main

/**测试null Object直接强转Integer, 强转成功，输出null*/
public static void main(String[] args) {
        Object data = null;
        Integer data1 = (Integer) data;
        System.out.println(data1);
    }

/**验证 null+"" 变成字符串"null"，确实如此*/
    public static void main(String[] args) {
        Integer num = null;
        String numStr = num + "";
        System.out.println(null == numStr);
        System.out.println("null".equalsIgnoreCase(numStr));
    }

/**验证String和Boolean对象相等问题，Boolean很特殊*/
public static void main(String[] args) {
        String s1 = new String("newString");
        String s2 = new String("newString");
        String s3 = "string";
        String s4 = "string";
        System.out.println(s1 == s2);//false
        System.out.println(s3 == s4);//true

        Boolean b1 = new Boolean("true");
        Boolean b2 = new Boolean("true");
        Boolean b3 = Boolean.valueOf("true");
        Boolean b4 = Boolean.valueOf("true");
        System.out.println(b1.equals(b2));//true
        System.out.println(b3.equals(b4));//true
    }

/**测试自动装箱带来的性能问题*/
public static void main(String[] args) {
        long startTime = System.currentTimeMillis();
        /**使用Long耗时9秒，自动装箱导致构造了大约2^31个多余Long实例*/
        Long sum = 0L;
        /**使用long耗时0秒*/
//        long sum = 0L;
        for (int i = 0; i < Integer.MAX_VALUE; i++) {
            sum += i;
        }
        long endTime = System.currentTimeMillis();
        System.out.println("总和: " + sum + "\n耗时: " + (endTime-startTime)/1000);
    }

/**instanceOf 和 getClass*/
class Super{}

class Sub extends Super{
    public static void main(String[] args) {
        Super parent = new Super();
        Sub child = new Sub();
        System.out.println(child instanceof Super);
        System.out.println(child.getClass() == parent.getClass());
    }
}