What is the use of indexof method in java

indexof method:

Note: The indexOf method returns an integer value indicating the starting position of the substring within the String object. If the substring is not found, -1 is returned.

public class IndexOf{
	public static void main(String[] args){
		String s="李宏#王海#林巧#陆寻#唐梅";
		String q="#"; //需要查找的字符串
		String err="*"; //不存在的字符串
		int i=0;
		for(int j=0;j<s.length();j++){ //循环所有字符串
			String get=s.substring(j,j+1); //打印所有字符串
			if(get.equals(q)){ //判断#字是否出现
				i++; //#字出现次数
			}
		}
		System.out.println("总共有"+s.length()+"个字符串");
		System.out.println("#字共出现了"+i+"次"); //#字符总共出现的次数
		System.out.println("第一个#字出现在字符串的"+s.indexOf(q)+"个位置"); 
		if(s.indexOf(err)==-1){ //返回-1则表示字符不存在字符串中
			System.out.println("*字在字符串中不存在");
		}
	}
}

Run results:

There are 14 strings in total.
# words appear 4 times in total.
The first # word appears in 2 of the strings. The word at position
* does not exist in the string

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