Testing a simple hello world method

Question

I have worked with junit test integration tests and controller tests in spring and usually we test the output of a method but when i tried to test a simple hello world in main method i had no idea how to go about it so will like to get any idea on what do write

public class App 
{
    public static void main( String[] args )
    {
        System.out.println( "Hello World!" );
    }
}

This is the simple java class any idea how i can test it I tried to write something like this

public void mainMethodTest() throws Exception{

        System.out.println("hello world");
        String[] args = null;


        Assert.assertEquals(System.out.println("hello world"),App.main(args));
    }

Answer 1

You could assign to the System.out variable a ByteArrayOutputStream object which you store the reference in a variable.
Then invoke your main() method and assert that the String content of the ByteArrayOutputStream object contains the expected String:

@Test
public void main() throws Exception{                
    PrintStream originalOut = System.out; // to have a way to undo the binding with your `ByteArrayOutputStream` 
    ByteArrayOutputStream bos = new ByteArrayOutputStream();
    System.setOut(new PrintStream(bos));
    // action
    App.main(null);
    // assertion
    Assert.assertEquals("hello world", bos.toString());
    // undo the binding in System
    System.setOut(originalOut);
}

Why does it work ?

bos.toString() returns the "Hello World!" String passed in the method under test:

System.out.println( "Hello World!" );

as after setting System.out in this way : System.setOut(new PrintStream(bos));, the out variable refers to a PrintStream object that decorates the ByteArrayOutputStream object referenced by the bos variable. So any System.out invocations will write bytes in the ByteArrayOutputStream object.