Java IO Tutorial - Java Scanner

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To read numbers from the standard input, we have to read it as a string and parse it to a number.

The Scanner class in java.util package reads and parses a text, based on a pattern, into primitive types and strings.

The text source can be an InputStream, a file, a String object, or a Readable object.

We can use a Scanner object to read primitive type values from the standard input System.in.

The following code illustrates how to use the Scanner class by building a trivial calculator to perform addition, subtraction, multiplication, and division.

import java.util.Scanner;
//from  ww  w  .  j a v a2  s  .c o m
public class Calculator {
  public static void main(String[] args) {
    System.out.println("type something like: 1+3");
    Scanner scanner = new Scanner(System.in);
    double n1 = Double.NaN;
    double n2 = Double.NaN;
    String operation = null;

    try {
      n1 = scanner.nextDouble();
      operation = scanner.next();
      n2 = scanner.nextDouble();
      double result = calculate(n1, n2, operation);
      System.out.printf("%s %s  %s  = %.2f%n", n1, operation, n2, result);
    }

    catch (Exception e) {
      System.out.println("An invalid expression.");
    }
  }

  public static double calculate(double op1, double op2, String operation) {
    switch (operation) {
    case "+":
      return op1 + op2;
    case "-":
      return op1 - op2;
    case "*":
      return op1 * op2;
    case "/":
      return op1 / op2;
    }

    return Double.NaN;
  }
}

The code above generates the following result.

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