autoboxing and unboxing

　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.