StringBuilder Basics

Part of the Logic & Flow section of Coddy's Java journey — lesson 44 of 59.

StringBuilder is used to create mutable (modifiable) strings. It's more efficient than regular String concatenation when you need to modify strings multiple times.

Create a StringBuilder with initial text:

StringBuilder sb = new StringBuilder("Hello");

Add more text using append():

sb.append(" World");

After executing the above code, sb contains:

Hello World

Let's use insert() to add text at a specific position:

StringBuilder sb = new StringBuilder("Java");
sb.append(" is");      // adds at end
sb.insert(0, "Hey ");  // adds at position 0 (start)

After executing the above code, sb contains:

Hey Java is

Converting StringBuilder to String:
When you need to return or store the result as a String, use toString():

StringBuilder sb = new StringBuilder("Hello");
sb.append(" World");

String result = sb.toString();  // Converts to String
System.out.println(result);     // Prints: Hello World

Why use toString()? Methods that return String type cannot return a StringBuilder directly - you must convert it first using toString().

Challenge

Easy

Create a method named buildPhrase that takes three arguments:

A String (start) for the beginning of the phrase
A String (middle) for the middle part
A String (end) for the ending

The method should:

Create a StringBuilder with the start string
Add a space and the middle string
Add a space and the end string
Add an exclamation mark at the end
Return the final string

Return messages should be:

If any input is null: return "Invalid input"
For successful operation: return the built phrase

Cheat sheet

StringBuilder is used to create mutable strings, more efficient than String concatenation for multiple modifications.

Create a StringBuilder with initial text:

StringBuilder sb = new StringBuilder("Hello");

Add text at the end using append():

sb.append(" World");

Insert text at a specific position using insert():

sb.insert(0, "Hey ");  // adds at position 0 (start)

Convert StringBuilder to String:

String result = sb.toString();

Try it yourself

import java.util.Scanner;

public class Main {
    public static String buildPhrase(String start, String middle, String end) {
        // Write your code here
    }
    
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String start = scanner.nextLine();
        String middle = scanner.nextLine();
        String end = scanner.nextLine();
        
        if (start.equals("null")) start = null;
        if (middle.equals("null")) middle = null;
        if (end.equals("null")) end = null;
        
        System.out.println(buildPhrase(start, middle, end));
    }
}