How to Use String Format in Java: Simple Guide

In Java, you use String.format() to create formatted strings by specifying a format string with placeholders and providing values to fill them. This method returns a new string with the values inserted according to the format specifiers.

Syntax

The basic syntax of String.format() is:

• String.format(formatString, arguments...)

Here, formatString contains text and format specifiers like %s for strings, %d for integers, and %f for floating-point numbers. The arguments are the values that replace these specifiers in order.

java

String formatted = String.format("Hello, %s! You have %d new messages.", "Alice", 5);

Example

This example shows how to use String.format() to insert a name and a number into a string with proper formatting.

java

public class Main {
    public static void main(String[] args) {
        String name = "Alice";
        int messages = 5;
        String formatted = String.format("Hello, %s! You have %d new messages.", name, messages);
        System.out.println(formatted);
    }
}

Output

Hello, Alice! You have 5 new messages.

Common Pitfalls

Common mistakes include:

• Using the wrong format specifier for the data type (e.g., %d for a string).
• Not matching the number of specifiers with the number of arguments.
• Forgetting that String.format() returns a new string and does not change the original string.

java

public class Main {
    public static void main(String[] args) {
        // Wrong: Using %d for a string
        // String result = String.format("Name: %d", "Alice"); // Causes runtime error

        // Correct:
        String result = String.format("Name: %s", "Alice");
        System.out.println(result);
    }
}

Output

Name: Alice

Quick Reference

Format Specifier	Description	Example
%s	String	"Hello"
%d	Integer (decimal)	123
%f	Floating-point number	3.14
%c	Character	'A'
%%	Literal percent sign	%

Key Takeaways

Use String.format() with format specifiers like %s, %d, and %f to create formatted strings.

Always match the number and type of format specifiers with the arguments provided.

String.format() returns a new string; it does not modify the original string.

Common errors come from mismatched specifiers and argument types.

Use %% to include a literal percent sign in the output.