The String.contains() method is used to search for a sequence of characters within a string. In this article, we will learn how to effectively use the string contains functionality in Java.

Example:

In this example, we check if a specific substring is present in the given string.

// Java Program to demonstrate working contains() method
class Gfg {

    public static void main(String args[]) {
      
        String s = "My name is GFG";

        System.out.println(s.contains("GFG"));
        System.out.println(s.contains("geeks"));
    }
}

true
false

Syntax of contains() method

public boolean contains(CharSequence sequence);

Parameter:

• sequence: This is the sequence of characters to be searched.

Return Value:

• Returns true, if the sequence is found.
• Returns false, if not found.

Implementation of contains() method

public boolean contains(CharSequence sequence)
{
   return indexOf(sequence.toString()) > -1;
}

Here, the contains() method internally converts the CharSequence to a String and uses the indexOf() method to check for the substring. If indexOf() returns 0 or a positive value, contains() returns true; otherwise, it returns false.

Points to Remember:

• Case-Sensitive: The contains() method is case-sensitive, so "Java" and "java" are treated differently. because, first "J" is in upper case and the second "j" is in lower case.
• Null Handling: Passing null as a parameter will throw a NullPointerException.
• Supports CharSequence: It works with String, StringBuilder, and StringBuffer.

Examples of String.contains() Method

Now, we will explore different examples to understand how the contains() method works in Java.

Case-Sensitive Check Using contains() Method

Here, we demonstrate that the contains() method is case-sensitive when searching for substrings.

// Java Program to demonstrate case sensitivity of contains() method
class Gfg1 {

    public static void main(String args[])
    {
        String s = "Welcome! to GFG";

        System.out.println(s.contains("Gfg"));
        System.out.println(s.contains("GFG"));
    }
}

false
true

Null Handling Using contains() Method

This approach is used to safely handle scenarios, where a null value might be passed to the contains() method, preventing unexpected runtime exceptions.

// Java program to handle null 
// using contains() method of String class
public class NullHandling {
  
    public static void main(String[] args) {
      
        String s = "Null check";

        try {
            System.out.println(s.contains(null)); // This will throw an exception
        } catch (NullPointerException e) {
            System.out.println("NullPointerException caught: Substring cannot be null.");
        }
    }
}

NullPointerException caught: Substring cannot be null.

Using StringBuilder

This approach is used when we need to check if a String contains content from a StringBuilder.

// Java program to check if a String 
// contains a substring constructed using StringBuilder
public class StringBuilderExample {
  
    public static void main(String[] args) {
      
        String s1 = "Welcome to GFG";
        StringBuilder s2 = new StringBuilder("GFG");

        // Check if mainString contains substring
        System.out.println(s1.contains(s2)); // true
    }
}