Mutable Strings: StringBuilder vs StringBuffer for Performance0%

Mutable Strings: StringBuilder vs StringBuffer for Performance

Beginner13 min read•Updated: 2026-10-22

⚡ Mutable Strings: StringBuilder vs StringBuffer

Introduction

Because Java String is immutable, concatenating strings in a loop (str += i) creates a brand-new object on every iteration, destroying performance and triggering heavy garbage collection. For repetitive modifications, use StringBuilder.

⚠️ The String Concatenation Loop Trap

1// ❌ HORRIBLE: Allocates 10,000 temporary String objects on the heap!
2String s = "";
3for (int i = 0; i < 10000; i++) {
4    s += i; // Incurs O(N^2) memory copying!
5}
6 
7// ✅ OPTIMAL: Mutates a single internal char array buffer in O(N) time!
8StringBuilder sb = new StringBuilder();
9for (int i = 0; i < 10000; i++) {
10    sb.append(i);
11}
12String result = sb.toString();

🆚 StringBuilder vs StringBuffer

Feature	`StringBuilder`	`StringBuffer`
Introduced	Java 5	Java 1.0
Thread Safety	Not Thread-Safe (No synchronization)	Thread-Safe (Synchronized methods)
Performance	Fastest (Recommended for 99% of code)	Slower due to locking overhead

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Modern Text Blocks (""") in Java 15+

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Essential String Methods (substring, indexOf, replace, split, trim)	Modern Text Blocks (""") in Java 15+

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Modern Text Blocks (""") in Java 15+

Mutable Strings: StringBuilder vs StringBuffer for Performance

Beginner13 min read•Updated: 2026-10-22

Study Materials

⚡ Mutable Strings: StringBuilder vs StringBuffer

Introduction

⚠️ The String Concatenation Loop Trap

1// ❌ HORRIBLE: Allocates 10,000 temporary String objects on the heap!
2String s = "";
3for (int i = 0; i < 10000; i++) {
4    s += i; // Incurs O(N^2) memory copying!
5}
6 
7// ✅ OPTIMAL: Mutates a single internal char array buffer in O(N) time!
8StringBuilder sb = new StringBuilder();
9for (int i = 0; i < 10000; i++) {
10    sb.append(i);
11}
12String result = sb.toString();

🆚 StringBuilder vs StringBuffer

Feature	`StringBuilder`	`StringBuffer`
Introduced	Java 5	Java 1.0
Thread Safety	Not Thread-Safe (No synchronization)	Thread-Safe (Synchronized methods)
Performance	Fastest (Recommended for 99% of code)	Slower due to locking overhead

Next Lesson

Modern Text Blocks (""") in Java 15+

Continue learning with hands-on practice, examples, and exercises in the upcoming topic.

Related Lessons

Previous Lesson	Next Lesson
Essential String Methods (substring, indexOf, replace, split, trim)	Modern Text Blocks (""") in Java 15+

Practice Quiz

Test your understanding of this lesson with 1 questions. Each question has one correct answer.

Previous Next