StringBuilder Methods

StringBuilder is used to create and modify mutable strings.

Unlike String, its content can be changed without creating a new object every time.

StringBuilder sb = new StringBuilder("Hello");

sb.append(" World");

System.out.println(sb);

Output:

Hello World

When to use?

Use StringBuilder when you need to modify a string many times, especially inside loops.

1. length()

Returns the current number of characters.

StringBuilder sb = new StringBuilder("Hello");

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

Output:

5

2. capacity()

Returns the amount of character space currently allocated.

StringBuilder sb = new StringBuilder();

System.out.println(sb.capacity());

Output:

16

The capacity can automatically increase when needed.

3. append()

Adds something to the end.

StringBuilder sb = new StringBuilder("Hello");

sb.append(" World");
sb.append("!");

System.out.println(sb);

Output:

Hello World!

4. insert()

Inserts something at a specific index.

StringBuilder sb = new StringBuilder("Helo");

sb.insert(2, "l");

System.out.println(sb);

Output:

Hello

5. delete()

Removes characters from start to end - 1.

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

sb.delete(5, 11);

System.out.println(sb);

Output:

Hello

6. deleteCharAt()

Removes one character.

StringBuilder sb = new StringBuilder("Hello");

sb.deleteCharAt(1);

System.out.println(sb);

Output:

Hllo

7. replace()

Replaces a range with another string.

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

sb.replace(6, 11, "Java");

System.out.println(sb);

Output:

Hello Java

8. setCharAt()

Changes a character at a specific index.

StringBuilder sb = new StringBuilder("Hollo");

sb.setCharAt(1, 'e');

System.out.println(sb);

Output:

Hello

9. reverse()

Reverses the entire sequence.

StringBuilder sb = new StringBuilder("Hello");

sb.reverse();

System.out.println(sb);

Output:

olleH

This is particularly useful in DSA:

StringBuilder sb = new StringBuilder("madam");

sb.reverse();

System.out.println(sb.toString());

10. toString()

Converts a StringBuilder into a String.

StringBuilder sb = new StringBuilder("Hello");

String text = sb.toString();

System.out.println(text);

This is useful when an API requires a String.

11. setLength()

Changes the length of the StringBuilder.

StringBuilder sb = new StringBuilder("Hello");

sb.setLength(3);

System.out.println(sb);

Output:

Hel

12. ensureCapacity()

Ensures that the builder has at least the specified capacity.

StringBuilder sb = new StringBuilder();

sb.ensureCapacity(100);

System.out.println(sb.capacity());

The capacity will be at least 100.

String vs StringBuilder

String

String text = "Hello";

text = text + " World";

String is immutable.

StringBuilder

StringBuilder text = new StringBuilder("Hello");

text.append(" World");

StringBuilder is mutable.

String ↔ StringBuilder

String → StringBuilder

String text = "Hello";

StringBuilder sb = new StringBuilder(text);

StringBuilder → String

String text = sb.toString();

StringBuilder vs StringBuffer

Use StringBuilder by default. Use StringBuffer when you specifically need its synchronization.

Quick Revision

Easy rule

String = immutable
StringBuilder = mutable and generally preferred for repeated string modifications