How to Find Substring in Java: Syntax and Examples

In Java, you can find a substring using the indexOf method which returns the position of the substring or -1 if not found. Alternatively, use contains to check if a substring exists, returning a boolean value.

Syntax

The main methods to find a substring in Java are:

• int indexOf(String substring): Returns the starting index of the substring or -1 if not found.
• boolean contains(CharSequence sequence): Returns true if the substring exists, otherwise false.

java

String str = "Hello, world!";
int position = str.indexOf("world");
boolean exists = str.contains("Hello");

Example

This example shows how to find the position of a substring and check if it exists in a string.

java

public class SubstringExample {
    public static void main(String[] args) {
        String text = "Welcome to Java programming!";
        String search = "Java";

        int index = text.indexOf(search);
        if (index != -1) {
            System.out.println("Substring '" + search + "' found at index: " + index);
        } else {
            System.out.println("Substring not found.");
        }

        boolean contains = text.contains(search);
        System.out.println("Does the text contain '" + search + "'? " + contains);
    }
}

Output

Substring 'Java' found at index: 11
Does the text contain 'Java'? true

Common Pitfalls

Common mistakes when finding substrings in Java include:

• Not checking if indexOf returns -1 before using the index.
• Confusing indexOf with contains: indexOf returns an index, contains returns a boolean.
• Case sensitivity: these methods are case sensitive, so "java" and "Java" are different.

java

String text = "Hello World";

// Wrong: Using index without checking
int pos = text.indexOf("world");
System.out.println("Position: " + pos); // Outputs -1, but no check

// Right: Check before using
if (pos != -1) {
    System.out.println("Found at: " + pos);
} else {
    System.out.println("Not found");
}

Output

Position: -1
Not found

Quick Reference

Method	Description	Return Type	Example
indexOf(String)	Finds starting index of substring	int	str.indexOf("abc")
contains(CharSequence)	Checks if substring exists	boolean	str.contains("abc")
substring(int, int)	Extracts substring by index range	String	str.substring(2,5)

Key Takeaways

Use indexOf to get the position of a substring; it returns -1 if not found.

Use contains to check if a substring exists; it returns true or false.

Always check indexOf result before using it to avoid errors.

Remember substring search is case sensitive in Java.

Use substring method to extract parts of a string by index.