Java String Methods

Java string methods are built-in actions you can call on a String value. They help you inspect text, search inside it, extract pieces, compare it, and create changed versions.

You call a string method with dot syntax, such as message.length() or name.toUpperCase().

Inspecting A String

Use length() to count characters and charAt() to read one character at a specific index. String indexes start at 0, so the first character is at index 0.

public class Main {
    public static void main(String[] args) {
        String word = "Library";

        System.out.println("Text: " + word);
        System.out.println("Length: " + word.length());
        System.out.println("First: " + word.charAt(0));
        System.out.println("Last: " + word.charAt(word.length() - 1));
    }
}

Output:

Text: Library
Length: 7
First: L
Last: y

The expression word.length() - 1 gives the last valid index. For "Library", the length is 7, so the last index is 6.

Searching Inside Text

Use contains() when you only need a true or false answer. Use indexOf() when you need to know where the text starts.

Method	What it returns
contains(text)	true if the string contains the text, otherwise false.
indexOf(text)	The first matching index, or -1 if the text is not found.
startsWith(text)	true if the string begins with the text.
endsWith(text)	true if the string ends with the text.

public class Main {
    public static void main(String[] args) {
        String fileName = "report-final.pdf";

        System.out.println(fileName.contains("final"));
        System.out.println(fileName.indexOf("final"));
        System.out.println(fileName.startsWith("report"));
        System.out.println(fileName.endsWith(".pdf"));
        System.out.println(fileName.indexOf("draft"));
    }
}

Output:

true
7
true
true
-1

These searches are case-sensitive. For example, contains("Final") would be false for "report-final.pdf".

Extracting Part Of A String

The substring() method returns part of a string. With two indexes, substring(start, end) starts at start and stops before end.

public class Main {
    public static void main(String[] args) {
        String code = "JAVA-2026-BASICS";

        String topic = code.substring(0, 4);
        String year = code.substring(5, 9);
        String level = code.substring(10);

        System.out.println("Topic: " + topic);
        System.out.println("Year: " + year);
        System.out.println("Level: " + level);
    }
}

Output:

Topic: JAVA
Year: 2026
Level: BASICS

The one-argument version, such as substring(10), returns everything from that index to the end of the string.

Creating Changed Versions

Strings in Java are immutable, which means a method does not edit the original string. Methods such as trim(), replace(), toLowerCase(), and toUpperCase() return new strings.

public class Main {
    public static void main(String[] args) {
        String input = "  Java Basics  ";

        String cleaned = input.trim();
        String lower = cleaned.toLowerCase();
        String updated = lower.replace("basics", "methods");

        System.out.println("Original: [" + input + "]");
        System.out.println("Cleaned: [" + cleaned + "]");
        System.out.println("Updated: " + updated);
    }
}

Output:

Original: [  Java Basics  ]
Cleaned: [Java Basics]
Updated: java methods

Comparing Text

Use equals() to compare strings with matching case. Use equalsIgnoreCase() when uppercase and lowercase differences should not matter.

public class Main {
    public static void main(String[] args) {
        String savedAnswer = "java";
        String userAnswer = "Java";

        System.out.println(savedAnswer.equals(userAnswer));
        System.out.println(savedAnswer.equalsIgnoreCase(userAnswer));
    }
}

Output:

false
true

Common Mistakes

• Do not use an index less than 0 or greater than the last valid index.
• Remember that substring(start, end) stops before end.
• Assign the return value when you want to keep a changed version of a string.
• Use equals() or equalsIgnoreCase() for text comparison, not ==.

Takeaway: Java string methods let you read, search, extract, compare, and transform text while keeping the original string unchanged.