Java Interview Questions (Series) - StringBuffer vs StringBuilder

Change is inevitable except from a vending machine. - Robert C. Gallagher

Brief

If you are a Java developer you definitely changed a String's value before.

❓ But did you do it correctly❓

We found out in this previous post, that the String class is immutable in Java and saw what actually happens behind the scenes.

❓  In this case, how should we go about String alteration?

After asking themselves this question, the Java creators came up with a solution.

Actually, they came up with two ways for modifying a String.

1️⃣  StringBuffer which is with us since the first version of Java

2️⃣  StringBuilder which came around in Java 1.5

Implementation

StringBuffer

🧩  It is a class that can hold a sequence of characters.

All its public methods are synchronized that means it's thread-safe.

🚦  Let's see some practical examples of using StringBuffer :

🔸 initialize

StringBuffer sb1 = new StringBuffer();

StringBuffer sb2 = new StringBuffer("Init"); // initializing with value

🔸  append values at the end

StringBuffer sb = new StringBuffer("1234");
sb.append(5);    // append "5" at the end

String value = sb.toString();  // "12345"

append method is overloaded to be able to accept a lot of types of input ( int, float, char, etc.)

🔸  insert values at a certain position

 StringBuffer sb = new StringBuffer("1234");
 sb.insert(2,5);  // insert "5" at index 2
 
 String value = sb.toString(); // "12543"

insert method is also overloaded to be able to accept different types of input ( int, float, char, etc.)

🔸 remove values

 StringBuffer sb = new StringBuffer("1234");
 sb.delete(1,3);  // specify the index range of chars to be removed
 
 String value = sb.toString(); // 14
 
 sb.deleteCharAt(0);  // specify the index of the char to be removed
 
 String newValue = sb.toString(); // 4

StringBuilder

🧩  It's also a class capable of holding and manipulating a character sequence.

It is really similar to StringBuffer in terms of methods, only lacking some that are already found at String class level, like length(), substring(), etc.

❗️  The main difference between them is that the methods on StringBuilder are not synchronized!

This means that StringBuilder is not safe to be used in a multi-threaded application.

I'll do a post on multi-threaded Java applications where I'll go into a lot more details about the dragons that sleep in there.

🏁  On the other hand, the advantage of the methods not being synchronized is that they are a lot faster.  

Because of that, if you don't care about thread-safety, StringBuilder is the way to go for your String manipulations.

Bonus

Going back, you probably used neither of the two.

You can alter the String using the + operator like this:

String address = "My " + "Street";

And it is a totally fine way of doing it. But what is going on in there?

The + operator between two Strings is transformed by the compiler in:

String address = (new StringBuilder()).append("My ").append(" Street").toString();  

That means, even if that + is more straight-forward you might have problems with multiple threads doing it.