为什么整数常量池的行为在127处变化？ [英] Why does the behavior of the Integer constant pool change at 127?

问题描述

我无法理解Java Constant Pool for Integer是如何工作的。

I am not able to understand how the Java Constant Pool for Integer works.

我理解字符串的行为，因此能够证明自己是正确的同样使用整数常量的情况。

I understand the behavior of Strings, and hence able to justify myself that it is the same case with Integer Constants also.

因此，对于整数

Integer i1 = 127;
Integer i2 = 127;
System.out.println(i1==i2); // True

&

Integer i1 = new Integer(127);
Integer i2 = new Integer(127);
System.out.println(i1==i2); // False

直到这里，一切都在我脑海中。

Till here everything goes in my head.

我无法消化的是，当我从127增加整数时，它的行为有所不同。此行为在127之后发生变化，下面是代码片段

What I am not able to digest is, it behaves differently when I increase the integer from 127. This behavior changes after 127, below is the code snippet

Integer i1 = 128;
Integer i2 = 128;
System.out.println(i1==i2); // False. WHY?????

有人可以帮我理解吗？

推荐答案

不，数字的常量池与字符串的工作方式不同。对于字符串，只有编译时常量被插入 - 而对于整数类型的包装类型，任何装箱操作总是使用该池，如果它适用于该值。例如：

No, the constant pool for numbers doesn't work the same way as for strings. For strings, only compile-time constants are interned - whereas for the wrapper types for integer types, any boxing operation will always use the pool if it's applicable for that value. So for example:

int x = 10;
int y = x + 1;
Integer z = y; // Not a compile-time constant!
Integer constant = 11;
System.out.println(z == constant); // true; reference comparison

JLS保证一小部分池化值，但实现可以使用更广泛的范围希望。

The JLS guarantees a small range of pooled values, but implementations can use a wider range if they wish.

请注意，虽然不能保证，但我看过的每个实现都使用 Integer.valueOf 执行装箱操作 - 所以你可以在没有语言帮助的情况下获得相同的效果：

Note that although it's not guaranteed, every implementation I've looked at uses Integer.valueOf to perform boxing operations - so you can get the same effect without the language's help:

Integer x = Integer.valueOf(100);
Integer y = Integer.valueOf(100);
System.out.println(x == y); // true

来自 JLS的第5.1.7节：

如果被装箱的值p为真，假，一个字节，或者在\ u0000到\ u007f范围内的字符，或者在-128到127之间的整数或短数字（包含），然后让r1和r2成为p的任意两次拳击转换的结果。总是这样的情况是r1 == r2。

If the value p being boxed is true, false, a byte, or a char in the range \u0000 to \u007f, or an int or short number between -128 and 127 (inclusive), then let r1 and r2 be the results of any two boxing conversions of p. It is always the case that r1 == r2.

理想情况下，装箱给定的原始值p总会产生相同的参考。实际上，使用现有的实现技术可能不可行。上述规则是务实的妥协。上面的最后一个条款要求将某些常见值装入无法区分的对象中。实现可以懒惰地或急切地缓存这些。对于其他值，此公式不允许对程序员的盒装值的身份进行任何假设。这将允许（但不要求）共享部分或全部这些引用。

Ideally, boxing a given primitive value p, would always yield an identical reference. In practice, this may not be feasible using existing implementation techniques. The rules above are a pragmatic compromise. The final clause above requires that certain common values always be boxed into indistinguishable objects. The implementation may cache these, lazily or eagerly. For other values, this formulation disallows any assumptions about the identity of the boxed values on the programmer's part. This would allow (but not require) sharing of some or all of these references.

这可以确保在大多数情况下，行为将是所需的行为，而不会强加不当的性能损失，特别是在小型设备上。例如，较少内存限制的实现可以缓存所有char和short值，以及-32K到+ 32K范围内的int和long值。

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.

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