Marker Interface

Here is the 'marker' interface
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public interface IProblemChild{
}
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Here is the 'marked' class. It is marked by implementing
the IProblemChild interface.
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class AngusYoung implements IProblemChild{
}
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This is the test run. Notice that the some of the objects 
in the array are 'marked' by implementing (being an instance 
of) the IProblemChild interface.
This allows a program to treat these classes in a special
manner or to take some special action based on this
'mark'.
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class Test{

    public static void main(String[] args){
        System.out.println("class: Test, method: main");

        Object[] myArray = new Object[5];

        for(int i =0; i<myArray.length;i++){
            if(i%2==0)
                myArray[i]= new Integer(3);
            else
                myArray[i] = new AngusYoung();
        }
        
        for(int i =0; i<myArray.length;i++){
            if(myArray[i] instanceof IProblemChild)
                System.out.println("cause im a problem child");
        }
    }
}
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Here is a tar with the above java classes in it. 
markerInterface.tar