Re: Wrapper for multiple instanceof

Lew <>
Tue, 21 Aug 2007 21:47:20 -0400
<> wrote:

Recently I have been trying to do a multiple instanceof and thought it
would be better to write a function than do those ugly || with
repeated instanceofs'.

Most instanceof's can be replaced with good OOP design.

public class MyInstance
    public static boolean check(Object obj, String...classes) throws
        boolean result = false;
        for(String str : classes)
            Class c = Class.forName(str);
            result = result | (obj instanceof c);
            if(result) break;

This idiom is a red flag that polymorphism should be used instead.

Define an interface, I'll call it "DoesSomething", with a method, say,

public interface DoesSomething
  public void doSomething();

Then define implementing classes specific to your situation:

public class HelloWorld implements DoesSomething
   private final String something;
   public HelloWorld( String s )
     something = s;
   public void doSomething()
     System.out.println( something );

In some client class you would have code like this:

   DoesSomething doer = Factory.getDoesSomething();
   // this factory method will deliver some implementation
   // of DoesSomething depending on environment or data


Notice the client code doesn't "know" that doer is "really" a HelloWorld, or
some other implementing class. It just lets the object "know" what to do for
itself. No run-time type checking needed on your part.

This idiom is at the heart of object-oriented programming.


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