Integer IntegerCache源码阅读

/*public static void main(String[] args) {
    Integer a = 128, b = 128;
    Integer c = 127, d = 127;
    System.out.println(a == b);//false
    System.out.println(c == d);//true

  }*/
  
  /*public static void main(String[] args) {
    Integer int1 = Integer.valueOf("100");
    Integer int2 = Integer.valueOf("100");
    System.out.println(int1 == int2);//true
  }*/

  public static void main(String[] args) {
    Integer int1 = Integer.valueOf("300");
    Integer int2 = Integer.valueOf("300");
    System.out.println(int1 == int2);//false
  }

public static Integer valueOf(String s) throws NumberFormatException {
    return Integer.valueOf(parseInt(s, 10));
  }

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

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() {}
  }