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作者: 阿桃_28e7 | 来源:发表于2018-12-25 17:36 被阅读0次
    /**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());
        }
    }
    

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