StringBuffer vs StringBuilder

StringBuffer and StringBuilder look identical: same methods, same results. They differ in one thing only: StringBuffer locks itself on every call so several threads can use it safely, and StringBuilder doesn't. That single difference decides which you should use, and the answer for almost all new code is StringBuilder.

One Difference

You can confirm it by asking Java whether the append methods are synchronized:

import java.lang.reflect.Modifier;
 
public class Locks {
    public static void main(String[] args) throws Exception {
        int buffer = StringBuffer.class.getMethod("append", String.class)
            .getModifiers();
        int builder = StringBuilder.class.getMethod("append", String.class)
            .getModifiers();
        System.out.println("StringBuffer.append synchronized: "
            + Modifier.isSynchronized(buffer));
        System.out.println("StringBuilder.append synchronized: "
            + Modifier.isSynchronized(builder));
    }
}

StringBuffer.append synchronized: true
StringBuilder.append synchronized: false

This uses reflection (covered later in the course) to read the method's modifiers. The point is simple: the lock is the only real difference.

Why StringBuilder Is Faster

Taking and releasing a lock costs a little time on every call, even when no other thread is around. In one timing run of 5 million single-character appends, StringBuffer took about 21 ms and StringBuilder about 14 ms. Modern JVMs remove much of the locking cost when they can prove only one thread uses the object, so the gap is usually small; but StringBuilder is never slower.

String, StringBuffer and StringBuilder

Common Mistakes

Picking StringBuffer by habit from old tutorials. Default to StringBuilder.

Sharing a StringBuilder field between threads. That's the one case it breaks; use a local builder per call, or StringBuffer.

Using either one when + on one line would do. Builders are for many steps.

Interview Corner

Practice

1. Easy: Replace every StringBuffer in the "StringBuffer in Java" examples with StringBuilder and check that the single-threaded output doesn't change.
2. Harder: Write a method that builds a comma-separated line from an int[], using a local StringBuilder. Explain why it's thread-safe even if many threads call the method at once.

Key Takeaways