String Builder Example in Java

Introduction

String Builder is a classic Java console program that demonstrates the concept with complete source code and sample output. Strings are immutable objects in Java; the examples show comparison, searching and transformation.

This tutorial walks through the program line by line, explains how the logic works, and highlights best practices you can apply in your own code.

Definition

To avoid unnecessary memory usage, StringBuilder class is used. It provides similar functionality to Strings, but stores its data in a mutable way. Only one StringBuilder object is created. Also because object creation is time consuming, using StringBuilder produces much faster code.

Syntax

StringBuilder Variable_name=new StringBuilder("String_sequence");

String Builder Example Program

public class StringBuilderExample {
    public static void main(String[] args) {
        String str = "She sells sea shells on the sea shore";
		System.out.println("The input string is: "+str);
        int len = str.length();
        char[] arr1 = new char[len];
        char[] arr2 = new char[len];
        for (int i = 0; i < len; i++) {
            arr1[i] = 
                str.charAt(i);
        } 
        for (int j = 0; j < len; j++) {
            arr2[j] =
                arr1[len - 1 - j];
        }
        String revstr =
            new String(arr2);
        System.out.println("After operation, the input string becomes: "+revstr);
    }
}

Sample Output

The input string is: She sells sea shells on the sea shore
After operation, the input string becomes: erohs aes eht no sllehs aes slles ehS

When to use

Use string manipulation when cleaning user input, parsing text files or formatting messages.

How it works

1. Execution begins in the main method — the JVM calls this method when you run the class.

2. String str = "She sells sea shells on the sea shore"; updates a variable used in the calculation or output.

3. A println / print call writes text to the console — part of the sample output below.

4. int len = str.length(); updates a variable used in the calculation or output.

5. char[] arr1 = new char[len]; updates a variable used in the calculation or output.

6. char[] arr2 = new char[len]; updates a variable used in the calculation or output.

7. for (int i = 0; i < len; i++) { updates a variable used in the calculation or output.

8. A println / print call writes text to the console — part of the sample output below.

Best Practices

• Use meaningful variable and class names that describe their purpose.
• Compile and run the program locally — modify values to see how output changes.
• Read compiler errors carefully; they usually point to the exact line to fix.

Common Mistakes

• Copying code without understanding each line — practice by changing one statement at a time.
• Mismatching the public class name and the .java filename.
• Forgetting semicolons at the end of statements.