Unit 6 (Event Handlers)

Question 1

What is the role of an event listener in Java?

A) To generate events
B) To handle events and perform actions in response
C) To display events to the user
D) To store event data

Answer 1

B) To handle events and perform actions in response

Question 2

What does the callback mechanism involve in GUI programming?

A) A component calling methods from the operating system
B) Two classes interacting where one class notifies the other about certain events
C) A loop where events are constantly checked
D) Automatic event generation without user interaction

Answer 2

B) Two classes interacting where one class notifies the other about certain events

Question 3

Which class in Java is typically used to handle simple button click events?

A) JButton
B) ActionListener
C) ActionEvent
D) JFrame

Answer 3

B) ActionListener

Question 4

What is required to handle an event from a JButton in Java?

A) Implementing the ActionListener interface and overriding the actionPerformed method
B) Extending the JFrame class and overriding the actionPerformed method
C) Implementing the ActionEvent class and creating a custom method
D) Directly modifying the JButton class

Answer 4

A) Implementing the ActionListener interface and overriding the actionPerformed method

Question 5

Which method is used to determine the source of an event when multiple components use the same listener?

A) getSource()
B) getComponent()
C) getListener()
D) getEventSource()

Answer 5

A) getSource()

Question 6

What will be the output of the following code when the ‘Exit’ button is clicked?

public class MyFrameWithExit extends JFrame {
private JButton exitButton = new JButton(“Exit”);
public MyFrameWithExit() {
super(“Exit Frame”);
exitButton.addActionListener(e -> System.exit(0));
add(exitButton);
setSize(300, 200);
setVisible(true);
}
}
A) The frame will close.
B) Nothing happens.
C) An error will occur.
D) The button will disable itself.

Answer 6

A) The frame will close.

Question 7

In the context of Java GUI, what does subscribing a listener to a button involve?

A) Calling the addActionListener method on the button
B) Modifying the button’s properties
C) Overriding the button’s default methods
D) Implementing all methods in the ActionListener interface in the button class

Answer 7

A) Calling the addActionListener method on the button

Question 8

What is the purpose of implementing interfaces like ActionListener in Java GUI components?

A) To inherit properties from superclass components
B) To handle events like button clicks
C) To improve the look and feel of the GUI
D) To increase the processing speed of events

Answer 8

B) To handle events like button clicks

Question 9

What happens if a registered listener does not handle an event?

A) The event is stored for later use.
B) The event is discarded.
C) The event is sent to another listener.
D) The GUI throws an exception.

Answer 9

B) The event is discarded.

Question 10

How do you unsubscribe a listener from a JButton in Java?

A) Using the removeActionListener method
B) By setting the listener to null
C) By deleting the JButton object
D) It’s not possible to unsubscribe a listener.

Answer 10

A) Using the removeActionListener method

Question 11

What is the role of the actionPerformed method in event handling?

A) It initializes the components that generate actions.
B) It executes in response to an action event.
C) It generates action events.
D) It registers components to receive action events.

Answer 11

B) It executes in response to an action event.

Question 12

Consider the following code snippet. What is the likely issue?

JButton button = new JButton(“Submit”);
button.addActionListener(e -> doSomething());
A) The button does not have a valid ActionListener.
B) The lambda expression is incorrectly implemented.
C) The doSomething() method must accept an ActionEvent parameter.
D) There is no issue with the code.

Answer 12

C) The doSomething() method must accept an ActionEvent parameter. (Note: This depends on the implementation of doSomething(). If it doesn’t need event data and is accessible in the context, D could also be correct.)

Question 13

Which of these is not a valid way to handle events in Java?

A) Using inner classes
B) Using anonymous classes
C) Using lambda expressions
D) Direct method calls without interface implementation

Answer 13

D) Direct method calls without interface implementation

Question 14

What would be the effect of clicking a JButton configured with the following ActionListener?

button.addActionListener(e -> System.out.println(“Button clicked!”));
A) It prints “Button clicked!” to the console.
B) It displays “Button clicked!” in a new window.
C) It causes the application to terminate.
D) It clears the text on the button.

Answer 14

A) It prints “Button clicked!” to the console.

Question 15

Which interface must be implemented to handle item state changes in checkboxes?

A) ItemListener
B) ActionListener
C) ChangeListener
D) ComponentListener

Answer 15

A) ItemListener

Question 16

In event-driven programming, which Java feature is used to handle events like mouse clicks or key presses?

A) Polymorphism
B) Encapsulation
C) Callbacks
D) Inheritance

Answer 16

C) Callbacks

Question 17

What is the output of this code if the button is clicked multiple times?

public class CounterFrame extends JFrame {
private int count = 0;
private JButton countButton = new JButton(“Count”);
private JLabel label = new JLabel(“Count: 0”);

public CounterFrame() {
    countButton.addActionListener(e -> {
        count++;
        label.setText("Count: " + count);
    });
    add(countButton, BorderLayout.NORTH);
    add(label, BorderLayout.SOUTH);
    setSize(200, 100);
    setVisible(true);
} } A) The label updates to show the increasing count each time the button is clicked. B) The label displays "Count: 1" no matter how many times the button is clicked. C) The count increases but the label does not update. D) A runtime error occurs because the label text is not properly formatted.

Answer 17

A) The label updates to show the increasing count each time the button is clicked.

Question 18

Which interface must be implemented to handle changes in item selection, such as in a checkbox?
A) ActionListener
B) ItemListener
C) WindowListener
D) KeyListener

Answer 18

B) ItemListener

Question 19

How would you handle multiple different types of events in a single GUI component?
A) Implement multiple listeners
B) Use a single universal listener interface
C) Implement ActionListener for all event types
D) Java does not support handling multiple event types in a single component

Answer 19

A) Implement multiple listeners

Question 20

Why would you unsubscribe an event listener from a component in a Java GUI application?
A) To free up resources when they are no longer needed
B) To increase the performance of the application
C) It’s mandatory to unsubscribe all listeners before closing the application
D) Unsubscribing is not supported in Java

Answer 20

A) To free up resources when they are no longer needed

Question 21

What is a common use of anonymous classes in Java GUI event handling?
A) To create temporary components
B) To implement listener interfaces for event handling directly within the method where components are created
C) To declare new Java standard classes
D) To replace the functionality of lambda expressions

Answer 21

B) To implement listener interfaces for event handling directly within the method where components are created