Java Swing - GUIs
TOPIC 02 – INTERACTIVITY WITH ACTIONLISTENER

LESSON NOTE

ACTIONS

For now, actions (events) will consist of one of two things:

• Clicking a button;
• Hitting <enter> while inside a text field.

EXAMPLE 0 - BASIC GUI

To understand how to add interactivity, we will revisit the code for a basic GUI. Our GUI will have only a button on it.

Here is the code for our starting GUI:

The code above creates:

ACTIONLISTENER INTERFACE

Java’s ActionListener interface is designed to listen for events and automatically execute the actionPerformed method when such an action occurs. As programmers, we implement this interface and override the actionPerformed method to specify which response we want for this action.

We can make any class implement the ActionListener interface and take care of the listening duty. However, it makes sense to do this in the main application class because the datafields are accessible there.

So here are the steps to include the interactivity:

• Add implements ActionListener on the first line.
  
  
• In the constructor, make this class be the Listener by adding:
  
  componentName.addActionListener(this);
  
  where the componentName is the name of a button or text field.

• Add the actionPerformed(ActionEvent e) method in the class. This method is required by the interface that we implemented in the first point above.

• Inside the actionPerformed method, respond however you'd like to the action that has occurred. More details on this later.

EXAMPLE 1 - OUR FIRST INTERACTIVE GUI

We can place both the visual components of the GUI and the actionPerformed method inside a single class. This does allow for a one-stop-shop programming approach but does make the code a little messy.

REMEMBERING INFORMATION

If we want to remember information between events, then that information has to be stored in a datafield. As always, we declare the datafield at the top, we initialize it in the constructor and then we update it and use it as we want in the actionPerformed method.

EXAMPLE 2 – COUNTING THE CLICKS

Here’s the same example as above but counting the number of clicks. Notice that we can actually set the text of the button with the setText method.

MONITORING MULTIPLE COMPONENTS

We can monitor more than one component by simply adding action listeners to each component that we want to monitor. Of course, any of the components will trigger the same actionPerformed method.

EXAMPLE 3 – GREETING APP

In this application, we will allow both the button and the text field to generate the response. Both will generate the same response.

DIFFERENTIATING BETWEEN COMPONENTS

When there are more than one components that could trigger an action event, we often need to know which component actually did trigger the event. This can easily be done by using the getSource method from ActionEvent.

EXAMPLE 4 – DIFFERENT RESPONSES

GUIs usually require different responses depending on the source of the event. We can do this quite easily using e.getSource() inside the actionPerformed method.