StringBuilder vs StringBuffer in Java

In Java, when working with mutable strings (strings that can be modified), you may need to choose between StringBuilder and StringBuffer. While both are mutable classes that allow modification of their values, they differ significantly in terms of thread safety, performance, and application. Here, we’ll compare their characteristics and provide code examples to illustrate when to use each one.

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Key Differences: StringBuilder vs. StringBuffer

Feature

StringBuilder

StringBuffer

Feature	StringBuilder	StringBuffer
Mutability	Mutable	Mutable
Stored in	Heap (does not use String Pool)	Heap (does not use String Pool)
Thread Safety	Not thread-safe	Thread-safe
Synchronization	Not synchronized	Synchronized
Performance	Faster due to lack of synchronization	Slower due to synchronization overhead
Use Case	Single-threaded scenarios	Multi-threaded scenarios where thread-safety is required

Mutability

Mutable

Mutable

Stored in

Heap (does not use String Pool)

Heap (does not use String Pool)

Thread Safety

Not thread-safe

Thread-safe

Synchronization

Not synchronized

Synchronized

Performance

Faster due to lack of synchronization

Slower due to synchronization overhead

Use Case

Single-threaded scenarios

Multi-threaded scenarios where thread-safety is required

Let’s explore each class in more detail.

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1. StringBuilder: The Efficient Choice for Single-Threaded Environments

• StringBuilder is a mutable class, meaning it allows modifications to its content.

• It is thread-unsafe, so it’s ideal for single-threaded scenarios.

• Not synchronized: StringBuilder is faster than StringBuffer due to the absence of synchronization overhead.

• Multi-threaded limitation: Using StringBuilder in multi-threaded environments without additional safety measures can lead to race conditions and other concurrency issues.

Example: Demonstrating Thread Unsafety in StringBuilder

In this example, we use two threads to append characters to a StringBuilder instance. However, due to the lack of synchronization, we encounter race conditions:

public class StringBuilderBasics {

    public void threadUnsafe() {
        // Common resource being shared
        StringBuilder builder = new StringBuilder();

        // Thread appending "A" 1000 times
        Thread t1 = new Thread(() -> {
            for (int i = 0; i < 1000; i++) {
                builder.append("A");
            }
        });

        // Thread appending "B" 1000 times
        Thread t2 = new Thread(() -> {
            for (int i = 0; i < 1000; i++) {
                builder.append("B");
            }
        });

        t1.start();
        t2.start();

        try {
            t1.join();
            t2.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        // Result: 1840 (unpredictable)
        System.out.println("Length: " + builder.toString().length());
    }

    public static void main(String[] args) {
        new StringBuilderBasics().threadUnsafe();
    }
}

Explanation:

• Due to thread-unsafety, the final length of the StringBuilder output is unpredictable (e.g., 1840 instead of 2000).

• This happens because both threads attempt to append characters simultaneously, leading to overwrites or dropped operations.

Takeaway: Use StringBuilder only in single-threaded environments or when thread-safety is handled externally.

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2. StringBuffer: The Safe Option for Multi-Threaded Environments

• StringBuffer is mutable, allowing modifications to its content.

• It is synchronized, which makes it thread-safe.

• Ideal for multi-threaded environments where thread safety is necessary.

• Performance cost: Synchronization introduces overhead, so StringBuffer is slower than StringBuilder.

Example: Thread Safety in StringBuffer

Here’s the same example as above, but this time using StringBuffer:

public class StringBufferBasics {

    public void threadSafe() {
        // Common resource being shared
        StringBuffer buffer = new StringBuffer();

        // Thread appending "A" 1000 times
        Thread t1 = new Thread(() -> {
            for (int i = 0; i < 1000; i++) {
                buffer.append("A");
            }
        });

        // Thread appending "B" 1000 times
        Thread t2 = new Thread(() -> {
            for (int i = 0; i < 1000; i++) {
                buffer.append("B");
            }
        });

        t1.start();
        t2.start();

        try {
            t1.join();
            t2.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        // Result: 2000
        System.out.println("Length: " + buffer.toString().length());
    }

    public static void main(String[] args) {
        new StringBufferBasics().threadSafe();
    }
}

Explanation:

• StringBuffer ensures that both threads append safely, achieving the expected length of 2000.

• While the final string is thread-safe, the output may be interleaved (e.g., “AAABBB...” mixed together) as thread execution order is non-deterministic.

Takeaway: Use StringBuffer for multi-threaded applications where data consistency is crucial and synchronization is needed.

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Choosing the Right Class

To decide between StringBuilder and StringBuffer, consider the following:

• Use StringBuilder in single-threaded scenarios where performance is critical and thread-safety isn’t a concern.

• Use StringBuffer in multi-threaded scenarios where you need mutable string operations and require thread safety to avoid race conditions.

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Conclusion

This comparison should help you make an informed choice between StringBuilder and StringBuffer. Understanding the trade-offs in mutability, performance, and thread safety can lead to better decision-making when working with strings in Java.

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• Java Fundamentals
• Array Interview Essentials
• Java Memory Essentials
• Java Keywords Essentials
• Java OOPs Essentials
• Collections Framework Essentials

Happy Coding!

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