Computer Science 252
Problem Solving with Java

Spring 2014, The College of Saint Rose

Lab 1: Java Review and Objectdraw Practice
Due: 11:59 PM, Friday, January 24, 2014

The goal of our first lab is to get everyone some practice with Java and Objectdraw.

You may help each other with this lab, but the work you turn in must be substantially your own. If you have previous Objectdraw experience, please help your classmates get up to speed.

There are a number of lab questions, and two practice programs, but no programming assignment in this lab. Please refer to the "Submission Guidelines" on the course home page and syllabus for the requirements for each of these items.

Getting Set Up

To get your BlueJ environment set up for this week's lab assignment, start BlueJ and choose "New Project" from the "Project" menu. Navigate to your folder for this course and choose the name "Lab1" (no spaces) for the project.

Create a document where you will record your answers to the lab questions. If you use plain text, call it "lab1.txt". If it's a Word document, you can call it whatever you'd like, but when you submit, be sure you convert it to a PDF document "lab1.pdf" before you submit it.

Also be sure you are able to download, run, and modify class examples.

Working solutions to the two programs can be downloaded here.

Lab Procedure

Recall that our class examples to date have introduced the Objectdraw graphics primitives Text, FilledRect, FilledOval, FramedRect, FramedOval, and Line. We have also seen the WindowController methods begin, onMousePress, onMouseRelease, onMouseClick, onMouseMove, onMouseDrag, onMouseExit and onMouseEnter.

A Summer Scene

Create a new "WindowController" class named Picture within your project. Before going further, put your name at the top and replace the default comment with one describing that this file is for this lab. Your development of the Picture program is a "Practice Program" as described in the submission guidelines.

Use the Objectdraw graphics primitives to create the following picture:

For this first version, you should have the entire picture appear when the program starts, so all of your graphical objects will be created in the begin method. A few notes about this:

Question 1: Paste the contents of your begin method at this point (4 points).

Adding Mouse Events

We will now make some modifications to the program to make it respond to some mouse events.

First, modify the program so the "leaves" of the tree (OK, the FilledOval) turns red when the mouse leaves the window. It should turn back to green when the mouse re-enters the window. (3 points)

Since you now need to refer to the object after you construct it, you will need to give it a name. Since the object's name will be needed in three different methods, it should be declared as an instance variable at the top of the class.

When you introduce the new name (let's say you call it leaves), (incorrectly) assign its value to the result of the construction with the .setColor as part of the same statement.

  leaves = new FilledOval(75, 175, 70, 70, canvas).setColor(;

Question 2: When you do this, what error does Java indicate? (1 point)

The reason for this is that your instance variable holds a FilledOval, but the result of the statement on the right hand side of the assignment is the result of the setColor call. Since setColor is a mutator method, it does not return a value.

In this situation, you need to break the construction and the setting of its color into separate lines.

  leaves = new FilledOval(75, 175, 70, 70, canvas);

Our next modification is to delay the addition of the tree to the picture until the first time the mouse is pressed within the window. Move the code that create the tree to an onMousePress method.

Question 3: What happens if you move the mouse out of the window before you press it in the window? (1 point)

The problem here is that there is a chance that you'll try to change the color of the leaves before they are first created. In this circumstance, the instance variable that refers to the leaves will be null. We can use this fact to make sure we only attempt to change the color of the object when it exists.

Insert appropriate conditionals to ensure that the color is changed only if the object exists. (2 points)

There is still a problem, but it is less obvious. Now, every time you press the mouse, a new tree is created. This results in the objects that make up the tree being constructed and stacked on top of each other. While this does not affect the correctness in terms of the picture produced, it is inefficient. In some other cases, constructing multiple copies of the "same" object would cause more visible problems.

Insert a conditional that ensure that the tree is only created on a mouse press if the tree objects do not yet exist. (2 points)


Next, we will add the capability for our sun and cloud to move. Our goal is to have each of these objects move a bit each time the mouse is pressed within that object.

Let's start with the sun. We are going to do something very similar to what you can see in the NudgeBall example. Modify your program's onMousePress method to move the sun (all three of its components) to the right by 3 each time the mouse is pressed within the sun. Note that you can check if the sun object contains the mouse press point by querying either the FilledOval for the yellow part of the sun or the FramedOval that represents its black outline. (3 points)

Question 4: Why do we not need to check both the filled part of the sun and its frame for containment of the point? Why do we not need to check containment of the point within the "smiley" text? (1 point)

Now, we'll do the same with the cloud, except it should move to the left by 4 when any part of the cloud contains the mouse press point. (3 points)

Question 5: Here, you will need to check if the point is contained in any of the graphical objects that make up the cloud. Why do we need to do this here if we didn't with the sun? (1 point)

Let's now make it so the sun and cloud go back to their original positions when the mouse leaves the window. Here, you will use the moveTo method on each object, which does an absolute move rather than the relative move of the move method. (3 points)

Our last task is to place a solid "sidewalk" between the lines on the grass, but it should only appear when the mouse is being moved and its position is over the grass. When the mouse is moved over the sky (or tree, or sun, or clouds), the sidewalk should be invisible.

There are two ways to make a single object disappear from the screen: the hide and removeFromCanvas methods that are available for all Objectdraw graphical objects. As a rule, we use hide when we would like an object to disappear but would later like it to reappear (with the show method). We use removeFromCanvas when it is to go away permanently. If we remove an object from the canvas and want to see it again, we would have to construct a new instance of the object.

In this case, we should use the hide/show technique. When the program starts, create the FilledRect for the sidewalk, but immediately hide it. Then in the onMouseDrag method, check if the mouse location is inside of the object representing the grass. If so, we show the sidewalk. If not, we hide it. Note that there is no harm in calling show on an object that is already visible or hide on an object that is already hidden. (2 points)

Some Dragging

One of the operations we will use most frequently with our graphical programs is dragging objects around on the canvas. The DragABall example shows the basics of dragging an object around.

Create a second Java class within your project for this lab called "DragBoxAndBall".

Practice Program: Write a program DragBoxAndBall, which is much like "DragABall", except that it creates both a box and a ball, and lets you drag around either. (8 points)


Before 11:59 PM, Friday, January 24, 2014, submit your lab for grading. There are three things you need to do to complete the submission: (i) Copy your file with the answers to the lab questions into your project directory. Be sure to use the correct file name. If you prepared your answers in Word, export to a PDF file and submit that. (ii) Upload a copy of your lab (a .7z or .zip file containing your project directory) using Submission Box under assignment "Lab1". (iii) Demonstrate the execution of your programs for your instructor (2 business day grace period for demos).


This assignment is worth 35 points, which are distributed as follows:

> FeatureValueScore
Lab questions 8
Picture leaves change color on entry/exit 2
Picture color change only if object exists 2
Picture tree created only if not existing 2
Picture sun moves 3
Picture cloud moves 3
Picture sun and cloud move back on exit 3
Picture show/hide sidewalk 2
DragBoxAndBall functionality 8
Total 35