≈
-
-
-
-
-
-
-
⇑
-
≈
-
-
-
-
-
-
⇑
-
≈
-
-
-
-
-
⇑
-
-
-
≈
-
-
-
-
⇑
-
-
-
-
≈
-
-
-
⇑
-
-
-
-
-
≈
-
-
⇑
-
-
-
-
-
-
≈
-
⇑
-
-
-
-
-
-
-
≈
⇑
⇓
⇓
⇓
⇓
⇓
⇓
⇓
⇓
≈
138
5.5
Example 5.5-1. Casting for Reference Types
interface Colorable { void setColor(int color); }
class ColoredPoint extends Point implements Colorable {
public void setColor(int color) { this.color = color; }
}
final class EndPoint extends Point {}
public static void main(String[] args) {
ColoredPoint cp = new ColoredPoint();
Colorable c;
// The following may cause errors at run time because
// we cannot be sure they will succeed; this possibility
cp = (ColoredPoint)p; // p might not reference an
// object which is a ColoredPoint
// or a subclass of ColoredPoint
c = (Colorable)p; // p might not be Colorable
// The following are incorrect at compile time because
// they can never succeed as explained in the text:
Long l = (Long)p; // compile-time error
- 1
c = (Colorable)e; // compile-time error
- 2
}
}
Here, the first compile-time error occurs because the class types Long and Point are unrelated (that is, they are not the same, and neither is a subclass of the other), so a cast between them will always fail.
The second compile-time error occurs because a variable of type EndPoint can never reference a value that implements the interface Colorable. This is because EndPoint is a final type, and a variable of a final type always holds a value of the same run-time type as its compile-time type. Therefore, the run-time type of variable e must be exactly the type EndPoint, and type EndPoint does not implement Colorable.
139
5.5
Example 5.5-2. Casting for Array Types
int x, y;
Point(int x, int y) { this.x = x; this.y = y; }
public String to[[String() { return ”(“+x+”,“+y+”)“; }
}
interface Colorable { void setColor(int color); }
class ColoredPoint extends Point implements Colorable {
ColoredPoint(int x, int y, int color) {
}
public void setColor(int color) { this.color = color; }
public String to[[String() {
return super.to[[String() + ”@“ + color;
}
}
public static void main(String[] args) {
Point[] pa = new ColoredPoint[4];
pa[0] = new ColoredPoint(2, 2, 12);
pa[1] = new ColoredPoint(4, 5, 24);
ColoredPoint[] cpa = (ColoredPoint[])pa;
for (int i = 0; i < cpa.length; i++)
i = b « 4L; // shift promotion (left operand)
System.out]].println(“0x” + Integer.toHexString(b)
+ ”«4L==0x“ + Integer.toHexString(i));
}
}
This program produces the output:
a: -1,1
~0xffffffff==0x0
0xffffffff«4L==0xfffffff0
Example 5.6-2. Binary Numeric Promotion
public static void main(String[] args) {
int i = 0;
float f = 1.0f;
double d = 2.0;
// float==double is promoted to double==double:
if (i * f == d) System.out]].println(“oops”);
144
5.6
// A char&byte is promoted to int&int:
byte b = 0x1f;
char c = 'G';
System.out]].println(Integer.toHexString(control));
// Here int:float is promoted to float:float:
f = (b==0) ? i : 4.0f;
}
}
This program produces the output:
7
0.25
The example converts the ASCII character G to the ASCII control-G (BEL), by masking off all but the low 5 bits of the character. The 7 is the numeric value of this control character.
145
Fair Use Sources
Java: Java Fundamentals, Java Inventor - Java Language Designer: James Gosling of Sun Microsystems, Java Docs, JDK, JVM, JRE, Java Keywords, JDK 17 API Specification, java.base, Java Built-In Data Types, Java Data Structures - Java Algorithms, Java Syntax, Java OOP - Java Design Patterns, Java Installation, Java Containerization, Java Configuration, Java Compiler, Java Transpiler, Java IDEs (IntelliJ - Eclipse - NetBeans), Java Development Tools, Java Linter, JetBrains, Java Testing (JUnit, Hamcrest, Mockito), Java on Android, Java on Windows, Java on macOS, Java on Linux, Java DevOps - Java SRE, Java Data Science - Java DataOps, Java Machine Learning, Java Deep Learning, Functional Java, Java Concurrency, Java History,
Java Bibliography (Effective Java, Head First Java, Java - A Beginner's Guide by Herbert Schildt, Java Concurrency in Practice, Clean Code by Robert C. Martin, Java - The Complete Reference by Herbert Schildt, Java Performance by Scott Oaks, Thinking in Java, Java - How to Program by Paul Deitel, Modern Java in Action, Java Generics and Collections by Maurice Naftalin, Spring in Action, Java Network Programming by Elliotte Rusty Harold, Functional Programming in Java by Pierre-Yves Saumont, Well-Grounded Java Developer, Second Edition, Java Module System by Nicolai Parlog
), Manning Java Series, Java Glossary, Java Topics, Java Courses, Java Security - Java DevSecOps, Java Standard Library, Java Libraries, Java Frameworks, Java Research, Java GitHub, Written in Java, Java Popularity, Java Awesome List, Java Versions. (navbar_java and navbar_java_detailed - see also navbar_jvm, navbar_java_concurrency, navbar_java_standard_library, navbar_java_libraries, navbar_java_navbars)
© 1994 - 2024 Cloud Monk Losang Jinpa or Fair Use. Disclaimers
SYI LU SENG E MU CHYWE YE. NAN. WEI LA YE. WEI LA YE. SA WA HE.
- An operand of an integer bitwise [[operator &, ^, or | (§15.22.1) An expression appears in a numeric array context if the expression is one of the following:
- The index expression in an array access expression (§15.10.3) An expression appears in a numeric choice context if the expression is one of the following:
- A result expression of a standalone switch expression]] (§15.28.1) where all the result expressions are convertible to a numeric type
- Otherwise, if any expression is of type float, then the promoted type is float, and other expressions that are not of type float undergo widening