StringBuilder vs StringBuffer in Java: Key Differences, Performance, and When to Use Each

When working with mutable strings in Java, two classes stand out: StringBuilder and StringBuffer. Both are designed for efficient string manipulation, but they differ in critical ways that impact performance and thread safety. In this guide, we’ll break down their differences, benchmark their performance, and explain when to use each.

StringBuilder vs StringBuffer: Key Differences

Why Does Synchronization Matter?

• StringBuffer: All its methods are synchronized, meaning only one thread can access it at a time. This prevents data corruption in multi-threaded apps but adds overhead.
• StringBuilder: No synchronization, making it faster but unsafe for concurrent threads.

Performance Benchmark

Let’s compare their speed with a simple test appending 77 million empty strings:

public class PerformanceTest {
    public static void main(String[] args) {
        int N = 77_777_777;
        long time;

        // StringBuffer Test
        StringBuffer sb = new StringBuffer();
        time = System.currentTimeMillis();
        for (int i = N; i > 0; i--) sb.append("");
        System.out.println("StringBuffer: " + (System.currentTimeMillis() - time) + " ms");

        // StringBuilder Test
        StringBuilder sbr = new StringBuilder();
        time = System.currentTimeMillis();
        for (int i = N; i > 0; i--) sbr.append("");
        System.out.println("StringBuilder: " + (System.currentTimeMillis() - time) + " ms");
    }
}

Sample Results:

• StringBuffer: ~2241 ms
• StringBuilder: ~753 ms

Conclusion: StringBuilder is 3x faster in single-threaded scenarios due to the absence of synchronization locks.

When to Use Each

Use StringBuilder If:

• Your code runs in a single thread.
• You prioritize performance (e.g., loops, large string manipulations).
• Example: Building a CSV file or JSON string in a non-concurrent app.

Use StringBuffer If:

• Multiple threads access/modify the same string buffer.
• Example: Shared logging systems in web servers.

Thread Safety: A Practical Example

Consider a multi-threaded app where two threads append data:

class ThreadSafetyTest {
    public static void main(String[] args) throws InterruptedException {
        // With StringBuffer (Safe)
        StringBuffer buffer = new StringBuffer();
        Runnable task = () -> {
            for (int i = 0; i < 1000; i++) buffer.append("x");
        };

        Thread t1 = new Thread(task);
        Thread t2 = new Thread(task);
        t1.start();
        t2.start();
        t1.join();
        t2.join();
        System.out.println("Buffer length: " + buffer.length()); // Correct: 2000

        // With StringBuilder (Unsafe)
        StringBuilder builder = new StringBuilder();
        Runnable unsafeTask = () -> {
            for (int i = 0; i < 1000; i++) builder.append("x");
        };
        Thread t3 = new Thread(unsafeTask);
        Thread t4 = new Thread(unsafeTask);
        t3.start();
        t4.start();
        t3.join();
        t4.join();
        System.out.println("Builder length: " + builder.length()); // May not be 2000!
    }
}

Result:

• StringBuffer reliably produces 2000 characters.
• StringBuilder may fail due to race conditions (e.g., ArrayIndexOutOfBoundsException).

Best Practices

• Prefer StringBuilder unless thread safety is necessary.
• Use StringBuffer only when dealing with multiple threads.
• Avoid + for loops, as it creates unnecessary intermediate objects.

Example:

// Inefficient: Creates multiple intermediate String objects
String s = "";
for (int i = 0; i < N; i++) s += "x";

// Efficient: Uses StringBuilder
StringBuilder sb = new StringBuilder();
for (int i = 0; i < N; i++) sb.append("x");

FAQ

1. Is StringBuffer obsolete?

No—it’s still useful in multi-threaded contexts. However, StringBuilder is preferred for most cases.

2. Can StringBuilder replace StringBuffer?

Yes, if you’re working in a single-threaded environment.

3. What about String concatenation (+)?

Avoid it in loops. It creates unnecessary intermediate String objects, leading to poor performance.

Conclusion

• StringBuilder: Faster, non-thread-safe. Use in single-threaded apps.
• StringBuffer: Slower, thread-safe. Use when multiple threads modify the same buffer.

By understanding these differences, you’ll write efficient and robust Java code. For most projects, StringBuilder is the right choice—reserve StringBuffer for scenarios demanding thread safety.

Further Reading:

• Oracle Java Docs: StringBuilder
• Oracle Java Docs: StringBuffer

Keywords: Java StringBuilder vs StringBuffer, StringBuffer performance, StringBuilder thread safety, Java string concatenation, mutable strings in Java