Java StringBuilder

A String in Java never changes after you create it. Every change builds a brand-new String and leaves the old one behind. StringBuilder solves this problem. It holds text that you can edit in place, which saves time and memory.

A Notebook and a Whiteboard

A String works like a printed notebook page. A mistake means you print a fresh page. A StringBuilder works like a whiteboard. You erase a word, write a new one, and keep using the same board.

Diagram: String and StringBuilder

String s = "Hi";
s = s + " there";
s = s + "!";

Memory:  "Hi"   "Hi there"   "Hi there!"      (3 objects)

StringBuilder sb = new StringBuilder("Hi");
sb.append(" there");
sb.append("!");

Memory:  [ Hi there! ]                        (1 object)

Creating a StringBuilder

StringBuilder a = new StringBuilder();
StringBuilder b = new StringBuilder("Java");
StringBuilder c = new StringBuilder(100);   // starting capacity

The capacity tells Java how much room to reserve. The builder grows by itself when the text gets longer.

Important Methods

append

Adds text to the end.

StringBuilder sb = new StringBuilder("Hello");
sb.append(" World");
sb.append(2026);
System.out.println(sb);   // Hello World2026

insert

Adds text at a chosen position.

StringBuilder sb = new StringBuilder("Hello World");
sb.insert(5, ",");
System.out.println(sb);   // Hello, World

replace and delete

Replace swaps a range of characters. Delete removes a range. Both methods take a start index and an end index. The end index stays outside the range.

StringBuilder sb = new StringBuilder("I like tea");
sb.replace(7, 10, "milk");
System.out.println(sb);   // I like milk
sb.delete(0, 2);
System.out.println(sb);   // like milk

reverse

Flips the whole text.

StringBuilder sb = new StringBuilder("stressed");
System.out.println(sb.reverse());   // desserts

Other Useful Methods

Method	Job
length()	Returns the number of characters
charAt(i)	Returns the character at index i
setCharAt(i, ch)	Changes one character
deleteCharAt(i)	Removes one character
toString()	Converts the builder to a String

Speed Test in a Loop

Joining text inside a loop shows the biggest difference. A String creates thousands of temporary objects. A StringBuilder reuses one object.

StringBuilder sb = new StringBuilder();
for (int i = 1; i <= 5; i++) {
    sb.append(i).append(" ");
}
System.out.println(sb.toString().trim());   // 1 2 3 4 5

Chaining works because each append call returns the same builder.

StringBuilder and StringBuffer

StringBuffer offers the same methods as StringBuilder. StringBuffer protects its data when many threads use it together, and this protection makes it slower. Choose StringBuilder for single-thread programs, which covers most beginner work.

When to Use Each Type

Situation	Best Choice
Text never changes	String
Many edits or loops	StringBuilder
Many threads edit the same text	StringBuffer

Common Mistakes

• Comparing two builders with equals, which checks addresses and not text
• Printing a builder when you need a String and forgetting toString
• Using an end index that falls outside the text

Real Example: Building a Report Line

A billing program creates one text line for each item. The program joins the item name, the quantity, and the price. A StringBuilder collects all parts and returns one clean line.

StringBuilder line = new StringBuilder();
line.append("Pen").append(" x ").append(3).append(" = ").append(45.0);
System.out.println(line);   // Pen x 3 = 45.0

The same approach builds long messages, file contents, and web page text. Each added piece costs almost nothing.

Capacity and Growth

A builder starts with room for 16 characters when you give no size. Java doubles the space and adds a little when the text outgrows the room. Setting a good starting capacity avoids repeated growth in large jobs.

Quick Summary

• StringBuilder edits text in place
• append, insert, replace, delete, and reverse handle most jobs
• Loops with many joins run faster with StringBuilder