Difference Between String and StringBuffer

Think of a String as a printed page and a StringBuffer as a whiteboard. To change a printed page, you have to print a whole new page; the whiteboard you just edit. Both hold text, and both can produce the same final words, but they suit very different jobs. This question ("String vs StringBuffer") appears in almost every Java interview, so here's the full comparison with an example that shows each difference.

Side by Side in Code

public class Versus {
    public static void main(String[] args) {
        String s = "Hi";
        s.concat(" Navika");
        System.out.println(s);
 
        StringBuffer sb = new StringBuffer("Hi");
        sb.append(" Navika");
        System.out.println(sb);
 
        System.out.println("Hi".equals("Hi"));
        StringBuffer x = new StringBuffer("Hi");
        System.out.println(x.equals(new StringBuffer("Hi")));
    }
}

Hi
Hi Navika
true
false

How it works

1. s.concat(" Navika") built a new string and threw it away; s is still "Hi". String is immutable.
2. sb.append(" Navika") changed the buffer itself. StringBuffer is mutable.
3. String overrides equals to compare text; StringBuffer doesn't, so two buffers with the same text aren't equals.

The Full Comparison

Common Mistakes

Storing a StringBuffer as a map key or in a set. Its equals and hashCode don't look at the text.

Returning a StringBuffer from a method that should return text. Return sb.toString(), so callers can't change it.

Using String with += in a big loop. That's the slow case; use a builder.

Interview Corner

Practice

1. Easy: Write the same 5-step text-building code with String and with StringBuffer, printing the variable after each step. Where do they differ?
2. Harder: Put two StringBuffer objects with the same text into a HashSet and print the set's size. Then put their toString() results in and compare.

Key Takeaways