Reference:

1. two types of array declaration: dataType[] arrayRefVar; and dataType arrayRefVar;. The former is preferred, while the latter is for the convenience of C++ programmers.

2. length length() size(). length属性针对数组，length()针对字符串string，size()针对泛型集合。

3. List lt; 会报错 List lt = new ArrayList(); 则不会报错。自定义对象如果不初始化，再访问时报错java.lang.NullPointerException。

4. 用==比较字符串实质是比较地址，永远不相等，应该使用equals()方法。

https://blog.csdn.net/GongchuangSu/article/details/51514389