Java Generics

Java generics allows an ArrayList object to be constrained to use a single specific type (e.g. ArrayList<Integer>). However, some applications require the ability to store objects of multiple unrelated types. In this question the aim is to store Integer objects alongside LinkedList<Integer> objects.

(a) One solution is to use ArrayList<Object>, since all Java objects extend Object.

Explain why this is bad practice. [2 marks]

(b) Seeking to provide a solution that allows an arbitrary set of constrained types, a programmer writes an abstract ConstrainedArray base class. To use it, the class is extended and a specialised void add(...) method should be provided for each acceptable type.

public abstract class ConstrainedArray {

protected ArrayList<Object> mArray =

new ArrayList<Object>();

public Object get(int idx) {return mArray.get(idx);}

public int size() { return mArray.size(); }


(i) Show how to create a class IntListArray that extends this base class

and accepts only Integer or LinkedList<Integer> objects. Where

appropriate, objects should be copied on insertion. [4 marks]

(ii) Describe a sequence of events that would allow external modification of an

object stored within an IntListArray, despite correct copying on insertion.

How could this be addressed in IntListArray? [3 marks]

(iii) By adding protected void add(Object o) {mArray.add(o);} to the

ConstrainedArray class, the mArray field can be made private. Show how

this would affect your IntListArray class and discuss the advantages of

the change from protected to private. [5 marks]

(c) The solutions in parts (a) and (b) both involve a get() method returning an Object reference.

(i) Explain why this is bad practice. [1 mark]

(ii) Propose an alternative solution for a constrained array of Integers.

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