autoboxing and unboxing

最新推荐文章于 2025-08-15 22:56:20 发布

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最新推荐文章于 2025-08-15 22:56:20 发布

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文章标签： java

原文链接：http://www.cnblogs.com/reynold-lei/p/4367540.html

探讨Java中自动装箱与拆箱的过程，并解析当数值在-128到127范围内时，Integer对象如何通过缓存机制实现引用相等的现象。文章通过示例代码说明了这一机制的具体实现。

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　Why does 128==128 return false but 127==127 return true

 1 public static void autoboxingUnboxing(){
 2         Integer ta = 127;
 3         Integer tb = 127;
 4         Integer tc = 128;
 5         Integer td = 128;
 6         if(ta == tb) System.out.println("ta equals tb");
 7         if(tc == td) System.out.println("tc equals td");
 8         
 9     }
10 
11 
12 Output:
13 ta equals tb

When you compile a number literal in Java and assign it to a Integer (capital I) the compile emits:

Integer b2 =Integer.valueOf(127)

This line of code is also generated when you use autoboxing.

valueOf is implemented such that certain numbers are "pooled", and it returns the same instance for values smaller than 128.

From the java 1.6 source code, line 621:

 public static Integer valueOf(int i) { if(i >= -128 && i <= IntegerCache.high) return IntegerCache.cache[i + 128]; else return new Integer(i); }

The value of high can be configured to another value, with the system property.

-Djava.lang.Integer.IntegerCache.high=999

If you run your program with that system property, it will output true!

The obvious conclusion: never rely on two references being identical, always compare them with .equals() method.

So b2.equals(b3) will print true for all logically equal values of b2,b3.

private static class IntegerCache {
    private IntegerCache(){}

    static final Integer cache[] = new Integer[-(-128) + 127 + 1];

    static {
        for(int i = 0; i < cache.length; i++)
            cache[i] = new Integer(i - 128);
    }
}

public static Integer valueOf(int i) {
    final int offset = 128;
    if (i >= -128 && i <= 127) { // must cache 
        return IntegerCache.cache[i + offset];
    }
    return new Integer(i);
}

As correctly inferred, this caching code was added to support the object identity semantics of autoboxing for values between -128 and 127 (inclusive), as required by the language specification. Since the same object must be returned for integral values in this range, once the autoboxed value for, say, 0 is created, it must be retained until the virtual machine exits. That leaves the issue of whether or not the values in question are created eagerly or lazily and whether or not any container for those values is created eagerly or lazily.

转载于:https://www.cnblogs.com/reynold-lei/p/4367540.html