Demo Dependency Injection

Example.java

public class Example { 

  private DatabaseThingie myDatabase; 

  public Example() { 
    myDatabase = new DatabaseThingie(); 
  } 

  public void doStuff() { 
    myDatabase.GetData(); 
  } 
}

Example02.java

public class Example { 
  private DatabaseThingie myDatabase; 

  public Example() { 
    myDatabase = new DatabaseThingie(); 
  } 

  public Example(DatabaseThingie useThisDatabaseInstead) { 
    myDatabase = useThisDatabaseInstead; 
  }

  public void DoStuff() { 
    myDatabase.GetData(); 
  } 
}

ExampleTest.java

public class ExampleTest { 
  TestDoStuff() { 
    MockDatabase mockDatabase = new MockDatabase(); 

    // MockDatabase is a subclass of DatabaseThingie, so we can 
    // "inject" it here: 
    Example example = new Example(mockDatabase); 

    example.DoStuff(); 
    mockDatabase.AssertGetDataWasCalled(); 
  } 
}