Implementation of a Set with an array.

Set.java

package us.kardol.datastructure;

/**
 * Created by Guillermo Kardolus on 2/24/16.
 */
public class Set {
    private int size = 0;
    private int capacity = 4;
    private Object[] elements = new Object[capacity];

    public boolean isEmpty(){
        return size == 0;
    }

    public void add(Object object){
        if(this.contains(object)){
            return;
        }
        if(size == capacity){
            elements = copy();
        }
        elements[size] = object;
        size++;
    }

    public void remove(Object object){
        int index = indexOf(object);
        if(index < 0){
            return;
        }
        elements[index] = elements[size - 1];
        elements[size - 1] = null;
        size--;
    }

    public int getSize(){
        return this.size;
    }

    public boolean contains(Object o){
        return indexOf(o) > -1;
    }

    private Object[] copy(){
        this.capacity = this.capacity * 2;
        Object[] output = new Object[capacity];
        for(int i = 0; i < elements.length; i++){
            output[i] = elements[i];
        }
        return output;
    }

    private int indexOf(Object o){
        for(int i = 0; i < size; i++){
            if(elements[i].equals(o)){
                return i;
            }
        }
        return -1;
    }
}

SetTest.java

package us.kardol.datastructure;

import org.junit.Before;
import org.junit.Test;

import static org.junit.Assert.*;

/**
 * Created by Guillermo Kardolus on 2/24/16.
 */
public class SetTest {
    Set emptySet;
    Set oneSet;
    Set manySet;

    @Before
    public void setUp(){
        emptySet = new Set();
        oneSet = new Set();
        manySet = new Set();

        oneSet.add("1");
        manySet.add("1");
        manySet.add("2");
    }

    @Test
    public void testIsEmptyWithoutElements() {
        assertTrue(emptySet.isEmpty());
    }

    @Test
    public void testIsEmptyOnNonEmptySet(){
        assertFalse(oneSet.isEmpty());
    }

    @Test
    public void testManySetSize(){
        assertTrue(manySet.getSize() > 1);
    }

    @Test
    public void testManySetContainsElement(){
        assertTrue(manySet.contains("1"));
    }

    @Test
    public void testAddTooMany(){
        manySet.add("5");
        manySet.add("3");
        manySet.add("7");
    }

    @Test
    public void testAddDuplicate(){
        oneSet.add("1");
        assertEquals(oneSet.getSize(), 1);
    }

    @Test
    public void testRemoveElement(){
        manySet.remove("1");
        assertEquals(manySet.getSize(), 1);
        assertFalse(manySet.contains("1"));
        assertTrue(manySet.contains("2"));
    }
}