Java Data Structure How to - Extend ArrayList to create ArrayList which only stores unique values

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Question

We would like to know how to extend ArrayList to create ArrayList which only stores unique values.

Answer

/* w  ww  .  j av  a 2 s .co  m*/
//package org.hisp.dhis.system.util;

/*
 * Copyright (c) 2004-2013, University of Oslo
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 * Redistributions of source code must retain the above copyright notice, this
 * list of conditions and the following disclaimer.
 *
 * Redistributions in binary form must reproduce the above copyright notice,
 * this list of conditions and the following disclaimer in the documentation
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 * be used to endorse or promote products derived from this software without
 * specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR
 * ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
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import java.util.ArrayList;
import java.util.Collection;

public class Main{
  public static void main(String[] argv){
    UniqueArrayList<String> l = new UniqueArrayList<>();
    l.add("a");
    l.add("a");
    l.add("a");
    System.out.println(l);
  }
}

/**
 * @author Lars Helge Overland
 */
 class UniqueArrayList<E>
    extends ArrayList<E>
{    
    @Override
    public boolean add( E e )
    {
        return super.contains( e ) ? false : super.add( e );
    }
    
    @Override
    public void add( int index, E e )
    {
        if ( !super.contains( e ) )
        {
            super.add( index, e );
        }
    }
    
    @Override
    public boolean addAll( Collection<? extends E> c )
    {
        boolean modified = false;
        
        if ( c != null )
        {        
            for ( E e : c )
            {
                if ( !super.contains( e ) )
                {
                    super.add( e );
                    modified = true;
                }
            }
        }
        
        return modified;
    }
    
    @Override
    public boolean addAll( int index, Collection<? extends E> c )
    {
        boolean modified = false;
        
        if ( c != null )
        {        
            for ( E e : c )
            {
                if ( !super.contains( e ) )
                {
                    super.add( index++, e );
                    modified = true;
                }
            }
        }
        
        return modified;
    }
}

The code above generates the following result.

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