== and equals

Integer 作为包装类型，数据在 -128 至 127 范围内的赋值， Integer 对象是在 IntegerCache. cache 产生，会复用已有对象

    @Test
    public void test15() {
        String a = "a"; //存在与栈中
        String b = "a"; //存在与栈中
        String A = new String("A"); //存在与堆中
        String B = new String("A"); //存在与堆中
        
        System.out.println(a == b); //true
        
        System.out.println(A == B); //false

        
        System.out.println(Objects.equals(a, b)); //true
        System.out.println(Objects.equals(A, B)); //true
    }

Integer 类型的比较

    public boolean equals(Object obj) {
        if (obj instanceof Integer) {
            return value == ((Integer)obj).intValue();
        }
        return false;
    }

String 类型的比较

    public boolean equals(Object anObject) {
        if (this == anObject) {
            return true;
        }
        if (anObject instanceof String) {
            String anotherString = (String)anObject;
            int n = value.length;
            if (n == anotherString.value.length) {
                char v1[] = value;
                char v2[] = anotherString.value;
                int i = 0;
                while (n-- != 0) {
                    if (v1[i] != v2[i])
                        return false;
                    i++;
                }
                return true;
            }
        }
        return false;
    }