There is one important concept to understand here, i.e, a reference variable can refer to any object that is a

__subclass__ of the declared reference variable type but not a __superclass__. Let's see why.

{% highlight java linenos%}

class Foo {

public void doFooStuff() { }

}

class Bar extends Foo {

public void doBarStuff() { }

}

class Test {

public static void main (String [] args) {

Foo reallyABar = new Bar(); // Legal because Bar is a

// subclass of Foo

Bar reallyAFoo = new Foo(); // Compiler error! Foo is not a

// subclass of Bar

}

}

{% endhighlight %}

In line 11, `reallyAFoo` is a `Bar` reference variable (child) so someone would call `reallyAFoo.doBarStuff()` but the

reference variable actually holds a `Foo` object (parent) which doesn't have a `doBarStuff()` method. So, the compiler

prevents this and gives a `Incompatible types` error.

In other words, a child class is nothing but the parent class with additional properties. So there is no issue in line 9,

where a `Foo` reference variable (parent) is holding a `Bar` object (child). Because everything a `Foo` object can do,

can also be done by a `Bar` object.