How to Use Format in Java String: Simple Guide

In Java, you can use String.format() to create formatted strings by specifying a format pattern and values. The format uses placeholders like %s for strings and %d for integers, which get replaced by the provided arguments.

Syntax

The basic syntax of String.format() is:

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

Here, formatString contains text with placeholders, and arguments are the values to insert.

Common placeholders include:

• %s - string
• %d - integer
• %f - floating-point number

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 message.

java

public class FormatExample {
    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 when using String.format() include:

• Mismatch between placeholders and argument types (e.g., using %d but passing a string).
• Wrong number of arguments compared to placeholders.
• Forgetting to use String.format() and trying to use placeholders directly in strings.

java

public class FormatPitfall {
    public static void main(String[] args) {
        // Wrong: passing string to %d placeholder
        // String result = String.format("Number: %d", "five"); // Causes runtime error

        // Correct usage:
        String result = String.format("Number: %d", 5);
        System.out.println(result);
    }
}

Output

Number: 5

Quick Reference

Placeholder	Description	Example
%s	String	"%s" -> "hello"
%d	Integer (decimal)	"%d" -> 42
%f	Floating-point number	"%.2f" -> 3.14
%c	Character	"%c" -> 'A'
%%	Literal percent sign	"%%" -> "%"

Key Takeaways

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

Ensure the number and types of arguments match the placeholders exactly.

Common placeholders include %s for strings, %d for integers, and %f for floats.

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

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