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Computational Problem Solving with Java and Interactive Object Graphics

Mark F. Russo, PhD

Section 3.4 The Scanner Class

The Scanner utility class provided by Java is useful for reading user responses from an input stream, such as the terminal or a file. When reading from the terminal, a Scanner will wait until the user enters a token and presses the Enter or Return key. Not only can Scanner read a response as text, but it also can parse a response and return a particular data type, when you know what to expect. A Scanner reads one token at a time. By default, tokens are delineated with spaces. We’ll use the Scanner class to make our terminal programs interactive.

Subsection 3.4.1 Instantiating a Scanner

The first thing to note is that, unlike String, the Scanner class is not available in Java by default. It first must be imported from the library that comes with the JDK. This is done by adding the following line to the top of your Java program.
import java.util.Scanner;
Instantiating a Scanner class follows the same pattern as other classes, using the new keyword. The Scanner constructor calls for a parameter that tells it from where to read responses. The terminal can be read using the precreated object at System.in. Compare this to the Java print statement System.out.println(…), which writes to the terminal. This statement is scoped by System.out. Following the standard constuctor syntax and using the System.in constructor parameter, we can declare a new Scanner variable and initialize it by instantiating a new Scanner object using the following statement.
Scanner scnr = new Scanner(System.in);

Subsection 3.4.2 Scanner Methods

Scanner provides a variety of methods to read data from the terminal and even check if there is data to read before reading. The followng table describes many of these methods.
Table 3.4.1. Common Scanner Methods
Method Returns Description
hasNext() boolean true if there is another token to read
hasNextBoolean() boolean true if there is a boolean waiting to be read
hasNextInt() boolean true if there is a int waiting to be read
hasNextDouble() boolean true if there is a double waiting to be read
hasNextLine() boolean true if there is a line in the input waiting to be read
next() String Read and return the next token as a String
nextLine() String Read and return the next complete line of text as a String
nextBoolean() boolean Read the next token as a boolean and return
nextInt() int Read the next token as an int and return
nextDouble() double Read the next token as a double and return
close() void Close when done to prevent resource leak.

Subsection 3.4.3 Scanner Examples

The following program demonstrates how to instantiate a Scanner object and invoke its methods. We use it to add an initial interactive step to our first Java Program in Listing 3.2.2. Once again, we are using the distanceBetween(…) method twice to compute and sum the distance between the first two points and the second two points. Don’t forget to close() the Scanner to avoid a resource leak. Closing a Scanner sets a flag in the object so other parts of your code know it is not safe to read, it closes any underlying resource that it may be using, and it also makes it suitable for garbage collection to avoid memory leaks, a concept that we will investigate in more detail later. Also note that you may close an already-closed Scanner multiple times — no harm is done.
// InteractiveDistance.java
import java.util.Scanner;                   // Import java.util.Scanner

public class InteractiveDistance {

  public static void main(String[] args) {  // Computation starts here
    double x1, y1, x2, y2, x3, y3;

    // Create a Scanner object that reads from the terminal
    Scanner scnr = new Scanner(System.in);

    // Prompt the user for two points
    System.out.print("Please enter x1 y1: ");
    x1 = scnr.nextDouble();
    y1 = scnr.nextDouble();

    System.out.print("Please enter x2 y2: ");
    x2 = scnr.nextDouble();
    y2 = scnr.nextDouble();

    System.out.print("Please enter x3 y3: ");
    x3 = scnr.nextDouble();
    y3 = scnr.nextDouble();

    // Close the scanner
    scnr.close();

    // Reuse the method to calculate and sum two distances
    double dist1 = distanceBetween(x1, y1, x2, y2);
    double dist2 = distanceBetween(x2, y2, x3, y3);
    System.out.println("Total distance: " + (dist1+dist2));
  }

  // New method that encapsulates the distance formula computation
  // Parameters are four doubles representing the two point coordinates
  public static double distanceBetween(double x1, double y1, 
                                       double x2, double y2) {

    double term1 = Math.pow(x1-x2, 2);      // Compute temporary terms
    double term2 = Math.pow(y1-y2, 2);
    
    // Apply the distance formula
    double distance = Math.sqrt( term1 + term2 );
    
    // Return the computed distance
    return distance;
  }
}
Compile, run, and enter data into your program. As you’ll see, using the Scanner object to read data from the terminal is a great way to make your programs interactive and more useful. By reading in data values, the computations performed by your programs will be more generally applicable.
javac InteractiveDistance.java 
java InteractiveDistance
Please enter x1 y1: 0.0 20.0
Please enter x2 y2: 30.0 0.0
Please enter x3 y3: 40.0 10.0
Total distance: 50.19764837837084
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