title: How to add different values stored in an ArrayList<String> as 'Strings'?
body: <p>I'm retrieving some numbers stored in <code>FirebaseDatabase</code> as <code>Strings</code> and storing them all in an <code>ArrayList &lt;String&gt; aList</code> in my app like this:</p>
  
  <pre><code>    mDatabase.child("child").child(uid).addValueEventListener(new ValueEventListener() {
              @Override
              public void onDataChange(DataSnapshot dataSnapshot) {
  
                  for (DataSnapshot childSnapshot: dataSnapshot.getChildren()) {
  
                      aList.add(childSnapshot.getValue().toString());
  
                  }
  
                  Log.d("aList", String.valueOf(aList));
  
              }
  
              @Override
              public void onCancelled(DatabaseError databaseError) {
  
              }
          });
  </code></pre>
  
  <p>Here's what <code>log</code> is showing:</p>
  
  <pre><code>D/aList: [5.0, 3.5, 2.0, 2.5, 4.0, 3.5]
  </code></pre>
  
  <p>I tried adding these numbers like this:</p>
  
  <pre><code>Double d = Double.parseDouble(aList.toString()) + Double.parseDouble(aList.toString());
  </code></pre>
  
  <p>but got the error: <code>java.lang.numberformatexception: invalid double: [5.0]</code></p>
  
  <p><strong>What I want to know</strong> is how can I add these numbers given in the <code>aList</code> above? </p>