StringBuilder vs StringBuffer in Java

Many beginners write code like this:

String s = "";

for (int i = 0; i < 10000; i++) {
s += i;
}

Looks harmless…

But internally:

• thousands of new String objects are created

That’s why Java provides:

• StringBuilder
• StringBuffer

Why Regular String Is Slow

Strings in Java are immutable.

That means:

s += "Java";

does NOT modify the existing string.

Instead:

• a completely new object is created.

1. StringBuilder

StringBuilder is:

• Mutable
• Fast
• Not thread-safe

StringBuilder sb = new StringBuilder();

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

String result = sb.toString();

Common Methods

sb.insert(5, " Java");
sb.delete(5, 10);
sb.reverse();

2. StringBuffer

StringBuffer is similar to StringBuilder.

Difference:

• It is synchronized (thread-safe).

StringBuffer sbuf = new StringBuffer();

sbuf.append("Thread-safe");

Performance Difference

Inefficient

String s = "";

for (int i = 0; i < 10000; i++) {
s += i;
}

Efficient

StringBuilder sb = new StringBuilder();

for (int i = 0; i < 10000; i++) {
sb.append(i);
}

String result = sb.toString();

The Real Insight

When strings change frequently:

• mutable objects are much more efficient.

That’s why:

• StringBuilder is preferred in most cases

• StringBuffer is used when thread safety matters

• String = immutable

• StringBuilder = mutable + fast

• StringBuffer = mutable + thread-safe

For More Learning: https://www.quipoin.com/tutorial/data-structure-with-java/stringbuilder-vs-stringbuffer