StringBuilder

Learn mutable text with StringBuilder.

StringBuilder: mutable text

In Java, String is immutable; every append creates a new object. If you do many concatenations, StringBuilder is more efficient.

StringBuilder sb = new StringBuilder();
sb.append("Hello");
sb.append(" ");
sb.append("world");
String result = sb.toString();
// "Hello world"

StringBuilder changes the text in place; it does not create a new object per append. When you are done, call toString to get the final String. Always prefer this when concatenating text inside a loop.

StringBuilder vs String

String is immutable; concatenating with + creates a new object every time. StringBuilder is mutable; append grows the same object.

String s = "a";
s = s + "b";  // new String object
StringBuilder sb = new StringBuilder("a");
sb.append("b");  // same object

With hundreds or thousands of appends, + gets very slow; because each step copies. StringBuilder avoids this cost. For a one-line concat, + is fine.

Exercise Concatenate the message

Create a StringBuilder; append "Hello", ", " and "world" with append. Take the result with toString and print it.

Show example solution

StringBuilder sb = new StringBuilder();
sb.append("Hello");
sb.append(", ");
sb.append("world");
System.out.println(sb.toString());

Test yourself

Match scenario to tool

One-shot concat

String +
Loop concat

StringBuilder
Final text

toString
Initial content

new StringBuilder("...")

Fill in the blank: declare StringBuilder

StringBuilder sb = new ____();

Show answer

StringBuilder sb = new StringBuilder();

Which of the following is true?
- StringBuilder is mutable
- String is mutable
- StringBuilder is immutable
- String grows in place
Show answer

StringBuilder is mutable — String is immutable; StringBuilder is mutable.

Next lesson: append

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies