1. Installing BlueJ¶

Follow the instructions on your course site on Canvas to Install BlueJ.

2. Introducing LightBot¶

LightBot is a very simple robot programming simulation game. It runs right in your web browser, and looks like this:

In the LightBot game, you create a program to control a little robot to solve a simple task in the miniature block-based world where the LightBot exists. The programming in LightBot is iconic. The icons at the top right of the game’s display represent instructions to the robot. Each icon instructs the robot to do something–go one square forward, turn right, turn left, jump forward, and light up the square the robot standing on. Of course the square will only light up if the robot is on a blue square. The speaker icon in the upper left corner will allow you to toggle the game sound on and off. The reset button in the lower left corner will delete all of your instructions and start the level over.

Your task on each level of the LightBot game is to get the robot to light up all of the blue squares. To create a program to control the robot simply drag the icons at the top and drop them on the main method squares left to right.

The LightBot game illustrates several basic programming concepts. The robot itself is an object, and we can request it to perform specific actions that it knows how to carry out. We can write a series of these requests down as a program. The series of actions that we write down is a plan for solving a specific problem (an algorithm).

3. From LightBot to Micro Worlds¶

The LightBot game is an example of a micro-world. A micro-world is a miniature simulated environment that is populated with one or more objects with clearly defined behaviors, and that can be visually represented so that you can see how the objects move around and behave over time.

Micro-worlds typically use a grid to represent locations in the simulated world. You can see this in the LightBot game, where the robot walks on square tiles. In this course, we will use the same approach. We can refer to any location on a grid using x and y coordinates:

We will use a variation of Cartesian coordinates to write any location as an (x, y) pair, where x (the first coordinate) represents the distance from the origin horizontally, and y (the second coordinate) represents the distance from the origin vertically. The only catch is that the y axis points downward in our grid, making this a left-handed Cartesian plane. We use a left-handed orientation for our axes because that convention is predominant in computer programming contexts (even though it is the opposite of what is typical in studying geometry!).

So the highlighted cell in the grid shown above is at location (4, 2), since it is 4 cells horizontally to the right of the origin, and 2 cells vertically down from the origin. Also notice that the origin (0, 0) is explicitly represented in our grid and is located at the top left corner. The grid above is 10 by 10 in size, where coordinates run from 0 up to and including 9. This means we will always used zero-based indexing when referring to grid positions–i.e., we always start counting from zero at the origin (top left), rather than starting with one.

4. Self Check: Micro-Worlds¶

5. A Bit More LightBot¶

If you only played the first two levels of LightBot, there’s definitely more interesting stuff you haven’t seen! As the levels progress, the puzzles get more complicated. They are harder to solve using the limited number of slots in the program memory for the little robot. Fortunately, the LightBot game provides a few more icons for you to use.

The original version of LightBot provided two smaller instruction areas : f1 and f2. The f1 and f2 icons represent the two functions (icon areas) below the main method in the LightBot game. These two functions are where you will place instructions that the robot will need to perform multiple times. Anywhere you place the f1 or f2 icon, the robot will perform the entire sequence of actions associated with that function. This will allow you to build more complex programs which are necessary to solve the more challenging tasks in the later levels.

6. Textually Representing Programs¶

Computer programs are most often written in textual form–in fact, one of the most important goals of programming is to write programs so that they communicate well to other people. So let’s look at how we can represent LightBot puzzle solutions as text.

At its most basic, a computer program is simply a sequence of instructions for a computer to follow. Our LightBot level solutions are just that: programs. All we need in order to write them textually is names that correspond to the icons. For example, if we call our little robot :code:andy, we could use the following names to correspond to his icons (from left to right):

Symbol	Textual Program Statement (Method Call)
	andy.move();
	andy.turnRight();
	andy.turnLeft();
	andy.jump();
	andy.turnLightOn();
	andy.f1();
	andy.f2();

Each action is phrased as a request to an object that performs the desired behavior. So each request goes to our robot, :code:andy. For example, consider Level 2:

One solution to this level is:

andy.move();
andy.turnRight();
andy.move();
andy.turnLeft();
andy.move();
andy.move();
andy.turnLeft();
andy.move();
andy.turnRight();
andy.move();
andy.turnLightOn();

7. Self-Check: Turning Micro-Worlds into Code¶

8. What Does LightBot Say About Programming?¶

The LightBot game illustrates several basic programming concepts:

• Object

• Method

• Method Call

• State

• Algorithm

• Program

• Source Code

• Interpreter

In this course, we are going to focus on learning object-oriented programming in Java. This section describes several general concepts about object-oriented programs that we have seen in this first example. As you read subsequent chapters, refer back to this chapter to review the meaning of important words and phrases.

9. A Programmable LightBot in Java¶

9.1. The LightBot World Top-Down¶

In the sections above, you played with the LightBot game, and even saw a textual representation for the graphical commands that the little robot obeys. Now it is time to turn those concepts into a full-fledged program that you can write yourself.

First, recall that LightBot is really just a form of micro-world: each “level” of the game is a miniature world that can be represented as a grid, and all of the objects–the robot, the blocks, the bue tiles that light up–occupy different locations on this grid. The flash game uses isometric projection to make the world look more three-dimensional, and animation to make it more attractive. For example, here is the second level of the LightBot game you saw in Chapter 1:

To simplify things somewhat, however, let’s use a plain top-down view of the same situation:

In this top-down view, it is much easier to see which direction is the x direction and which is the y direction, so we can keep our coordinates straight. Also, this picture highlights the fact that each LightBot level is an 8 x 8 grid. As another example, the third level of the LightBot game looks like this in the original:

But the same level can be represented top-down like this:

Now that we have our “world” figured out, we can talk about writing source code to control the robot in it.

9.3. Where Do We Write It?¶

All of the program code we write must go somewhere. But where do we put it? In an object-oriented program, all program statements go inside a method, and every method belongs to an object (or to a class, a family of objects that all understand the same methods).

The programming environment we will use in this course is called BlueJ. We are also using class libraries from Greenfoot, a framework that allows us to work with many kinds of micro-worlds.

When you open a project in BlueJ, you will see a diagram of the various Java classes you are working on. Here, we can see BlueJ’s main window looking at a simple Java project for solving LightBot Level 3.

You can double-click on any Java class in the project to open it in the editor and view its program code. For our micro-world assignments, you will also see a class called Application–right-click the Application class and choose Run JavaFX Application to execute your micro-world application.

When you run any micro-world program, you will see the world displayed. The main area of the display consists of a top-down view of the world, in this case Level 3 of the LightBot game. You will also see controls at the bottom to Act (cause objects to execute their next action), Run (cause objects to repeatedly act over and over), and control the speed of execution.

Each Java class that you write will have its source code stored in a text file. By double-clicking on the rounded rectangle representing the class in BlueJ’s project window, you can see (and edit) your class source code in an edit window. If you double-click on Level3Solution, you will see its contents:

In the editor notice that a method has already been provided to hold new code. This method is called myProgram(), and it represents the behavior that will be carried out when you press the Run button at the bottom of your program’s window. This method has the following structure:

10. Syntax Practice 1a¶

11. Creating New Objects¶

In order to call methods on an object to solve a problem, first we actually need an object! That means that creating the objects we need to solve a problem is part of our work in constructing a solution.

Sometimes, all we need to do is create and object, and that act by itself does what we need. But most of the time, we create an object so we can ask it to do things–call its methods. In order to call methods on an object, we need some way to refer to the object we are talking to. In most cases, we do this by introducing a name that refers to the object. These names are called variables (although we’ll get more precise about that later).

It is common for these two tasks–declaring a new name, and creating a new object–to go together, and Java provides a convenient way for us to express this two-part pattern:

The is the syntax for declaring a new name and creating a new LightBot object. The declaration portion indicates that the programmer plans to use a LightBot to help solve the problem at hand. The programmer must provide an identifier (or name) for the LightBot object.

The instantiation portion of the figure above is a request that the LightBot object be created. The crucial part of the instantiation is the constructor, which is a special method used only to create new objects. The constructor in this figure has the form LightBot(). It specifies the type of object we want to create by giving the name of a class–here, we are creating a new LightBot–followed by a pair of parentheses. In Java, parentheses are always used following a name when you are calling a method or calling a constructor. Later, we’ll see how values can be provided inside the parentheses to customize the object that is being constructed.

Suppose we wish to create a new LightBot and we want to refer to it by the name andy (a good name for an android). Then we could declare the name andy and create the LightBot with this statement:

LightBot andy = new LightBot();

We can place this inside our myProgram() method:

public void myProgram()
{
    LightBot andy = new LightBot();

    // ... more goes here ...
}

While this creates a new LightBot, we haven’t said where to place him in the world. Suppose we want to add andy to the world at location (0, 4). We can do that using a method that all world objects understand called add(), like this:

public void myProgram()
{
    LightBot andy = new LightBot();
    this.addObject(andy, 0, 4);

    // ... more goes here ...
}

Now it is time to try out your first program! If you have typed these lines into BlueJ’s editor, you may notice that the Level3Solution icon in BlueJ’s main window has changed so that it has diagonal lines across it. These indicate that the source code has been changed–because you typed new instructions. Before the computer can execute our program, however, we first need to convert the source code you wrote into a form that can be executed by the computer. Click the “Compile” button at the top of the editor window (or the “Compile” button on BlueJ’s main window). This will convert the source code into a form the computer can execute. Finally, right-click on the Application class and select “Run JavaFX Application”.

OK, it isn’t exciting, but you will see a new LightBot created and added to the level at the coordinates you specified. You can use the “Reset” button at the bottom of the window to throw away the current world and start with a brand new fresh copy, and you can use “Run” to run your program again. The “Act” button will single-step your program, executing one action at a time sequentially through myProgram() if you want to see what happens in super-slow-mo. Alternatively, use the speed slider to speed up or slow down how fast the actions are carried out while your code is running.

12. Calling Methods on an Object¶

OK, so we’ve created a LightBot, but it doesn’t do very much. How do we request it to carry out actions? An action statement is a request that an object perform a specific task. That task can be either one of the basic action methods that the object already knows, or it could be a new method that you wrote yourself. The syntax of an action statement is:

An action statement is also called a method call statement, since it calls a method on an object. The identifier to the left of the dot identifies which object will receive the message–that is, which object are we asking to perform the desired action. The object that is receiving the message (the object we are calling the method on) is called the receiver. The methodName indicates which method we are asking the object to perform. A method call in Java always includes a pair of parentheses after the method name. For methods that require extra information to do their job, we place the extra information between the parentheses (the parameters to the method). None of our LightBot methods require any extra information, so all of their parentheses will be empty–but the parentheses are still required.

So if we want andy to move forward, we can write the corresponding action statement like this:

andy.move();

This method call consists of three key parts: the receiver to the left of the dot, the method name to the right of the dot, and the parentheses after the method name. In addition to these three parts, notice the semicolon at the end of the line. Just like the declaration and instantiation statement we used to introduce the name andy, all statements in Java end in a semicolon. Don’t leave it off–it tells the computer where each statement ends.

13. Putting it All Together¶

We can add the method call we just wrote to myProgram() like this:

public void myProgram()
{
    LightBot andy = new LightBot();
    this.addObject(andy, 0, 4);

    andy.move();

    // ... more goes here ...
}

If you make this change, compile your code, and then run it, you will see the robot be created and placed in its starting location, and then move one square forward. To complete our solution, andy needs to jump up on the wall, and then jump down:

public void myProgram()
{
    LightBot andy = new LightBot();
    this.addObject(andy, 0, 4);

    andy.move();
    andy.jump();
    andy.jump();

    // ... more goes here ...
}

Finish writing the rest of the instructions for andy so that he can light up all the blue tiles.

14. A Word on Making Code Easy to Read¶

When programmers write source code, they must pay particular attention to making it easy to read for other people. Most programmers work in groups, and other programmers need to pick up and work with existing code that may have been written by others a while ago–weeks, months, or even many years ago. As a result, it is crucial that source code be easy for other people to read.

One way we make our code easy to read is by choosing meaningful names for all our identifiers. For example, Level3 represents the layout of Level 3 in the LightBot game, and Level3Solution represents a solution to that specific level layout. Similarly, the method names for the behaviors that LightBots can perform are things like move(), jump(), turnLeft(), and so on. The names are chosen to match the behaviors so that little or no explanation is necessary.

Another important tool in making source code easy to read is is commenting.

Some comments are critically important in source code because they explain to other programmers how to use a method or a class. In many programming languages, including Java, there are established conventions for writing these comments so that documentation about how to use your code can be automatically generated. You’ll see plenty such documentation later, but for now let’s get our toes wet with the basics.

// A pair of slashes side by side marks the beginning of a Java comment.
// Everything after them, including the two slashes themselves, is a comment.

// Sometimes, // is used to place a comment on the end of a line
// that also contains something else.  But more commonly, we'll see
// double-slash comments as single-line comments on a line by
// themselves.

/*
 * It is also possible to write a multi-line comment in Java by
 * starting it with a slash followed by an asterisk (/*).  Everything
 * from this marker onward, across as many lines as you want, makes up
 * a single comment.  The comment ends with a matching asterisk followed
 * by a slash, like this:
 */

// Finally, in Java there are specially marked comments that appear
// just before a name is declared, providing documentation about what
// that name means and how it is used.  All the comments above are
// plain old comments that mean nothing outside of the source file
// where they are written.  The following comment describes the purpose
// of a method, however, and can be used to generate documentation for
// it:

/**
 * This comment describes the purpose of this method.  It is called a
 * Javadoc comment.  You can tell it is different from other comments
 * because it starts with "/**", instead of just "/*".  You should
 * include this kind of comment just before each method you write, and
 * just before each class you write.  It should describe the meaning/purpose
 * of what immediately follows--here, the method called doSomething().
 * The Javadoc comment should also explain how to use the method or
 * class it describes.
 */
public void doSomething()
{
    // ...
}

For now, remember that you must include a Javadoc comment (that is, a description) for each method you write and for each class you write. We’ll try to make this easier for you as we start, but it is a habit you’ll have to become accustomed to (and that will serve you well!).

A third important tool for making code easy for others to read is indentation. Programmers use white space at the beginning of lines to show structure in their source code. You may have noticed that each of the methods shown in this Chapter starts with an opening brace ({), also called a left brace, a curly brace, a curly bracket, or just a curly. Each method also ends with a corresponding closing brace (}). Statements between these braces are inside the method–they are the statements that define what behavior occurs when the method is called. Because they are inside the braces (that is, inside the method body), we put extra spaces at the beginning of each line to show this nesting.

In this class, we will use all space characters to indent our code. We will indent lines that are “contained within” a larger structure by four spaces. You’ll see that the editor automatically tries to help you keep this indentation habit as you write code, but it won’t force you. Ultimately, you are responsible for making your code look clean, neat, and readable.

15. Syntax Practice 1b¶

16. Programming Practice 1¶

The following exercises provide basic practice in writing solutions to simple problems using the concepts in this module.

17. Check Your Understanding¶