Core Java

Java Input Handling: Clearing the Scanner Buffer

When working with the Scanner class in Java for input, especially from the console, it is common to encounter issues related to leftover newline characters in the buffer. These residual characters can cause problems with subsequent input reads, leading to unexpected behaviour in programs. This article explores methods to clear the buffer, ensuring predictable and consistent input processing.

1. Example Problem

The Scanner class uses delimiters to separate tokens. By default, it uses whitespace (including spaces, tabs, and newline characters) as delimiters. When reading input, certain methods such as nextInt(), nextDouble(), etc., do not consume the newline character after the input.

Consider the following code:

public class ScannerBufferIssue {

    public static void main(String[] args) {
        try (Scanner scanner = new Scanner(System.in)) {
            System.out.print("Enter an integer: ");
            int number = scanner.nextInt();
            
            System.out.print("Enter a string: ");
            String text = scanner.nextLine();
            
            System.out.println("Number: " + number);
            System.out.println("Text: " + text);
        }
    }
}

In this example, after entering an integer and pressing Enter, the nextInt() method reads the integer but leaves the newline character in the buffer. The subsequent nextLine() call then reads this newline character, resulting in an empty string being assigned to text.

When you run this code, the following sequence of events occurs:

  1. The program prompts the user to enter an integer.
  2. The user enters an integer (e.g., 42) and presses Enter.
  3. The program then prompts the user to enter a string.
  4. The user attempts to enter a string (e.g., javacodegeeks).

However, due to the leftover newline character from the nextInt() method, the nextLine() method immediately consumes this newline character instead of waiting for the user to enter a string. This results in unexpected output.

Enter an integer: 42
Enter a string: Number: 42
Text: 

2. Clearing the Buffer

2.1 Using nextLine()

To clear the buffer and remove the leftover newline character, we can use scanner.nextLine() after reading the integer. This consumes the newline character, allowing subsequent input to be processed correctly. Here’s how you can modify the previous example to clear the buffer:

public class ScannerBufferSolution {

    public static void main(String[] args) {
        try (Scanner scanner = new Scanner(System.in)) {
            System.out.print("Enter an integer: ");
            int number = scanner.nextInt();
            scanner.nextLine(); // Clear the newline character from the buffer
            
            System.out.print("Enter a string: ");
            String text = scanner.nextLine();
            
            System.out.println("Number: " + number);
            System.out.println("Text: " + text);
        }
    }
}

In this modified version, the scanner.nextLine() call after nextInt() consumes the newline character, ensuring that the next nextLine() call reads the actual input as expected.

The output from running the modified version is as follows:

Enter an integer: 42
Enter a string: javacodegeeks
Number: 42
Text: javacodegeeks

2.2 Solution with skip()

In addition to using nextLine() to clear the buffer, another approach to remove the newline character left in the buffer is to use the skip() method. The skip() method allows us to skip a specific pattern in the input, which can be useful for discarding unwanted characters.

Here’s how we can modify the previous example to clear the buffer using the skip() method:

public class ScannerBufferSkipSolution {

    public static void main(String[] args) {
        try (Scanner scanner = new Scanner(System.in)) {
            System.out.print("Enter an integer: ");
            int number = scanner.nextInt();
            scanner.skip("\n"); // Clear the newline character from the buffer
            
            System.out.print("Enter a string: ");
            String text = scanner.nextLine();
            
            System.out.println("Number: " + number);
            System.out.println("Text: " + text);
        }
    }
}

In this solution, the scanner.skip("\n") call is used to skip the newline character in the buffer after reading the integer. This ensures that the next nextLine() call reads the actual input string as expected.

3. Conclusion

In this article, we explored techniques for clearing the scanner buffer in Java to ensure predictable and consistent input processing. By using methods like nextLine() and skip(), we can effectively manage and clear the buffer, preventing input errors and enhancing the reliability of our Java programs.

4. Download the Source Code

This article discussed techniques for clearing the scanner buffer in Java.

Download
You can download the full source code of this example here: Java scanner buffer

Omozegie Aziegbe

Omos holds a Master degree in Information Engineering with Network Management from the Robert Gordon University, Aberdeen. Omos is currently a freelance web/application developer who is currently focused on developing Java enterprise applications with the Jakarta EE framework.
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