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Java Notes

Console Input: Scanner

The java.util.Scanner class (added in Java 5) allows simple console and file input. Of course, your program should eventually have a GUI user interface, but Scanner is very useful for reading data files.

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// File   : introductory/IntroScanner.java
// Purpose: Write to and read from the console.
// Author : Michael Maus
// Date   : 2006-01-20

import java.util.*;                //Note 1

public class IntroScanner {

    public static void main(String[] args) {
        //... Initialization
        String name;               // Declare a variable to hold the name.
        Scanner in = new Scanner(System.in);

        //... Prompt and read input.
        System.out.println("What's your name, Earthling?");
        name = in.nextLine();      // Read one line from the console.
        in.close();                //Note 2

        //... Display output
        System.out.println("Take me to your leader, " + name);
    }
}

Notes

  1. Altho we only need the Scanner class from the java.util package, the most common programming style is to make all classes (*) visible.
  2. Closing the console isn't really necessary, but it's a good habit. If we had been reading a file, which is common with Scanner, closing it would be important.

Example using Scanner in a loop

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// File   : introductory/ScannerLoop.java
// Purpose: Read from the console in a loop using Scanner.
// Author : Michael Maus
// Date   : 2006-01-20

import java.util.*;

public class ScannerLoop {

    public static void main(String[] args) {
        //... Initialization
        double n;                // Holds the next input number.
        double sum = 0;          // Sum of all input numbers.
        Scanner in = new Scanner(System.in);

        //... Prompt and read input in a loop.
        System.out.println("Will add numbers.  Non-number stops input.");
        
        while (in.hasNextDouble()) {
            n = in.nextDouble();
            sum = sum + n;
        }
        in.close(); 

        //... Display output
        System.out.println("The total is " + sum);
    }
}
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