How to test write to file in Java?

Question

I\'m beginner, and keep yourself in hands. I have some easy program, and I need do junit test for write method. I have some collection in input. How I can do this? This my

Answer 1

I'll suggest you making an interface wrapper around you IO classes (the PrintWriter class in your case) so you can use mock objects for output. You don't have to test Java PrintWriter, you want to test your functionality, right?

So your class will be

class MyClass {

    MyWriter out;

    public void setOut(MyWriter out) {
        this.out = out;
    }

    // write to file
    public void write(String fileName, List figuresList) {
        try {
            try {
                for (int i = 0; i < figuresList.size(); i++) {
                    out.println(figuresList.get(i).toString());
                }
            } finally {
                out.close();
            }
        } catch (IOException e) {
            System.out.println("Cannot write to file!");
        }
    }
}

The signature of the MyWriter interface is pretty straightforward.

interface MyWriter {

    void println(Object x); // You can add other println methods here.

    void close();

}

Then you can use EasyMock to write a test. The test method will be something like

@Test
public void testWrite() {
    MyWriter out = EasyMock.createMock(MyWriter.class);
    EasyMock.expect(mock.println(EasyMock.anyObject())).times(3);
    EasyMock.expect(mock.close()).times(1);

    List list = ...
    list.add(...);
    list.add(...);
    list.add(...);

    replay(mock);

    MyClass myClass = new MyClass();
    myClass.setOut(out);
    myClass.write("mockFileName", list);        

    verify(mock);
}