Lecture 8 - Swing continued

Question 1

Why do we need a Model-View-Controller style for programming?

Answer 1

Otherwise, code will become unreadable

Question 2

How does the use of Model-View-Controller style programming vary by size?

Answer 2

Most simple: MVC in one class

Medium size & complexity: Program may have the a class with the View and Controller & a main class that creates 2 objects (View-Controller object & Model object)

Longest & most complex programs may have the MVC separate. 
View (constructor to create display, helper methods to update display)
Controller (actionPerform & other helper methods for the interaction)
Main class creates 3 objects (Model, View, Controller)

Question 3

What does the event listener KeyListener do?

Answer 3

It detects keys

Question 4

What imports do we need for KeyListener?

Answer 4

import java.awt.event.KeyEvent;

import java.awt.event.KeyListener;

Question 5

What does charAt(0) do?

Answer 5

Returns the char value at the specified index.

An index ranges from 0 to length() - 1. The first char value of the sequence is at index 0, the next at index 1, and so on, as for array indexing

Question 6

Write a for loop that copies something into an arrray,

Answer 6

for(int i=0;i

Question 7

What is the class header for using a KeyListener and an ActionListener?

Answer 7

public class ex extends JFrame implements ActionListener{

Question 8

“b” handles events, but constructs the variables/array in another class “a”.

In “b” you want to get the variable String keyword that you are passing to the constructor and create a new object of class “a”.

How do you do this?

Answer 8

String text = keyword.getText();

a = new a (text);

Question 9

What is the “model” class?

Answer 9

Holds the data

Private variables, methods to perform calculations, getters & speaks to the controller but doesn’t know the view exists.

Question 10

What is the “view” class?

Answer 10

The interface.

• Swing stuff (ie., extends, components, panels …)
• Getters (ie. getTextField())
• Setters for the controller to use to set the calculations in the Model class with the input from the View class.
• • An actionListener is added to the button so when its clicked the controller is alerted
• Handle errors using a JOptionPane that shows an error message

Question 11

What is the “controller” class?

Answer 11

It handles interactions between the view and the model in an actionPerformed() method, but doesn’t actually do anything itself.

Implements ActionListener

Think of it as the SERVER in the client-database relationship.

Question 12

How would you get the input from a text field and return is as an Integer?

Answer 12

use a get()

return Integer.parseInt(.getText())

Question 13

What is typically in the “view” class?

Answer 13

• extends JFrame so it needs all the information to set up a JFrame
• private global variables ie.,JLabel
• a constructor that has the JFrame methods ie., setSIze etc, adds the variables to the JPanel and then adds them to this JPanel
• getters to access the variables in other classes (JTextField will need to be converted to an int if we are looking for numbers etc)
• an ActionListener method that lets the controller know that the button has been clicked etc
• JOptionPane that shows an error if the input is incorrect

Question 14

What is typically in the “controller” class?

Answer 14

• A constructor that takes in the Model & the View class
• Handles events by adding actionPerformed method
• Includes a try catch block incase there is an unchecked error eg., someone enters 1 number instead of 2

Question 15

How do you use a JPanel that you made in another class?

Answer 15

• in extends JFrame implements ActionListener
• create an object with the panel class name and pass the current class “this” into it. This means that everything will be constructed in the JPanel class.

Question 16

What is system.exit(0) used for?

Answer 16

If we enter 0 it exists the program.

If its another number it lets the OS know that something went wrong.

Question 17

What is a content pane?

Answer 17

Is the content displayed inside a JFrame window

Question 18

What will be the value of b after this code?

int a = 3; int b = a; a += 2;

Answer 18

b is initialised as the same value as a, but it is a different variable. When we change a, b doesn’t change

The correct answer is: 3

Question 19

What will be the contents of a at the end of this code? Drag the numbers into the correct order.

public class ArrayQuestion {
   public static void main(String[] args) {
      int[] a = {3,5,2,1};
      int temp = a[1];
      a[1] = a[2];
      int temp2 = a[3];
      a[3] = temp;
      a[2] = temp2;
   }
}

Answer 19

3,2,1,5

Question 20

What will be the output produced on line 16?

Public class QQ {
   private int aNumber;
   public QQ(int a) {
      aNumber = a;
   }
   public void increment() {
      aNumber += 1;
   }
   public int getNumber() {
      return aNumber;
   }
   public static void main(String[] args) {
      QQ a = new QQ(4);
      QQ b = a;
      a.increment();
      System.out.println(b.getNumber()); // LINE 16
   }
}

Answer 20

Both references a and b refer to the same object. When the value of aNumber is increased in the object, it will be seen from both references.

The correct answer is: 5

Question 21

Explain how this works?

public static void main(String[] args) {
for(int i=1;i<3;i*=2) {
for(int j=0;j<i></i>

Answer 21

i=0, checks it meets the condition i<3 and if it does it goes into the inner loop where j=0.
j=0 keeps looping while the condition j<i></i>

Question 22

What is the result of the following calculation in Java:

5/2*1.0

Answer 22

2.0

Question 23

Suppose you are writing a Java method to add two numbers together. Which of the following method names would best follow Java convention?

Select one:

a. addTogether
b. AddTogether
c. addtogether
d. Add_Together

Answer 23

addTogether

Question 24

What will be the value of e?

Public class Method {
   public static void main(String[] args) {
      int d = 1;
      int e = method1(d);
   }
   public static int method1(int a) {
      return method2(a) + 2;
   }
   public static int method2(int a) {
      return a + 3;
   }
}

Answer 24

6

Question 25

One of the lines in this code includes a syntax error. Which is it?

1. public class QQQ {
2. public static void main(String[] args) {
3. int a = 3;
4. {
5. System.out.println(a);
6. int b = 2;
7. }
8. System.out.println(b);
9. }
10. }

Answer 25

8

Question 26

There is a syntax error in this code. Which line?

1. public class AnimalWithLegs {
2. private int nLegs;
3. public AnimalWithLegs(int a) {
4. nLegs = a;
5. }
6. }
   7.
7. public class Dog extends AnimalWithLegs {
8. public Dog() {
9. super(4);
10. }
11. public String toString() {
12. return “I have “ + nLegs + “ legs”;
13. }
    15.}

Answer 26

13

Question 27

What is the result of the following calculation?

3/4) + (4/3) + 3*(2+1

Answer 27

10

Question 28

What will be the output of line 7?

public class QQ {
   public String toString(). {
      return "Hello";
   }
   public static void main(String[] args) {
      QQ[] objs new QQ[5];
      System.out.println(objs[0]); // LINE 7
   }
}

Answer 28

null

Question 29

How do you turn buttons off?

Answer 29

b.setEnabled(false);