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5.1.2 Widening Primitive Conversion
The following 19 specific conversions on primitive types are called the widening primitive conversions:
byte to short, int, long, float, or double
short to int, long, float, or double
char to int, long, float, or double
int to long, float, or double
long to float or double
float to double
Widening primitive conversions do not lose information about the overall magnitude of a numeric value. Indeed, conversions widening from an integral type to another integral type and from float to double do not lose any information at all; the numeric value is preserved exactly. Conversions widening from float to double in strictfp expressions also preserve the numeric value exactly; however, such conversions that are not strictfp may lose information about the overall magnitude of the converted value.
Conversion of an int or a long value to float, or of a long value 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 IEEE 754 round-to-nearest mode (§4.2.4).
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 character to an integral type T zero-extends the representation of the character value to fill the wider format.
Despite the fact that loss of precision may occur, widening conversions among primitive types never result in a run-time exception (§11).
Here is an example of a widening conversion that loses precision:
class Test {
public static void main(String[] args) {
int big = 1234567890;
float approx = big;
System.out.println(big - (int)approx);
}
}
which prints:
-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 Conversions
The following 23 specific conversions on primitive types are called the narrowing primitive conversions:
byte to char
short to byte or char
char to byte or short
int to byte, short, or char
long to byte, short, char, or int
float to byte, short, char, int, or long
double to byte, short, char, int, long, or float
Narrowing conversions may lose information about the overall magnitude of a numeric value and may also lose precision.
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 character 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 value, this may cause the resulting value to be a negative number, even though characters represent 16-bit unsigned integer values.
A narrowing conversion of a floating-point number to an integral type T takes two steps:
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.
Otherwise, if the floating-point number is not an infinity, the floating-point value is rounded to an integer value V, rounding toward zero using IEEE 754 round-toward-zero mode (§4.2.3). Then there are two cases:
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.
Otherwise, if this integer value can be represented as an int, then the result of the first step is the int value V.
Otherwise, one of the following two cases must be true:
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.
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.
In the second step:
If T is int or long, the result of the conversion is the result of the first step.
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.
The example:
class Test {
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 +
".." + (int)(char)fmax);
System.out.println("byte: " + (byte)fmin +
".." + (byte)fmax);
}
}
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.
Despite the fact that overflow, underflow, or other loss of information may occur, narrowing conversions among primitive types never result in a run-time exception (§11).
Here is a small test program that demonstrates a number of narrowing conversions that lose information:
class Test {
public static void main(String[] args) {
// A narrowing of int to short loses high bits:
System.out.println("(short)0x12345678==0x" +
Integer.toHexString((short)0x12345678));
// A int value not fitting in 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:
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 test program produces the following output:
(short)0x12345678==0x5678
(byte)255==-1
(int)1e20f==2147483647
(int)NaN==0
(float)-1e100==-Infinity
(float)1e-50==0.0
5.1.4 Widening Reference Conversions
The following conversions are called the widening reference conversions:
From any class type S to any class type T, provided that S is a subclass of T. (An important special case is that there is a widening conversion to the class type Object from any other class type.)
From any class type S to any interface type K, provided that S implements K.
From the null type to any class type, interface type, or array type.
From any interface type J to any interface type K, provided that J is a subinterface of K.
From any interface type to type Object.
From any array type to type Object.
From any array type to type Cloneable.
From any array type to type java.io.Serializable
From any array type SC[] to any array type TC[], provided that SC and TC are reference types and there is a widening conversion from SC to TC.
Such 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 compile time.
See §8 for the detailed specifications for classes, §9 for interfaces, and §10 for arrays.
5.1.5 Narrowing Reference Conversions
The following conversions are called the narrowing reference conversions:
From any class type S to any class type T, provided that S is a superclass of T. (An important special case is that there is a narrowing conversion from the class type Object to any other class type.)
From any class type S to any interface type K, provided that S is not final and does not implement K. (An important special case is that there is a narrowing conversion from the class type Object to any interface type.)
From type Object to any array type.
From type Object to any interface type.
From any interface type J to any class type T that is not final.
From any interface type J to any class type T that is final, provided that T implements J.
From any interface type J to any interface type K, provided that J is not a subinterface of K and there is no method name m such that J and K both contain a method named m with the same signature but different return types.
From any array type SC[] to any array type TC[], provided that SC and TC are reference types and there is a narrowing conversion from SC to TC.
Such conversions require a test at run time to find out whether the actual reference value is a legitimate value of the new type. If not, then a ClassCastException is thrown.
5.1.6 String Conversions
There is a string conversion to type String from every other type, including the null type.
5.1.7 Forbidden Conversions
There is no permitted conversion from any reference type to any primitive type.
Except for the string conversions, there is no permitted conversion from any primitive type to any reference type.
There is no permitted conversion from the null type to any primitive type.
There is no permitted conversion to the null type other than the identity conversion.
There is no permitted conversion to the type boolean other than the identity conversion.
There is no permitted conversion from the type boolean other than the identity conversion and string conversion.
There is no permitted conversion other than string conversion from class type S to a different class type T if S is not a subclass of T and T is not a subclass of S.
There is no permitted conversion from class type S to interface type K if S is final and does not implement K.
There is no permitted conversion from class type S to any array type if S is not Object.
There is no permitted conversion other than string conversion from interface type J to class type T if T is final and does not implement J.
There is no permitted conversion from interface type J to interface type K if J and K contain methods with the same signature but different return types.
There is no permitted conversion from any array type to any class type other than Object or String.
There is no permitted conversion from any array type to any interface type, except to the interface types java.io.Serializable and Cloneable, which are implemented by all arrays.
There is no permitted conversion from array type SC[] to array type TC[] if there is no permitted conversion other than a string conversion from SC to TC. |
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楼主: wangcunjiang
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发表于 2004-7-24 00:12
发表于 2004-7-24 08:11