今天做leetcode題目時，發現int mid = 536848900，但是mid*mid=484528144，我說為啥程式執行一直出錯呢！！

     int mid = 536848900;
        System.out.println(mid * mid);//484528144

        long mid2 = 536848900;//long * long
        System.out.println(mid2 * mid2);//288206741431210000

        System.out.println(Integer.MAX_VALUE);//2147483647
        System.out.println(Math.pow(2, 31) - 1);//2.147483647E9

        int a = Integer.MAX_VALUE;
//        a += 1;//-2147483648
        a += 100;//-2147483549

        long MonthNanoSeconds1 = 30 * 24 * 3600 * 1000 * 1000;//右邊的結果是int,已溢位

        //int * long,與最後一個long型的1000相乘之前就已經溢位，所以結果也不對
        long MonthNanoSeconds2 = 30 * 24 * 3600 * 1000 * 1000L;

        //這是正確的寫法
        long MonthNanoSeconds3 = 30L * 24 * 3600 * 1000 * 1000;//long * int

        System.out.println(MonthNanoSeconds1);//2134720512
        System.out.println(MonthNanoSeconds2);//-1702967296000
        System.out.println(MonthNanoSeconds3);//2592000000000
        System.out.println(30 * 24 * 3600 * 1000);//-1702967296
        System.out.println(30 * 24 * 3600 * 1000 * 1000);//2134720512
 

在Java Language Specifictionz中所述(JSL 15.7.1)

If an integer multiplication overflows, then the result is the low-order bits of the mathematical product as represented in some sufficiently large two’s-complement format. As a result, if overflow occurs, then the sign of the result may not be the same as the sign of the mathematical product of the two operand values.

也就是說int型整數相乘，結果只會保留低32位，高位會拋棄掉。

加減乘除的運算子優先順序都是從左向右。

參考：

《java int溢位總結》 https://njucz.github.io/2017/08/16/java-int%E6%BA%A2%E5%87%BA%E6%80%BB%E7%BB%93/ 
《java中short、int、long、float、double取值範圍》 https://blog.csdn.net/qfikh/article/details/52832087 
《[Java]運算子優先順序參考圖表》 https://blog.csdn.net/xiaoli_feng/article/details/4567184