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:

1. 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:
2. The FileHandler class, which is the clever one that knows how to obtain lines from a file. It implements the next item:
3. 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:

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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