Experiments on Objects

Table of contents
  1. Experiments on figures project
  2. Figures Challenge
  3. Experiments on house project
  4. Experiments on objects-practice project
  5. Experiments on lab-classes project
  6. BlueJ Challenges

Experiments on figures project

  1. Start BlueJ and open the project named figures. Right-click on one of the circle objects (not the class!), Choose makeVisible from the menu. Observe what happens…
    Try:
    • moveRight
    • moveDown
    • makeInvisible
    • makeVisible
  2. Experiment: What happens if you call moveDown twice? Or three times? What happens if you call makeInvisible twice?
  3. Invoke the moveHorizontal method. You will see a dialog appear that prompts you for some input. Type in 50 and click OK. Observe what happens…
    Discuss: How is it different from moveRight or moveLeft ?
  4. Try invoking the moveVertical, slowMoveVertical, and changeSize methods before you move on.
  5. Challenge: Find out how you can use moveHorizontal to move the circle 70 pixels to the left. Experiment if moveVertical moves the circle up in the same way.
  6. Try invoking the changeColor method and see what happens when you specify a color that is not known.
  7. Invoke the changeColor method, and write the color into the parameter field without the quotes. What happens?
  8. Create several circle objects on the object bench. You can do so by selecting new Circle() from the pop-up menu of the Circle class. Make them visible, then move them around on the screen using the “move” methods. Make one big and yellow; make another one small and green. Try the other shapes too: create a few triangles, squares, and persons. Change their positions, sizes, and colors.
    Discuss: Does changing an attribute of one object affect another object?
  9. Experiment: Make sure you have several objects on the object bench, and then inspect each of them in turn. Try changing the state of an object (for example, by calling the moveLeft method) while the object inspector is open. You should see the values in the object inspector change. Object Inspector
  10. Select Show Terminal from the View menu. This shows another window that BlueJ uses for text output. Then select Record method calls from the terminal’s Options menu. This function will cause all our method calls (in their textual form) to be written to the terminal. Now create a few objects, call some of their methods, and observe the output in the terminal window.
  11. Let’s now save this code somewhere safe and we will revisit it later.
  12. Select Show Code Pad from the View menu. This should display a new pane next to the object bench in your main BlueJ window. This pane is the Code Pad. You can type Java code here.
  13. Try copying the recorded lines one-by-one and paste in Code Pad and observe how those instructions are doing the same thing what we did using mouse-clicks.

Figures Challenge

Go to Practice to find the challenges.

Experiments on house project

  1. Go to Learn, download the house project ==> extract it to your projects folder.
  2. Open the house project using BlueJ. Create an instance of class Picture and invoke its draw method. Also, try out the setBlackAndWhite and setColor methods. How do you think the Picture class draws the picture?

Observing Picture class

  1. Look at the pop-up menu of class Picture again. You will see an option labeled Open Editor. Select it. This will open a text editor displaying the source code of the class.
  2. In the source code of class Picture, find the part that actually sets the picture to black and white and change it to red and yellow.
    Practice: Change them to any colors of your choice. Once satisfied and understood how it’s working, then turn it back to black and white.
  3. In the source code of class Picture, find the part that actually draws the picture. Change it so that the sun will be blue rather than yellow.

  4. Add a second sun to the picture. To do this, pay attention to the field definitions close to the top of the class. You will find this code:

    private Square wall;
private Square window;
private Triangle roof;
private Circle sun;

    You need to add a line here for the second sun. For example:
    private Circle sun2;
    Then write the appropriate code in two different places for creating the second sun and making it visible when the picture is drawn.

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Download objects-practice projects

  1. days
  2. rectangles
  3. pictures

Experiments on objects-practice project

  1. Download and extract days project in your bluej_projects folder. Open in BlueJ. There are two options to create instances. Explore all available methods.
  2. Which option of Day class creates a Day instance with today’s date, month, and year?
  3. Can you create a Day instance of any given year, month, and date? Try giving values like 99 for aDayOfMonth or aMonth or aYear. Does it accept?
  4. Find a way to calculate age of a person using Day object’s methods.
  5. Can the same process be used to find how long a person lived?
  6. Try checking the new Day value using String toString() method after adding some number of days. Verify if it is correct by referring to the calendar.
  7. Download and extract rectangles project in your bluej_projects folder. Open in BlueJ. Create an instance of Rectangle and draw it. What do you observe? Is the draw() method of Rectangle in this project different from the draw() method of Rectangle in figures project?
  8. Create an instance of Rectangle using new Rectangle() and create another Rectangle instance using new Rectangle(double x, double y, double width, double height). What do you observe when you try drawing both of these rectangles? What do you observe? What do you think is the reason behind the result?
  9. How is setColor() method of the Rectangle class in this project different from that of house project?
  10. What happens when you invoke fill() method on your Rectangle instance?
  11. Try translate() and grow() methods on Rectangle instances and observe the difference in behaviors. What is the difference?
  12. Download and extract pictures project in your bluej_projects folder. Open in BlueJ. Try different ways of creating Picture instances. Try calling various methods available and see how many methods you are able to use as expected.

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Experiments on lab-classes project

  1. Go to Learn, download the lab-classes project ==> extract it to your projects folder.
  2. Create an object of class Student. You will notice that this time you are prompted not only for a name of the instance, but also for some other parameters. Fill them in before clicking OK. (Remember that parameters of type String must be written within double quotes.)
  3. Create some Student objects. Call the getName method on each object. Explain what is happening.
  4. Create an object of class LabClass. As the signature indicates, you need to specify the maximum number of students in that class (an integer).
  5. Call the numberOfStudents method of that class. What does it do?
  6. Look at the signature of the enrollStudent method. You will notice that the type of the expected parameter is Student. Make sure you have two or three students and a LabClass object on the object bench, then call the enrollStudent method of the LabClass object. With the input cursor in the dialog entry field, click on one of the student objects; this enters the name of the student object into the parameter field of the enrollStudent method. Click OK and you have added the student to the LabClass. Add one or more other students as well.
  7. Call the printList method of the LabClass object. You will see a list of all the students in that class printed to the BlueJ terminal window

  8. Create three students with the following details:

    Suneetha, student ID: A00234, credits: 24
Lata Mangeshkar, student ID: C22044, credits: 56
Papon, student ID: A12003, credits: 6

    Then enter all three into a lab and print a list to the screen.

  9. Use the inspector on a LabClass object to discover what fields it has.
  10. Set the instructor, room, and time for a lab, and print the list to the terminal window to check that these new details appear.

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BlueJ Challenges

Go to Practice to find the challenges.

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