indexOf determines whether a string contains another string

The expression in jdk is as follows
indexOf
public int indexOf(String str) returns the index of the first occurrence of the specified substring in this string. The returned integer is the smallest value of k for which
this.startsWith(str, k)
is true.

Parameters:
str - an arbitrary string.
Returns:
If the string argument appears as a substring in this object, returns the index of the first character of the first such substring; if it does not appear as a substring, returns -1.

According to the description, this method can be used to determine whether a string is in another string.
The index starts from 0. If the string does not contain another string, it returns -1. The

following demo is easy to understand.

package test;


public class Test {
	public static void main(String[] args) {
		String x = "Hello World/XXX";
		String y = "Hello World/";
		System.out.println("Returns the index value of the first occurrence of y in x: "+x.indexOf(y));
		System.out.println("Returns the index value of the first occurrence of x in y: "+y.indexOf(x));
		if(x.indexOf(y)!=-1){
			System.out.println("x包含y");
		}else{
			System.out.println("x does not contain y");
		}
	}
}

Result:
Returns the index value of the first occurrence of y in x: 0
Returns the index value of the first occurrence of x in y: -1
x contains y