JAVA

Strings and String Methods

Text is one of the most common types of data used in software applications. Usernames, email addresses, product names, search queries, messages, and addresses are all examples of text data. In Java, text is represented using the String class.

Strings are used in almost every Java application. Whether you are building a website, a mobile application, or a backend service, you will frequently work with text data. Java provides many built-in methods that make it easy to manipulate, search, compare, and transform strings.

In this chapter, you will learn how to create strings, access individual characters, and use the most common String methods available in Java.

What is a String?

A String is a sequence of characters.

Examples

"Hello"
"Java"
"OpenAI"
"john@example.com"

Unlike primitive data types such as int and double, a String is an object in Java.

Creating Strings

Syntax

String variableName = "value";

Example

String name = "Rahul";

Here:

• String is the data type
• name is the variable name
• "Rahul" is the value

Displaying a String

Example

String company = "OpenAI";

System.out.println(company);

Output

OpenAI

String Length

The length() method returns the number of characters in a string.

Example

String language = "Java";

System.out.println(language.length());

Output

4

Explanation

The word Java contains four characters.

Converting to Uppercase

The toUpperCase() method converts all characters to uppercase.

Example

String language = "java";

System.out.println(language.toUpperCase());

Output

JAVA

Converting to Lowercase

The toLowerCase() method converts all characters to lowercase.

Example

String language = "JAVA";

System.out.println(language.toLowerCase());

Output

java

Accessing Individual Characters

The charAt() method returns the character at a specified index.

Example

String language = "Java";

System.out.println(language.charAt(0));

Output

J

Explanation

Indexes start from 0.

J  a  v  a
0  1  2  3

Finding a Character Position

The indexOf() method returns the position of a character or substring.

Example

String language = "Java";

System.out.println(language.indexOf('v'));

Output

2

Checking if Text Exists

The contains() method checks whether a string contains specific text.

Example

String email = "john@example.com";

System.out.println(email.contains("@"));

Output

true

Comparing Strings

To compare strings, use the equals() method.

Example

String password1 = "admin123";
String password2 = "admin123";

System.out.println(password1.equals(password2));

Output

true

Explanation

The values are identical, so the result is true.

Comparing Without Case Sensitivity

The equalsIgnoreCase() method ignores uppercase and lowercase differences.

Example

String city1 = "Mumbai";
String city2 = "mumbai";

System.out.println(city1.equalsIgnoreCase(city2));

Output

true

Extracting Part of a String

The substring() method extracts a portion of a string.

Example

String email = "john@example.com";

System.out.println(email.substring(0, 4));

Output

john

Replacing Characters or Words

The replace() method replaces characters or text.

Example

String text = "Java Programming";

System.out.println(text.replace("Java", "Python"));

Output

Python Programming

Removing Extra Spaces

The trim() method removes spaces from the beginning and end of a string.

Example

String username = "   Rahul   ";

System.out.println(username.trim());

Output

Rahul

Checking Start and End Text

startsWith()

Example

String website = "https://example.com";

System.out.println(website.startsWith("https"));

Output

true

endsWith()

Example

String file = "photo.jpg";

System.out.println(file.endsWith(".jpg"));

Output

true

Splitting a String

The split() method breaks a string into multiple parts.

Example

String tags = "Java,Python,JavaScript";

String[] result = tags.split(",");

System.out.println(result[0]);
System.out.println(result[1]);
System.out.println(result[2]);

Output

Java
Python
JavaScript

String Immutability

Strings in Java are immutable.

This means that once a String is created, its value cannot be changed.

Example

String name = "Rahul";

name.toUpperCase();

System.out.println(name);

Output

Rahul

The original string remains unchanged.

Correct

name = name.toUpperCase();

System.out.println(name);

Output

RAHUL

Common String Methods

Complete Program

Example

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);

        System.out.print("Enter your email: ");
        String email = sc.nextLine();

        if(email.contains("@")) {
            System.out.println("Valid Email Format");
        }
        else {
            System.out.println("Invalid Email Format");
        }

    }
}

Input

john@example.com

Output

Valid Email Format

Explanation

• The user enters an email address.
• The program checks whether the email contains @.
• If it does, a success message is displayed.
• Otherwise, an error message is displayed.

Common Mistakes

Using == for String Comparison

Wrong

String a = "Java";
String b = "Java";

System.out.println(a == b);

Correct

System.out.println(a.equals(b));

Always use equals() when comparing string values.

Accessing an Invalid Index

Wrong

String text = "Java";

System.out.println(text.charAt(4));

Problem

Valid indexes are:

0
1
2
3

Index 4 does not exist.

Correct

System.out.println(text.charAt(3));

Key Takeaways

• Strings are used to store text data.
• A String is an object in Java.
• Java provides many built-in methods for working with strings.
• length() returns the number of characters.
• charAt() accesses individual characters.
• equals() compares string values.
• substring() extracts text.
• replace() modifies content.
• Strings are immutable.
• String methods make text processing simple and efficient.