Java(TM) Boutique - Using Mock Objects in Java

Tutorials : Using Mock Objects in Java :

A Simple Example

To illustrate the various possibilities when using mock objects we need a simple example. Consider a case with these components:

An application--called ListFile--with one single feature: it can list the contents of a file to the console. This application is using the object that we want to mock:
The FileHandler class, which is the clever one that knows how to obtain lines from a file. It implements the next item:
An Interface--called FileInterface--defining the methods needed for working with a file.

FileInterface looks like this:

Figure 1: The FileInterface

public interface FileInterface {
  
  public void open(String filename) ;
  public void close() ;
  public String read() ;    
  public boolean fault();

}

You'll find the purpose of the methods evident:

open	Gets the name of the file and checks if it's available
close	Closes the file after use
read	Reads and returns the next line in the file
fault	Used instead of Exceptions, returns true if something bad has happened. Must be called after each call to open, close and read

The FileHandler implements the methods from the interface by using Java's file capabilities:

package hansen.playground.mock;

import java.io.*;

public class FileHandler implements FileInterface {
  
  private BufferedReader in;
  private boolean error = false; 
  
  public void open(String filename) {
    try {
      in = new BufferedReader(new FileReader(filename));
    } catch (FileNotFoundException e) {
      error = true;
    }
  }

  public void close() {
    try {
      in.close();
    } catch (IOException e) {
      // ignore ;-)
    }
  }

  public String read() {
    String s;
    try {
      s = in.readLine();
    } catch (IOException e) {
      error = true;
      s = "";
    }
    return s;
  }

  public boolean fault() {
    return error;
  }  
}

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