[Java] How to get the length of different data types

//一维数组长度
int[] arr = new arr[3];
int arrLength = arr.length;

//二维数组的行数
int[][] arrs = new arrs[3][3];
int arrsLength = arrs.length;

//二维数组中某一个一维数组的长度
int arrsLength1 = arrs[1].length;

//得到字符串长度
String str = "mystring";
int strLength = str.length();

//得到集合长度
List<String> a1=new ArrayList<String>();
a1.add("mystring");
System.out.println(a1.size());