Java 基础知识

基础

Integer类

public class Test {
    public static void main(String[] args){
      Integer a = new Integer(129);
      Integer b = new Integer(129);
      
      // 1
      System.out.println(a == b);
      
      
      Integer c = new Integer(127);
      Integer d = new Integer(127);
      // 2
      System.out.println(c == d);
    }
}

上述代码可以看出1处输出 false； 2 处输出 true。从Integer类源码中，有一个内部类，代码如下：

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() {}
    }