title: How to sum up all the elements with the same property in an ArrayList in Java?
body: <p>I have an ArrayList&lt; Item>, and I want to do a function that sum all the quantity of the items with same code, then display all the items with the quantity.</p>
  
  <pre><code>public class Item{
      private String code;
      private int quantity;
  
      public Item(String InputCode, int InputQuantity)
      {
          this.code= InputCode;
          this.quantity = InputQuantity;
      }
  }
  </code></pre>
  
  <p>Suppose i do the following</p>
  
  <pre><code>List&lt;Item&gt; items = new ArrayList&lt;Item&gt;();
  items.add(new Item("A01", 1));
  items.add(new Item("A02", 1));
  items.add(new Item("B05", 2));
  items.add(new Item("A01", 3));
  items.add(new Item("Z02", 2));
  items.add(new Item("A02", 2));
  display();
  </code></pre>
  
  <p>then the desired output is</p>
  
  <pre><code>A01 4
  A02 3
  B05 2
  Z02 2
  </code></pre>
  
  <p>I have no idea, can anyone give me some hints?</p>