如何通过源码了解Java的自动装箱拆箱详解

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

public static Integer valueOf(int i) {
    // -128 - 127
    if (i >= IntegerCache.low && i <= IntegerCache.high)
        return IntegerCache.cache[i + (-IntegerCache.low)];
    return new Integer(i);
}

public static void main(String[] args) {
    long startTime = System.currentTimeMillis();
    Integer count = 0;
    // int count = 0;
    for (int i = 0; i < 5000000; i++) {
        count += i;
    }
    System.out.println("计算时长：" + (System.currentTimeMillis() - startTime) + " ms");
}

// 执行结果：
// Integer 计算时长：51 ms
// int 计算时长：6 ms

public static void main(String[] args) {
// TODO 自动生成的方法存根
Integer a = 1;
Integer b = 2;
Integer c = 3;
Integer d = 3;
Integer e = 321;
Integer f = 321;
Long g = 3L;
System.out.println(c==d);//true
//包装类的==在没有遇到算术运算的情况下不会自动拆箱
System.out.println(e==f);//false
System.out.println(c==(a+b));//true
System.out.println(c.equals(a+b));//true
System.out.println(g==(a+b));//true
//equals方法不会处理数据转型关系
System.out.println(g.equals(a+b));//false
}

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