Strings in Java | Basics of Java - Software Development PDF Download

Introduction

Strings are an essential part of any programming language, and Java is no exception. In Java, a string is a sequence of characters that represents text. Whether you want to display messages, process user input, or manipulate textual data, understanding strings is crucial. In this beginner-friendly guide, we will delve into the world of strings in Java, covering important concepts, syntax, and providing multiple examples to solidify your understanding.

What is a String?

A string in Java is an object that represents a sequence of characters. It is a reference type, which means it's stored in the heap memory. String objects are immutable, meaning their values cannot be changed after they are created. Whenever you modify a string, a new string object is created in memory.

Creating and Initializing Strings

There are several ways to create and initialize strings in Java:
Using String Literals:

String message = "Hello, World!";

Using the new Keyword:

String name = new String("John Doe");

String Concatenation

Concatenation refers to combining strings together. In Java, you can concatenate strings using the + operator or the concat() method:

Using the + Operator:

String firstName = "John";

String lastName = "Doe";

String fullName = firstName + " " + lastName;

System.out.println(fullName);

Output

John Doe

Using the concat() Method:

String greeting = "Hello";

String name = "Alice";

String message = greeting.concat(", ").concat(name);

System.out.println(message);

Output

Hello, Alice

String Length

To determine the length of a string (i.e., the number of characters it contains), you can use the length() method:

String text = "Hello, World!";

int length = text.length();

System.out.println(length);

Output

13

Accessing Characters in a String

Individual characters within a string can be accessed using their indexes. In Java, the index of the first character is 0, and the index of the last character is length - 1:

String name = "John Doe";

char firstLetter = name.charAt(0);

char lastLetter = name.charAt(name.length() - 1);

System.out.println("First letter: " + firstLetter);

System.out.println("Last letter: " + lastLetter);

Output

First letter: J

Last letter: e

Modifying Strings

As mentioned earlier, strings are immutable in Java. Therefore, if you want to modify a string, you need to create a new string object. Here are some common string manipulation methods:

Changing Case

String message = "Hello, World!";

String upperCase = message.toUpperCase();

String lowerCase = message.toLowerCase();

System.out.println(upperCase);

System.out.println(lowerCase);

Output

HELLO, WORLD!

hello, world!

Trimming Whitespace

String text = "   Hello, World!   ";

String trimmedText = text.trim();

System.out.println(trimmedText);

Output

Hello, World!

Comparing Strings

To compare strings for equality, you can use the equals() method. This method checks if the contents of two strings are the same:

String password = "password123";

String userInput = "password123";

boolean isMatch = password.equals(userInput);

System.out.println(isMatch);

Output

true

Sample Problems

Problem 1: Write a Java program to count the number of vowels in a given string.

public class VowelCounter {

    public static int countVowels(String text) {

        int count = 0;

        for (int i = 0; i < text.length(); i++) {

            char c = Character.toLowerCase(text.charAt(i));

            if (c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u') {

                count++;

            }

        }

        return count;

    }

    public static void main(String[] args) {

        String text = "Hello, World!";

        int vowelCount = countVowels(text);

        System.out.println("Number of vowels: " + vowelCount);

    }

}

Output

Number of vowels: 3

Problem 2: Write a Java program to reverse a given string.

public class StringReverser {

    public static String reverseString(String text) {

        String reversed = "";

        for (int i = text.length() - 1; i >= 0; i--) {

            reversed += text.charAt(i);

        }

        return reversed;

    }

    public static void main(String[] args) {

        String text = "Hello, World!";

        String reversedText = reverseString(text);

        System.out.println("Reversed string: " + reversedText);

    }

}

Output

Reversed string: !dlroW ,olleH

Conclusion

Understanding strings is crucial for any Java programmer. In this article, we covered the basics of strings, including creation, concatenation, length, accessing characters, modifying strings, and comparing them. By practicing the provided examples and solving the sample problems, you will gain a solid foundation in working with strings in Java. Keep exploring and experimenting with strings to enhance your Java programming skills.