Java: arr==null vs arr.length==0

当 arr 是一个array时，写Java开始的corner case常常会写类似下面的语句：

if(arr == null || arr.length == 0){
    return 0;
}

其实这是两个条件， arr==null 和 arr.length==0 是不同的：

arr.length == 0说明 arr is an empty array, which means an empty array is assigned to arr; arr.length would yields 0.

arr == null means that null is assigned to arr, arr.length will throw out NullPointerException.

所以在写这两个条件时必须保证arr==null在前。