How to implement an interface on a protected java class

Question

I was looking to implement an interface on a package-private java class, but I am having some difficulty achieving this. Below is an example.

class Foo
{
    String something(String str)
    {
        return ""str+"!";
    }
}




public interface Bar
{
    String something(String str);
}

What is the best approach here? My end goal is to implement the Bar interface on the Foo class. I am hoping to be able to cast Foo as Bar: (Bar)Foo

The Bar interface and the Foo class are in separate packages. Is there a way to do this?

Please advise.

Answer 1

You can't. The point of having the package level access it to precisely avoid seeing that class outside. What you can do however ( granted Foo is not final ) something like this:

C:\>type *.java
//Foo.java
package foo;
class Foo {
  String something( String s ) {
    return s + "!";
  }
}  
//Bar.java
package bar;
public interface Bar {
  public String something( String s );
}    
//Baz.java
package foo;    
import bar.Bar;    
public class Baz extends Foo implements Bar {
  // make sure you're overriding
  @Override
  public String something ( String s ) {
    return super.something( s );
  }
}     
//Use it: Main.java
package bar;
import foo.Baz;   
class Main {
  public static void main( String ... args ) {
    Bar bar = new Baz();
    System.out.println( bar.something("like this?"));
  }
}

C:\>java bar.Main
like this?!

Da da!

The trick is to define the child in the same package as the parent so you can create a public version of it.

I hope this helps.