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Event Handling in Java: What is that and How Does it Work?

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20th Mar, 2020
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Event Handling in Java: What is that and How Does it Work?

Any program that is graphic user interface or GUI-based, including Java application for Windows, is driven by events. For a program like a Java application to be useful, it is required to provide a response to commands or inputs from the user. Java applications have events to capture user actions. Before we discuss event handling in Java, let’s start by discussing events.

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What is an Event?

Events hold the same meaning and importance in every programming language. They are external effects that are controlled by the user, and that make your application behave in line with the pre-defined condition put in place to record its response to different user actions. An event could even be termed as an object that springs to life when a change takes place within the GUI. 

Examples of events in Java include typing of characters or text, clicking on a combo box, clicking on a button, or any other action from the user’s side. Any such action triggers an event, which then results in the creation of the event object that is relevant for that event. 

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For instance, if a user enters some data (user action that triggers an event), the application produces an output by putting out a dialog box or displaying information (event object). These event-related behaviors are a part of the Event Handling Mechanism of Java and are present in the Swing GUI library. It is important to note that the source of every event and the event listener must be linked. If this isn’t the case, the triggering of an event will yield no action. 

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Why Does a Program or Application Need to be Event Driven?

Before event handling came into the picture, a program had to collect all the user information itself to know what it was doing at a given point in time. This means that after being run or initialized, a program was always in a big repeat loop that was waiting for the user to do something. 

So, the program was looking for any action – from the press of a button to slider movement. After it came to know that something has happened from the user’s side, it prepared itself to deliver the appropriate response. This is referred to as polling. Although polling gets the job done, more often than not, it comes across as too unmanageable and time-consuming a task.

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If we consider using it for modern-day applications, it doesn’t really fit the requirements. Two primary reasons make polling unsuitable for modern applications – Polling puts all the event-related code inside the big repeat loop, and the interactions that take place inside this location are too complex. Also, polling makes a program enter a never-ending loop, which results in the exhaustion of CPU cycles without any guarantee of action coming from the user. 

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The Abstract Window Toolkit or AWT has gone ahead and struck association with a different working model for solving the issue discussed above. This new model is event-driven programming. With AWT, there is no need for the program to look out for user-generated events. It is the Java run time that does this job. It intimates the program as soon as an event takes place. It saves a valuable resource from exhaustion and handles user interaction better. 

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What are Event Handlers in Java and How do They Work?

Let’s now discuss the primary topic that we set out to deal with. How does event handling in Java work? 

As mentioned earlier, events are user actions or external effects that make an application behave in a certain way. In Java, AWT components, including textbox, button, and others, are responsible for the generation of events. As soon as an event is generated, the listener handles it and performs the appropriate action. 

Event handling in Java comprises four elements. These could even be termed as event handlers.

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1. Event source: An event source that generates an event is mostly an AWT component. You can run java.awt.component command to learn about components that may or may not have any role in generating events. The components are the subclass of the original source class. Event sources could be anything from text boxes and combo boxes to buttons, and more. 

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2. Event classes: In Java, these are classes that account for almost every component that has anything to do with the generation of events. These are also called event types. Here are a few of the most common event classes:

  • ActionEvent: This event class or event type represents an event that involves the clicking of a graphical element, such as a button or a list item. The listener related to this class is ActionListener.
  • KeyEvent: This event class represents an event that involves the pressing and releasing of a key. The listener associated with this class is KeyListener.
  • ContainerEvent: This event type represents an event that happens with the GUI container. This class is associated with any event where user action involves the addition or removal of object(s) from the GUI. The related listener for this class is ContainerListener.  
  • MouseEvent: This class represents all those events that involve the clicking or pressing of the mouse. The listener for this class is MouseListener.
  • WindowEvent: This event class or type represents events that involve any action related to a window. Closing, activating, or deactivating a window come under this class. The related listener for this class is WindowListener. 

3. Event Listeners: These are interfaces of Java that provides various methods that can be used in different implemented classes. The job of event listeners involves listening for events and then processing them appropriately when they take place. Almost every component in Java has a dedicated listener that handles any event that that component generates. ActionListener handles events that involve lists, buttons, text fields, and menus. 

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4. Event Adapters: If a program presents too many abstract methods for the listener to override, it can get difficult to compile it. For example, if we want to close a frame, there are seven WindowListener abstract methods that we need to override. To reduce complexity and heavy coding, Java has event adapters. Event adapters are already overridden abstract methods. 

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It is important to remember that multiple event sources and listeners can interact with each other. So, multiple events belonging to the same class can be handled by a single listener. This means that one listener can handle all those events that involve the same components that perform similar actions. On similar lines, a single event could be handled by more than one listener, only if the program’s design allows this. 

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Arjun Mathur

Blog Author
Arjun is Program marketing manager at UpGrad for the Software development program. Prior to UpGrad, he was a part of the French ride-sharing unicorn "BlaBlaCar" in India. He is a B.Tech in Computers Science from IIT Delhi and loves writing about technology.

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