Why does this work ? (Question from ExamLab)

[John Stark]

This works:

public class Main 
{
    	public static void main(String args[]) 
	{
		System.out.println(new A().getInner());
	}
}

class A
{
	private class Inner
	{
	}

	Inner getInner()
	{
		return new Inner();
	}
}

The return type of getInner() is something which does not exists for class Main as Inner is private in class A. However the code compiles fine and gives an output like A$Inner@hgdyeu. But when I replace

System.out.println(new A().getInner());

by

System.out.println(new A().getInner().toString());

then I get the compilation error:

Main.java:12: error: toString() in Object is defined in an inaccessible class or interface
System.out.println(new A().getInner().toString());

toString() is obviously successfully called in System.out.println(new A().getInner()); as I get the corresponding output. Why does this work and System.out.println(new A().getInner().toString()); does not work?

Thanks,

John

[Stephan van Hulst]

You can't call methods directly on inaccessible types. However, if you cast them to one that's accessible, then you can. This is exactly what happens when you pass the Inner object to the println() method. Its signature uses the Object class, so the Inner object gets cast to Object before its toString() method is called.

Try to do the same by assigning getInner() to a variable of type Object first, before calling toString().

[John Stark]

Yes, indeed this works:

System.out.println(((Object)new A().getInner()).toString());

Thanks a lot for the clarification.

John