ArrayList (Inheritance)

**urpalshu** · October 10th, 2011, 01:18 PM

I am new to Java, I am woring on learning inheritance.

I am creating an abstract class called Thing.

I have to put this requirement in, Please could somebody help me with Syntax

Thing will have the following private class field:
• Things, an ArrayList defined with a Thing generic. This ArrayList must be maintained as Thing objects are constructed. This ArrayList must contain all unique objects for which (object instanceof Thing) is true. Each time an object for which (object instanceof Thing) is true is constructed, a reference to that object must be put in the ArrayList, if that object does not already exist in the ArrayList. The Thing equals method will be used to test if the object is not unique, i.e., already exists in the ArrayList. Your code must not useany object casts and must not use any instance of operator.

Java Code:

 
 import java.util.*;   
public abstract class Thing { 
public String iName = new String();
 private ArrayList<Thing> iThings = new ArrayList<Thing>();   
 
public static void main(String[] args) { Thing vehicle1 = new Vehicle();   }   
public Thing(String name) {   iName = name;   }   public String getName() {   return iName; }     public abstract void show();   
public abstract boolean equals(Thing b);   
public void showThings() { System.out.println("Things");  
show();  
 } 
 }

Thank you,
urpalshu

**kenster421** · October 10th, 2011, 03:57 PM

Maybe I'm a stickler for OOP, but I'll ask anyway.

Why is there a collection of Things inside of class Thing? It's like putting a collection of Computers inside of class Computer. It's possible I could be wrong, but I don't think there are tiny computers inside my laptop...

I think what you need is a ThingList class that has a collection of Things inside of it.

In that case you can add a class member using the standard ArrayList definition and make sure you parameterize ArrayList with Thing (also called generics).

**copeg** · October 10th, 2011, 04:42 PM

With regards to syntax, I recommend reading the following links:
Code Conventions for the Java Programming Language
Lesson: Generics (The Java™ Tutorials > Learning the Java Language)

Should you have a more specific question as to what you mean by syntax, whether you are getting compile or runtime errors, or program misbehavior, I recommend your question providing a bit more information

Originally Posted by kenster421

Why is there a collection of Things inside of class Thing? It's like putting a collection of Computers inside of class Computer. It's possible I could be wrong, but I don't think there are tiny computers inside my laptop..

While I can't speak for why the original poster is doing this, I do it all the time for all sorts of data structures (Trees, Graphs, etc...), although I typically wouldn't name the class 'Thing'

**kenster421** · October 11th, 2011, 12:22 PM

While I can't speak for why the original poster is doing this, I do it all the time for all sorts of data structures (Trees, Graphs, etc...), although I typically wouldn't name the class 'Thing'

While I will concede that the case of a Tree is a good reason to employ this, I cannot think of any reason that a Graph should contain a collection of Graphs. If this is the functionality needed, use the Composite Design Pattern. Generally speaking (from a design perspective) an ideologically singular object like Car, should not contain a bunch of Cars within it.

Composite pattern - Wikipedia, the free encyclopedia

**copeg** · October 11th, 2011, 12:29 PM

Originally Posted by kenster421

While I will concede that the case of a Tree is a good reason to employ this, I cannot think of any reason that a Graph should contain a collection of Graphs. If this is the functionality needed, use the Composite Design Pattern. Generally speaking (from a design perspective) an ideologically singular object like Car, should not contain a bunch of Cars within it.

Composite pattern - Wikipedia, the free encyclopedia

Perhaps I'm not writing clearly enough - not a Graph containing a collection of Graphs or a Tree containing a collection of Trees - A Node containing a Collection of Nodes, the Set of all Nodes being the Tree or Graph. In the case of a Graph, often the Nodes contain collections of Edges rather than Nodes, but not always.

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