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<FigureGeneral> 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<FigureGeneral> list = ...
    list.add(...);
    list.add(...);
    list.add(...);

    replay(mock);

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

    verify(mock);
}

Answer 2

You probably don't need mocks at all. Try using StringWriter in your tests.

// write to file
public void write(String fileName, List<FigureGeneral> figuresList) {
  try {
    Writer out = new FileWriter(new File(fileName).getAbsoluteFile());
    write(out, figuresList);
  } catch (IOException e) {
    System.out.println("Cannot write to file!");
  }
}

@VisibleForTesting void write(Writer writer, List<FigureGeneral> figuresList) {
  PrintWriter out = new PrintWriter(writer);
  try {
    for (int i = 0; i < figuresList.size(); i++) {
      out.println(figuresList.get(i).toString());
    }
  } finally {
    out.close();
  }
}

@Test public void testWrite() {
  List<FigureGeneral> list = Lists.newArrayList();
  list.add(...); // A
  list.add(...); // B
  list.add(...); // C
  StringWriter stringWriter = new StringWriter();
  write(stringWriter, list);
  assertEquals("A.\nB.\nC.\n", stringWriter.toString()); 
}

Answer 3

You seem to have the right idea so I am not sure what you need help with exactly...

can we join both tests(write/read)

yes.

or better do this individually(coz if our test fall we don't know where is problem - in read or write)?

Better but harder to maintain.

How should make this correctly at junit(with prepare to test, and test itself)?

Create some dummy data, in code or in a text file.

In the first case, write out the file and read it back in as check it is the same.

In the second case, you can read the text file and write it out again, and check it is the same.