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Variable and method. Describe an algorithm that gets them down i. Algorithm and method. Pseudocode and Java code. Method definition and method invocation. Justify your answers. It should store the current temperature in an instance variable of type if they are left alone on the river bank. Write a class header as public Class MyClass.

Define the init header as public vid init. Print a string of five asterisks by System. A farmer lent the the instance variable. Use the Riddle class as a model. Forget the semicolon at the end of a println statement. The good news is that, according to the sales tax for a purchase. It should store the current tax rate as an instance mechanic, it is still possible to use the four pieces to weigh any quantity between variable.

How much did each of the four pieces using a TaxWhiz method. This class should have one public method, Test. Describe the compilation and execution process for this program, weigh? Hint: You can weigh a 4-pound object on a balance by putting a 5-pound calcTax double purchase , which returns a double, whose value is naming any other files that would be created.

For example, if the tax rate is 4 percent, 0. Suppose N is 6. Describe an algorithm that she can use to find the average of 10 homework ing statements are executed? Note that the z has wrapped around to the beginning of the alphabet.

What value would be reported by the you know to be a Caesar cipher. All you have to work with is 1. It has one attribute, a 3. It has one operation, 4.

All he can do is read the numbers to you. Describe an algorithm that calculateArea , which returns a double. Its attributes should be. Draw a UML diagram to represent this triangle. Then, we examine the UML diagram shown in Figure 1.

We focus on the basic Java language Figure 1. By the end of these sections, you should know how to identify the key elements that make up a Java program. We then present a detailed example of the programming development Objects: Using, Creating, process by designing a class that models a certain two person game and implements the class. The design is represented using UML notation. Objects belong to classes, which serve as templates or blueprints for creating objects.

Think again of the cookie cutter analogy. Just as a cookie cutter is used to shape and After studying this chapter, you will create individual cookies, a class definition is used to shape and create individual objects. They 2. Therefore, it 2. Figure 2. Scanner Figure 2. Recall from Chapter 0 that in order to get things done in a program we send messages to objects.

Sending a message to an object is a matter of calling one of its instance methods. For example, if we have a String, the default value for boolean is false. On the other hand, variables Otherwise it will return false.

For example, consider the following code named str and we want to find out how many characters it contains, we that are declared to be of a type equal to a class name are designed to store segment: can call its length method, using the expression str. If a reference to an object of that type.

Reference variables have a special In this case, the expression s1. It is possible to check whether or not a reference vari- It is important to note that the empty string is not the same as a String able contains a reference to an actual object by checking whether or not it variable that contains null.

Executing the statements: objectName. Thus, we first mention the name of the System. For So, to use one of the String methods in a program, we must first create example, the following statements print the lengths of our two strings: will not only not print out true; it will cause the the program to terminate a String object.

It is an error to use the method of a String variable, or any To create a String object in a program, we first declare a String System. This method takes a String refer to an object. On the other hand, the empty string is a perfectly good We then create a String object by using argument. It returns a String that combines the String argument to the String object which just happens to contain zero characters.

Consider this example: Figure 2. We can also use a constructor with an empty parameter list. Note that System. The System. Because the empty string has no System. This difference is related to an important Figure 2. Another useful String method is the equals method. If both itive type are stored. As soon as they are declared they are assigned a Strings have the same characters, in the same order, it will return true.

The default value for int is 0 and Figure 2. The programs declares a number of string variables, At this point we will not worry about the language features that en- drawn at coordin named s, s1, and so on, and it instantiates a String object for each vari- able the paint method to draw on the Java Swing window.

It then prints out a top-five list using the concatenation will focus instead on the information needed to make good use of the the JFrame. Can you figure out what it prints without g.

The first thing you should know is that, when running it? More gen- as the drawString method from the previous example.

The length method return an int value, and the concat ginning at a point which is x pixels from the left edge of the window and the rectangle as being x pixels from the left edge of the window and y method returned a String.

It is also very common for classes to define y pixels down from the top edge of the window. In a graphics window, pixels down from the top edge of the window. The width and height instance methods that perform actions but do not return a value.

The the point with coordinates 0,0 is at the top-left corner. The horizontal arguments specify the width and height of the rectangle in pixels. The vertical axis grows positively drawRect method also takes the same four arguments. The program is reproduced in Figure 2.

Notice that increasing the value of y will cause str to be displayed displaying strings with drawString. This is the opposite of the usual x and y coordinate system used in methods in the Graphics class.

The setColor With this information about g. Graphics ; method takes, as an argument, an object of type Color. All we need import j a v a.

Canvas ; where to display any message in the program window. For example, if to know about the Color class is that it is contained in the java. Table 2. The result of g. These meth- f. Using the Graphics object, g, each of these is called in the same way gle and the second inside a colored oval. We can choose some arbitrary HelloWorldGraphic program import j a v a.

JFrame ; for the string itself. In order to have the message visible we should fill a rectangle with the color cyan first, then draw the border of the rectangle public c l a s s HelloWorldGraphic extends Canvas in blue and, finally, display the message in black.

Notice in Figure 2. An oval is inscribed within an enclosing rectangle. And the width and height arguments give the enclosing g. We could specify one rectangle as having its upper left corner 25 g. A medium sized rectangle could have a width of pixels g. The statement g. Thus, the statement f. We can use similar other objects.

Thus, we now have sufficient information to finish the paint Figure 2. The completed program is displayed shows how to use color and drawing methods. Note how we repeatedly use the g. The Figure 2. To experiment with to process the information.

To design an object you need to answer five public methods and variables of an object make up its interface. Additional drawing capa- 1. What role will the object perform in the program? What data or information will it need? It contains the two private instance covered or skipped. What actions will it take?

What interface will it present to other objects? In a Java class definition, access to a class element, such as a vari- 5. What information will it hide from other objects? An access modifier is a declaration that controls Access modifier 2. Note also that the Riddle class Recall our definition of the Riddle class from Chapter 1, which is sum- itself is declared public.

This lets other classes have access to the class To program in Java the main thing you do is write class definitions for the The class as template marized in the UML diagram in Figure 2. A Riddle has two attributes, and to its public variables and methods. A class definition encapsu- question and answer. Each of these variables stores a string of charac- Recall that a class is like a blueprint or a cookie cutter. Once a class has been defined, it serves ters, which Java treats as data of type String.

The Riddle class contains defines the type of information attributes that each individual Riddle as a template, or blueprint, for creating individual objects or instances of the Figure 2.

It defines the methods op- class. In short, a class serves as a template, providing a detailed ods. Methods are used Variables and methods answer respectively.

Class as blueprint. In us either of its values. The following expression is an example of a method this case the computations are the simple Riddle methods that we have call: 2. To do this we need to define a main method, which can be Method call Calling or invoking a method is a means of executing its code.

The above defined either within the Riddle class itself or in a second class named 2. User interface One advantage of using a second class is that it gets us in the habit of which serves as a very simple user interface. It creates two Riddle ob- thinking about the need for a separate class to serve as a user interface, jects, named riddle1 and riddle2. Thus, Figure 2. Thus, System.

Note that we characterize the relationship between Riddle System. One way to think about this approach is as a division of labor Figure 2. In this chapter, we will focus on choosing appropriate instance variables and on designing methods as blocks of reusable code. Recall that a parameter is a variable that tem- 2. In this chapter, we will restrict our design to methods that do not have parameters and do not return values.

We will return to In this section, we will design and write the definition for a class that keeps the problem of designing changes to this class in the next chapter after an track of the details of a well known, two-person game.

We will focus on in-depth discussion of method parameters and return values. One is the number of sticks remaining in the without necessarily understanding why it works the way it does. We will row and the other is which player has the next turn. One suitable name for such a variable is nSticks.

The players alternate turns. A player must For this chapter, let us assume that the game starts with 7 sticks, rather remove one, two, or three sticks from the row on his or her turn. The than 21, to simplify discussion of the program. The game can be Data designating which player takes the next turn could be stored in played with any number of sticks but starting with twenty one sticks is different ways.

One way to do this is to think of the players as player one quite common. After 2. An object constructed with this class Method Decomposition should manage data that corresponds to having some specified number of sticks when the game begins. It should keep track of whose turn it is Now that we have decided what information the OneRowNim object and it should allow a player to diminish the number of sticks remaining by should manage, we need to decide what actions it should be able to per- one, two, or three.

Finally, a OneRowNim object should be able to decide form. We should think of methods that would be needed to communicate when the game is over and which player has won. Clearly, methods are needed for taking a What methods do we need? If a message to a OneRowNim object has no argument Problem Decomposition The Java class definition determines what information will be stored in to indicate the number of sticks taken, there will need to be three meth- each object and what methods each object can perform.

The method names ates an instance and associates a name with it in the program. The ob- user interfaces. Another tion. Each of these methods will be responsible for reducing the value of certain tasks. Reporting the number of sticks remain- stances. We can use report as a name for this action. VariableDeclaration2 variable. To designate a variable as a class variable it must be declared Figure 2.

OneRowNim class. These will be the MethodDefinition1 examples of valid declarations of instance variables: methods that other objects will use to interact with it. Given our design of the OneRowNim class as described in Figure 2. The rules associated with these access OneRowNim class. They both will store integer values, in which it is declared.

Like all classes that are designed to so they should be declared as variables of type int. Because the fied by any other class. Thus, the class header for the following: belong to the same package. Java determines accessibility in a top-down manner. Access to classes, in- To summarize, despite its apparent simplicity, a class level variable stance variables, and methods is defined according to the rules shown in Variables and methods The body of a class definition consists of two parts: the class-level vari- declaration actually accomplishes five tasks: Table 2.

A class-level variable is a variable 1. Specifies the type of data that can be stored in that location. Element Modifier Rule 3. Associates an identifier or name with that location. In general, a class definition will take the form shown in Figure 2. Class public Accessible if its package is accessible. Although Java does not impose any particular order on variable and 5.

Assigns an initial value to the location. Class-level variables are distinguished from local variables. A local or protected Accessible to its subclasses and to variable is a variable that is defined within a method.

Examples would instance method other classes in its package. Designing and defining methods is a form of abstraction. By defining a be the variables q and a that were defined in the Riddle String q, private Accessible only within the class. As we will see better in Chapter 3, by default Accessible only within the package.

Instead of Java handles each type of variable differently. As you recall from Chapter 1, a method def- declaring variables that were described in Section 1.

A class variable is associated with the class it- inition consists of two parts, the method header and the method body. The method header declares the name of the method and other general is necessary to perform some kind calculation before objects of the class player from 2 to 1 or from 1 to 2.

The first change is accomplished in a information about the method. The method body contains the executable are created. We will see examples of class methods when we discuss the straightforward way by the assignment: statements that the method performs. No modifier is needed to designate an instance we do not know whether its current value is 1 or 2. If its current value is 1, method. Class methods, which are used infrequently compared to The Method Header its new value should be 2; if its current value is 2, its new value should be instance methods, must be declared static.

Notice, however, that in both cases the current value plus the desired The method header follows a general format that consists of one or new value are equal to 3.

They all perform actions associated with a particular instance amples of method headers that we have already encountered. That is, they are all used to manage a particular One Row in which the method is defined and to other classes in the same package, Nim game.

Moreover, all four methods should be declared public, be- The takeTwo and takeThree methods are completely analogous and public methods are accessible to all other classes. Three of the methods are described subtracted from nSticks. If a as changing the values of the instance variables nSticks and player The body of the report method must merely print the cur- method is used to communicate with an object, or if it passes and the fourth, report , writes information to the console.

All four rent values of the instance variables to the console window with information to or from an object, it should be declared public. If a methods will receive no data when being called and will not return any System. To be understandable to someone using a method is intended to be used solely for internal operations within the values. Thus they should all have void as a return type and should all OneRowNim object, the values should be clearly labeled. Thus the body object, it should be declared private.

These methods are sometimes have empty parameter lists. The figure displays the class System. Methods declared at the class level are assumed to be in- stance methods unless they are also declared static. The static modifier This completes the method bodies of the OneRowNim class.

The com- The Method Body pleted class definition is shown in Figure 2. We will discuss alterna- is used to declare that a class method or variable is associated with the class itself, rather than with its instances. Just as for static variables, Designing a method is an application The body of a method definition is a block of Java statements enclosed tive methods for this class in the next chapter.

In Chapter 4, we will de- methods that are declared static are associated with the class and are of the encapsulation principle. As its name implies, an instance method can The description of the action required of the takeOne method is typ- indicating certain moves to make.

Most ical of many methods that change the state of an object. The body of of the class-level methods we declare will be instance methods. Class the takeOne method should use a series of assignment statements methods are used only rarely in Java and mainly in situations where it to reduce the value stored in nSticks by one and change the value in.

Declare a variable of type OneRowNim named game. Instantiate a OneRowNim object to which game refers. Command game to report.

What should the result type be for this method? Command game to remove three sticks. Write the headers for three methods. One method 8.

Command game to remove one stick. One public void takeTwo 9. The resulting main method is shown with the 2. We will encounter it repeatedly. A game. We make no assumptions about 2. They could be defined Next turn by p l a y e r 1 Number o f sticks left : 4 in the same class or in different classes. The method1 method executes Next turn by p l a y e r 2 sequentially until it calls method2.

This transfers control to the first Recall our define, create, and use mantra from Section 2. Now that we Number o f sticks left : 1 statement in method2. Execution continues sequentially through the have defined the OneRowNim class, we can test whether it works correctly Next turn by p l a y e r 1 statements in method2 until the return statement is executed. The return a main method. Following the design we used in the riddle example, statement causes a method to return control to the calling we will locate the main method in separate, user interface class, named This output indicates that player 1 removed the final stick and so player 2 statement—that is, to the statement that called the method in the first OneRowNimTester.

The body of main should declare a variable of type OneRowNim and create an object for it to refer to. For example, three moves taking 3, 3, and 1 sticks respectively private String instance variable named hint. Assign the variable method executes its last statement. Also, executing an initial value of "This riddle is too easy for a hint". Assume that this method requires no parameters a trace of its execution.

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