为什么1000==1000返回false，100==100返回true？

public static void main(String[] args) {
    Integer a = 1000, b = 1000;
    System.out.println(a == b);
    Integer c = 100, d = 100;
    System.out.println(c == d);
}

false
true

Integer a = 1000;

Integer i = Integer.valueOf(100);

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

Integer a = 1000, b = 1000;