Contents
1. Programming overview
2. Variables and objects
3. More on objects
4. Using numbers
5. Repetition
6. Graduating to a bigger sandbox
7. Conditional execution
8. Strings
9. More on loops
10. More on classes
11. Subclasses
12. Defining methods
13. User interaction
14. Defining classes
15. Making subclasses
16. Abstract classes & interfaces
17. Recursion
18. Recursion examples
19. Arrays
20. Sorting
21. Other control constructs
22. File I/O
23. Beyond the sandbox
24. Swing basics
25. More with Swing

Chapter 4. Using numbers

We now turn to working with numbers in computer programs: In this chapter, we see how a program might ask the user for a number and use that number in doing its task. We'll close with a related, important discussion of run-time exceptions.

4.1. Numeric types

We've glanced at the double type in Java, which we can use to represent a number. Using this type, we can, if we want, make a variable to refer to a number.

double scale;


This line creates a variable named scale, of type double. We can assign a number to the variable using an assignment statement.

scale = 2.4;


Notice that we don't have to use new here; we use new only in conjunction with constructors, and the numeric types don't have constructors.

The numeric types don't have constructors because these types are technically not objects. Instead, double is one of a few primitive types — that is, types that are built into the core of the Java language. There are a total of eight primitive types in Java, of which we will study three in this book: boolean, double, and int. (The other five are char, byte, short, long, and float.) You can see that the primitive types are all spelled entirely with lower-case letters. By contrast, classes are conventionally named starting with capital letters, as in Turtle or Color.

In this chapter, we'll examine both the double and the int types. The int type is for representing integers. (An integer is a number with no fractional part, like 0 or −3, but not 1.4.)

int i;
i = 5;


You cannot make an int variable reference a double value, even if the value has no fractional part. For example, the below fragment is illegal because it attempts to assign a double value to a variable whose type is int. Java regards any number containing a decimal point as a double value, including 5.0.

i = 5.0;       // Illegal!!


Java will happily allow a double variable to be assigned an int value, however, on the grounds that int is more restricted than double; any int value can be converted to a legitimate double value.

scale = 2;     // Ok


You may be wondering: Why would you ever use int instead of double? Why limit yourself? There are three reasons for this. First, computers use scientific notation with limited precision to remember double values. While this allows the computer to remember a wider range of numbers, it can only approximate most of the numbers in that range. An int value always represents the integer exactly. Second, computers compute using doubles somewhat more slowly. (Think about how much more difficult it is to add two numbers in scientific notation than to add two simple integers.) And, finally, making the distinction rarely causes much trouble: After a bit of practice, the distinction is second nature. In practice, most programs use very few double values.

4.2. Return values

We've already seen that we can send a value to a method via a parameter. But, often, a method needs to provide some sort of response; for this, the method can use a return value.

The methods we have seen so far have not had return values. But the TurtleProgram class includes the following two instance methods with return values.

double readDouble()

Shows the user a dialog box prompting for a number and returns the entered number.

int readInt()

Shows the user a dialog box prompting for an integer and returns the entered integer.

The type written before the method name in the method descriptor (double for readDouble and int for readInt) specifies what type of value the method will return. Although a method can accept several values as parameters, it can have only one return value. (It happens that neither of these methods takes any parameters.)

In the methods we examined last chapter (such as forward), you'll recall the word void preceded the name of the method.

void forward(int dist)


The word void as a return type indicates that the method does not have a return value.

Before we use any of the TurtleProgram methods, we first must address the question: How can we find a TurtleProgram object? As it happens, we won't use a constructor, as we have with the other classes Turtle and Color. In fact, when the computer is told to run our programs, it creates a TurtleProgram object and then execute its run method: The extends TurtleProgram and void run() bits at the top of the programs we've been writing are really saying that we are writing a run method for a class that is a slightly modified version of TurtleProgram. You needn't understand that fully, because we'll study it much more thoroughly later; the upshot, though, is that the computer has already created a TurtleProgram object by the time it executes our program, which is the object that we want to use. We can access this object by typing, simply, this. Thus, to tell our TurtleProgram object to execute its readDouble method, we will write this.readDouble().

But we won't write that exactly, because then the return value will be lost. To be able to reference the return value later in the program, we will use an assignment statement to assign a variable to remember whatever value the method returns.

scale = this.readDouble();


Here, we've assigned the name scale to refer to the value returned when we tell this to read a double. Since readDouble returns what the user types into a dialog box, this statement assigns scale to be a name for whatever number the user entered.

The DrawSquare program of Figure 4.1 illustrates this at work.

Figure 4.1: The DrawSquare program.

  1  import turtles.*;
  2  
  3  public class DrawSquare extends TurtleProgram {
  4      public void run() {
  5          double sideLength;
  6          sideLength = this.readDouble();
  7  
  8          Turtle boxTurtle;
  9          boxTurtle = new Turtle(10, 10);
 10          boxTurtle.forward(sideLength);
 11          boxTurtle.right(90);
 12          boxTurtle.forward(sideLength);
 13          boxTurtle.right(90);
 14          boxTurtle.forward(sideLength);
 15          boxTurtle.right(90);
 16          boxTurtle.forward(sideLength);
 17          boxTurtle.hide();
 18      }
 19  }


You can see that line 6 tells this to read a number, with sideLength assigned to remember the number entered by the user. We can then use the user's number as a parameter to forward in line 10 to indicate that the turtle should move forward as many pixels as the user indicated.

4.3. Arithmetic

Since numbers are not objects in Java, and their types are not classes, they cannot perform instance methods. Instead, to work with primitive values, we use operators, special symbols that can be used to compute a value. Java includes many operators, including several arithmetic operators for numeric values.

+	addition
-	subtraction
*	multiplication
/	division
%	modulus (remainder)

(The modulus operator (%) yields the remainder. For example, 11 % 3 would yield the integer 2, since when you divide 11 by 3, the remainder is 2.)

We can combine these operators as we like. Java will follow the normal algebraic order of operations: Multiplication and division (and modulus) have priority over addition and subtraction. And, within the same level of precedence, the computation proceeds left to right. Suppose I write something like the following.

start = 100 - length / 2.0;


The first thing that happens is that length is divided by 2; then this result is subtracted from 100; and finally start is assigned to refer to this result. (The value of length itself remains unchanged by this statement.) If we really wanted to do the subtraction first, then we can use parentheses.

start = (100 - length) / 2.0;


When you operate on two integers, Java will produce an integer result. But if either side is a double, Java will produce a double result. This works quite smoothly in practice, except for one thing: When you divide two integers, of course the true result is not necessarily an integer. In order to get an integer result, Java will simply ignore any remainder. Thus, 14 / 3 has the value 4. (Its true value is 4.666…, but the remainder of 2 is ignored since both 14 and 3 are int values.)

Figure 4.2 contains a program that uses operators. It asks the user for a number (Figure 4.3(a)) and then draws a square exactly that big in the center of the window (Figure 4.3(b) and (c)).

Figure 4.2: The DrawCenterSquare program.

  1  import turtles.*;
  2  
  3  public class DrawCenterSquare extends TurtleProgram {
  4      public void run() {
  5          double sideLength;
  6          sideLength = this.readDouble();
  7  
  8          double start;
  9          start = 100.0 - sideLength / 2.0;
 10  
 11          Turtle boxTurtle;
 12          boxTurtle = new Turtle(start, start);
 13          boxTurtle.forward(sideLength);
 14          boxTurtle.right(90);
 15          boxTurtle.forward(sideLength);
 16          boxTurtle.right(90);
 17          boxTurtle.forward(sideLength);
 18          boxTurtle.right(90);
 19          boxTurtle.forward(sideLength);
 20          boxTurtle.hide();
 21      }
 22  }


Figure 4.3: Running DrawCenterSquare.

(a)	(b)	(c)

When run, the computer begins at line 5 and works through the lines one by one.

1. Lines 5–6: Creates a double variable called sideLength for referring to the length of each side of the square to be drawn. Then it asks the user for a number and assigns sideLength to refer to this number. In Figure 4.3(a), the user types 60, so in the example of Figure 4.3, sideLength will represent the number 60.

2. Lines 8–9: Creates another variable called start that will represent the x-coordinate of the top left corner of the square. (Since the x- and y-coordinates of this point are equal, this variable will do double-duty as the y-coordinate also.) Its value is computed in line 9 using an arithmetic expression. Since the window is 200 pixels wide, 100 is halfway between the left and right sides; we want half of the side length to extend to the left of this, so we want to subtract sideLength / 2 from 100. Thus, this is the expression you see in line 9, whose value is assigned to start. In our example, where the user had typed 60 for sideLength, start would be assigned the value 70.

3. Lines 11–12: Creates another variable boxTurtle for referring to a Turtle object. The turtle's initial x- and y-coordinates being start. Thus, this turtle would be placed at (70, 70) facing east.

4. Lines 13–20: Tells the turtle to trace a square with each side being sideLength pixels long, and then to hide, leaving the square behind.

In line 12, we passed a variable's value as a parameter, instead of simply a number. More generally, we could write any expression for a parameter, and the computer will compute its value before calling the method. When we create the turtle, we could write the following instead.

boxTurtle = new Turtle(100 - sideLength / 2, 100 - sideLength / 2);


This would remove the need for the start variable, so lines 8 and 9 could be removed.

4.4. Exceptions

Hopefully, you've been writing your own programs as you've been going along. Those programs have surely sometimes had problems. There are three categories of problems that a program might have. (We alluded to these before in Section 1.2, but we're now prepared to review them specifically with reference to Java.)

• Compile errors: The compiler identifies a problem within your program, and it refuses to compile the program, so that the computer is unable to attempt executing the program. For beginning programmers, one of the most common compile errors is omitting a semicolon at the end of a statement.

• Logic errors: The computer successfully compiles and executes the program, but it doesn't quite do what you expected. An example would be omitting the hide invocation on a turtle, so that the turtle remains on the screen even though you wanted the turtle to go away once it completed its task.

• Exceptions: The computer identifies a fatal problem while running the program, and it terminates the program with a message telling about the problem. One way this could occur is if the program attempts somehow to perform integer division where the divisor is zero.

You may well not have encountered any exceptions yet, but as we go on you will encounter them more frequently, and now is a good time to learn how to recognize them. Consider the DrawFraction program of Figure 4.4. It reads a number n from the user, and it draws a line that extends an nth of the way across the window.

Figure 4.4: The DrawFraction program.

  1  import turtles.*;
  2  
  3  public class DrawFraction extends TurtleProgram {
  4      public void run() {
  5          int divisor;
  6          divisor = this.readInt();
  7  
  8          Turtle lineTurtle;
  9          lineTurtle = new Turtle(0, 100);
 10          lineTurtle.forward(200 / divisor);
 11          lineTurtle.hide();
 12      }
 13  }


Now suppose that the user enters 0 as the number. Then when the computer reaches line 10, the computer will have to divide the two integers 200 and 0. At this point, the computer will give up and display a message like the following. (You may have to look to find the message; it won't appear in the turtles' window, and it probably appear in a dialog box. You will probably have to look somewhere else to get the information.)

Exception in thread "main" java.lang.ArithmeticException: / by zero
        at DrawFraction.run(DrawFraction.java:10)
        at acm.program.Program.runHook(Program.java)
        at acm.program.Program.startRun(Program.java)
        at acm.program.Program.start(Program.java)
        at acm.program.Program.start(Program.java)
        at acm.program.Program.main(Program.java)

This message is rather cryptic, but in fact it provides quite a bit of information that's helpful for identifying what went wrong with the program. The first thing it tells you is the sort of thing that went wrong. To find this out, you examne the first line and look for a long mixed-case word, probably ending in Exception. Here, it says ArithmeticException. This word indicates the category of problem that occurred. Sometimes, there will be a colon after this long word with additional helpful information; here, the information, / by zero, describes exactly the problem.

The next lines are also very useful, as they describe where the computer was in the program it encountered the problem. Notice the parentheses where it says DrawFraction.java:10. This identifies exactly which line of the program contained the error.

Sometimes the first line beginning with at will indicate a line that is not in your own code, but instead in some method that you've invoked. You should keep looking down through the at lines until you find a line that identifies a line in your code. In this case, we would look for the first line mentioning DrawFraction.java, since our program was named DrawFraction in line 3.

Exercise 4.1

Modify the DrawTriangle program of Figure 3.1 so that it reads two numbers from the user, x and y, and places the triangle with its lower left corner at (x, y).