String indexOf() method example in Java

The indexOf() method of String class determines the index of the first occurrence of a particular String or a character within a String object. If the argument is not found in string, the method returns -1.

There are 4 versions of String indexOf method

1). indexOf(char ch) - returns index position for the given char value.
2). indexOf(char ch, int fromIndex ) - returns index position for the given char value and from fromIndex location.
3). indexOf(String substring, int char) - returns index position for the given char value and from fromIndex location.
4). indexOf(String str) - returns index position for the given str.

Passing null argument

Passing null argument is not allowed to indexOf() method. It will result in a NullPointerException exception. 



public class StringIndexOfExample0 {
	public static void main(String[] arr) {
		String str = new String("Welcome to Java programming.The Java is good programming language.");

		System.out.println(str.indexOf(null));
	}
}
output:
Exception in thread "main" java.lang.NullPointerException
	at java.lang.String.indexOf(String.java:1718)
	at java.lang.String.indexOf(String.java:1698)
	at StringIndexOfExample0.main(StringIndexOfExample0.java:5)

1). indexOf(char ch)

It returns the first index of the char ch found in the String object.



 public class StringIndexOfExample1 {
	public static void main(String[] arr) {
		String str1 = new String("Welcome to java programming!!!");

		int index1 = str1.indexOf('a');
		int index2 = str1.indexOf('p');
		int index3 = str1.indexOf('o');

		System.out.println("First Index of 'a' in " + str1 + " : " + index1);
		System.out.println("First Index of 'p' in " + str1 + " : " + index2);
		System.out.println("First Index of 'o' in " + str1 + " : " + index3);
	}
}

output:
First Index of 'a' in Welcome to java programming!!! : 12
First Index of 'p' in Welcome to java programming!!! : 16
First Index of 'o' in Welcome to java programming!!! : 4

2). indexOf(char ch, int fromIndex )

This method returns the index position for the given char value and from fromIndex location.



public class StringIndexOfExample2
{
public static void main(String[] arr)
{
String str = new String("Welcome to Java programming.The Java is good programming language.");
 
System.out.println("the String is : "+str);
int index1 = str.indexOf("p",1);
int index2 = str.indexOf("J",4);
int index3 = str.indexOf("W",0);
int index4 = str.indexOf("g",12);
int index5 = str.indexOf("x",0);

System.out.println("\nFirst Index of programming is : "+ index1);
System.out.println("First Index of java is : "+ index2);
System.out.println("First Index of Welcome is : "+ index3);
System.out.println("First Index of good is : "+ index4);
System.out.println("First Index of Hello is : "+ index5);
}
}
output:

the String is : Welcome to Java programming.The Java is good programming language.

First Index of programming is : 16
First Index of java is : 11
First Index of Welcome is : 0
First Index of good is : 19
First Index of Hello is : -1

3). indexOf(String substring, int char)

This method returns the index position for the given char value and from fromIndex location.


public class StringIndexOfExample3 {
	public static void main(String[] arr) {
		String str = new String("Welcome to Java programming.The Java is good programming language.");

		System.out.println("the String is : " + str);
		int index1 = str.indexOf("programming", 7);
		int index2 = str.indexOf("Java", 4);
		int index3 = str.indexOf("Welcome", 0);
		int index4 = str.indexOf("good", 12);
		int index5 = str.indexOf("Hello", 0);

		System.out.println("\nFirst Index of programming is : " + index1);
		System.out.println("First Index of java is : " + index2);
		System.out.println("First Index of Welcome is : " + index3);
		System.out.println("First Index of good is : " + index4);
		System.out.println("First Index of Hello is : " + index5);
	}
}

output:

 the String is : Welcome to Java programming.The Java is good programming language.

First Index of programming is : 16
First Index of java is : 11
First Index of Welcome is : 0
First Index of good is : 40
First Index of Hello is : -1

4). indexOf(String str)

This method returns the index position for the given str.

Here we are searching for the index of a particular String value in a String object using the indexOf() method. If the searched String is found, its first index is returned, otherwise, if the String is not found then -1 is returned.


public class StringIndexOfExample4
{
public static void main(String[] arr)
{
String str = new String("Welcome to Java programming.The Java is good programming language.");
 
System.out.println("the String is : "+str);
int index1 = str.indexOf("programming");
int index2 = str.indexOf("Java");
int index3 = str.indexOf("Welcome");
int index4 = str.indexOf("good");
int index5 = str.indexOf("Hello");

System.out.println("\nFirst Index of programming is : "+ index1);
System.out.println("First Index of java is : "+ index2);
System.out.println("First Index of Welcome is : "+ index3);
System.out.println("First Index of good is : "+ index4);
System.out.println("First Index of Hello is : "+ index5);
}
}
output:
 the String is : Welcome to Java programming.The Java is good programming language.

First Index of programming is : 16
First Index of java is : 11
First Index of The is : 28
First Index of good is : 40
First Index of Hello is : -1

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In this article, we have seen String indexOf() method example in Java. All source code in the article can be found in the GitHub repository.