Teste de unidade de uma classe Stack

Stack.java

import java.util.ArrayList;
import java.util.EmptyStackException;

public class Stack<T> {
  private ArrayList<T> elements = new ArrayList<T>();
  private int size = 0;

  public int size() {
    return size;
  }

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

  public void push(T elem) {
    elements.add(elem);
    size++;
  }

  public T pop() throws EmptyStackException {
     if (isEmpty())
        throw new EmptyStackException();
     T elem = elements.remove(size-1);
     size--; 
     return elem;
  }
}

StackTest.java

import org.junit.Test;
import org.junit.Before;
import static org.junit.Assert.assertTrue;
import static org.junit.Assert.assertFalse;
import static org.junit.Assert.assertEquals;
 
public class StackTest {
   Stack<Integer> stack;
	 
   @Before
   public void init() {
	  stack = new Stack<Integer>();
   }
 
  @Test
  public void testEmptyStack() {
    assertTrue(stack.isEmpty());
  }
 
  @Test
  public void testNotEmptyStack() {
    stack.push(10);
    assertFalse(stack.isEmpty()); 
  }
 
  @Test
  public void testSizeStack() {
    stack.push(10);
    stack.push(20);
    stack.push(30);
    int size = stack.size();
    assertEquals(3,size); 
  }
 
  @Test
  public void testPushPopStack() {
    stack.push(10);
    stack.push(20);
    stack.push(30);
    int result = stack.pop();
    result = stack.pop();
    assertEquals(20,result); 
  }
 
  @Test(expected = java.util.EmptyStackException.class)
  public void testEmptyStackException() {
    stack.push(10);
    int result = stack.pop();
    result = stack.pop(); 
  }
  
}