详解Java 自动装箱与自动拆箱

ArrayList<Integer> list = new ArrayList<>();

list.add(3);

int x = list.get(0);

list.add(3);

//实际上面的代码会被编译器给自动的变成下面的这个代码
list.add(Integer.valueOf(3))

int x = list.get(0);

//实际上面的代码会被编译器给自动的变成下面的这个代码
int x = list.get(0).intValue();

Integer i1 = 100;
Integer i2 = 100;
Integer i3 = 300;
Integer i4 = 300;

System.out.println(i1 == i2);
System.out.println(i3 == i4);

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() {}
}