Integer internal cache

Integer Internal Cache

stay Java in , Every time you create an object, you need to allocate memory , To reduce the frequent creation of objects , Many places use pools to store objects , Such as String String pool in , For basic types , There are caches in the corresponding wrapper classes to avoid frequent creation of objects , Such as Integer.

Integer Medium IntegerCache Static inner class ：

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;
        String integerCacheHighPropValue =
            sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
        if (integerCacheHighPropValue != null) {
    
            try {
    
                int i = parseInt(integerCacheHighPropValue);
                i = Math.max(i, 127);
                // Maximum array size is Integer.MAX_VALUE
                h = Math.min(i, Integer.MAX_VALUE - (-low) -1);
            } catch( NumberFormatException nfe) {
    
                // If the property cannot be parsed into an int, ignore it.
            }
        }
        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() {
    }
}

The function of this class is to cache -128—127 These commonly used values , In the use of Integer.valueOf() When the method is used , We will first check whether it is within this range , If it's in this range , Point directly to the numeric address in the pool , If not, create a new object

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

But if directly new object , No checking operation

Integer it1 = new Integer(10);
Integer it2 = new Integer(10);
System.out.println(it1 == it2);
//  The output is false

Integer it1 = 10 ;
Integer it2 = 10 ;
System.out.println(it1 == it2);
// true

Integer it1 = Integer.valueOf(127);
Integer it2 = Integer.valueOf(127);
System.out.println(it1 == it2);
// true

Integer it1 = Integer.valueOf(128);
Integer it2 = Integer.valueOf(128);
System.out.println(it1 == it2);
// false

Besides , You can also set vm Parameter to set the maximum value of the cache , Such as ：

-Djava.lang.Integer.IntegerCache.high=2000