Anything wrong about my Intset Class?
<p>I design new IntSet Class that use ArrayList. first, i extends Intset by ArrayList and i start implement method. i face some problem in my union() method. here is my code...</p>

<pre><code>public class IntSet extends ArrayList&lt;Integer&gt;{

    private static final long serialVersionUID = 1L;
    private ArrayList&lt;Integer&gt; intset;

    public IntSet(){
        this.intset = new ArrayList&lt;Integer&gt;();
    }
    public IntSet(ArrayList&lt;Integer&gt; intset){
        this.intset = intset;
    }

    public void insert(int x){
        this.intset.add(x);
    }

    @Override
    public Integer remove(int x){
        int index = intset.indexOf(x);
        this.intset.remove(index);
        return 1;
    }

    @Override
    public int size(){
        return this.intset.size();
    }

    @Override
    public Integer get(int index){
        return this.intset.get(index);
    }

    public boolean member(int x){
        if(intset.indexOf(x)==-1) return false;
        else return true;
    }

    public IntSet union(IntSet a){
        IntSet intersectSet = new IntSet();
        intersectSet.insert(0);
        intersectSet.insert(1);
        System.out.println(intersectSet.size());
        System.out.println(intersectSet.contains(1));
        for(int i=0; i&lt;a.size(); i++){
        }
        return intersectSet;
    }

    public String toString(){
        if(intset.size()==0) return "[]";
        String s = "[" + intset.get(0).toString();
        for(int i=1; i&lt;intset.size(); i++){
            s += "," + intset.get(i).toString();
        }
        return s += "]";
    }

}
</code></pre>

<p>In method </p>

<pre><code>union(IntSet a);
</code></pre>

<p>I constract new Intset object then add 2 value (0, 1) into <strong>intersectSet</strong> variable.</p>

<pre><code>intersectSet.insert(0);
intersectSet.insert(1);
</code></pre>

<p>then i print size of <strong>intersectSet</strong> it shown me 2 that is correct!</p>

<p>but when i need to check that there is 1 in <strong>intersectSet</strong> or not? it shown me fasle.</p>

<pre><code>System.out.println(intersectSet.contains(1));
</code></pre>

<p>In fact it should show me true because in <strong>intersectSet</strong> have interger 1.</p>

<p>anything wrong about my code and should i extends ArrayList for IntSet class?</p>
