Instance + source code = see through 128 traps

Catalog

What is? 128 trap

Source code analysis

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What is? 128 trap

What is? 128 What about the trap ？ as follows ：

Integer a1 = 10;
Integer a2 = 10;
System.out.println(a1 == a2);  true
Integer b1 = 128;
Integer b2 = 128;
System.out.println(b1 == b2);  false

You can see , The results returned are not two true. Accurately speaking , Compare according to the above form , In the interval

-128 To 127 All the results are true

Why is there this phenomenon ？ Let's analyze first ：

• Integer Is a wrapper class , Is a class , Reference data type .
• == Compare its address .

Thus we can see that a1 and a2 The same address as ,b1 and b2 Is different .

Source code analysis

Here is Integer An internal code , Some parts are omitted by me , Let's take a few lines to analyze .

private static class IntegerCache {
        static final int low = -128;
        static final int high;
        static final Integer cache[];

        static {
            // high value may be configured by property
            int h = 127;
            …………

            high = h;

            cache = new Integer[(high - low) + 1];
            int j = low;
            for(int k = 0; k < cache.length; k++)
                cache[k] = new Integer(j++);

            // range [-128, 127] must be interned (JLS7 5.1.7)
            assert IntegerCache.high >= 127;
        }

        private IntegerCache() {}
    }

         It's not hard to see. , The above code implements , take -127 To 128 The number of is placed in one cache Array , Is already created .

         If in this range , It will directly return the address in the array created in advance . So use == To compare is equal .

Not in this range b1 and b2, Will open up new memory space .

Keep early hours , take care , The king of early rising wishes you good luck ——

sorry , I saw through your trap .