ch12.htm

Java_by_Example,初级经典例子哦,珍藏版本

        guesses = 0;

        number = CreateNumber();

    }



    public void paint(Graphics g)

    {

        DrawInstructions(g);

        int guess = GetGuess();

        ShowMessage(g, guess);

    }



    public boolean action(Event event, Object arg)

    {

        ++guesses;

        repaint();

        return true;

    }



    ////////////////////////////////////////

    // Private methods.

    ////////////////////////////////////////

    void DrawInstructions(Graphics g)

    {

        g.drawString("Try to guess the number I am", 48, 65);

        g.drawString("thinking of. The number will be", 48, 80);

        g.drawString("between 0 and 100. You have an", 48, 95);

        g.drawString("unlimited number of tries.", 48, 110);

        g.drawString("Good Luck.", 95, 140);

    }



    int GetGuess()

    {

        String s = textField1.getText();

        int num = Integer.parseInt(s);

        return num;

    }



    int CreateNumber()

    {

        float n = (float)Math.random();

        number = (int)(n * 100 + 1);

        return number;

    }



    void ShowMessage(Graphics g, int guess)

    {

        String s = "Guesses so far: ";

        s += String.valueOf(guesses);

        g.drawString(s, 80, 170);



        if (guess < number)

            g.drawString("Your guess is too low.", 70, 185);

        else if (guess > number)

            g.drawString("Your guess is too high.", 70, 185);

        else

            g.drawString("You guessed the number!", 65, 185);

    }

}

</PRE>

</BLOCKQUOTE>

<HR>

<P>

<IMG ALIGN=RIGHT SRC="pseudo.gif" HEIGHT=94 WIDTH=94 BORDER=1>

<BLOCKQUOTE>

Tell Java that the program uses classes in the <TT>awt</TT> package.

<BR>

Tell Java that the program uses classes in the <TT>applet</TT>

package.<BR>

Tell Java that the program uses the lang package's <TT>Math</TT>

class.<BR>

Derive the <TT>Applet15</TT> class from Java's <TT>Applet</TT>

class.<BR>

    Declare the class's data fields.<BR>

    Override the <TT>Applet</TT> class's <TT>init()</TT> method.

<BR>

        Create the <TT>TextField</TT> object.<BR>

        Add the <TT>TextField</TT> object to the applet.<BR>

        Initialize the text in the <TT>TextField</TT> object to

&quot;50.&quot;<BR>

        Initialize the guess counter to zero.<BR>

        Create the number that the player must guess.<BR>

    Override the <TT>Applet</TT> class's <TT>paint()</TT> method.

<BR>

        Print the game's instructions.<BR>

        Get the player's guess.<BR>

        Show the appropriate message based on the guess.<BR>

    Override the <TT>Applet</TT> class's <TT>action()</TT> method.

<BR>

        Increment the guess counter.<BR>

        Tell Java to redraw the applet's display area.<BR>

        Tell Java that the <TT>action()</TT> method finished successfully.

<BR>

    Define the private <TT>DrawInstructions()</TT> method.<BR>

        Display the game instructions in the applet.<BR>

    Define the private <TT>GetGuess()</TT> method.<BR>

        Get the text from the <TT>TextField</TT> object.<BR>

        Convert the text to an integer.<BR>

        Return the integer to the calling function.<BR>

    Define the private <TT>CreateNumber() </TT>method.<BR>

        Calculate a random number from 0 to 1.<BR>

        Convert the random number to an integer between 1 and

100.<BR>

        Return the random number to the calling function.<BR>

    Define the private <TT>ShowMessage() </TT>method.<BR>

        Display the number of guesses so far.<BR>

        Display the results of the player's latest guess.

</BLOCKQUOTE>

<HR>

<BLOCKQUOTE>

<B>Listing 12.6 &nbsp;&nbsp;APPLET15.htmL: Applet15's HTML Document.

<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

&lt;title&gt;Applet Test Page&lt;/title&gt;

&lt;h1&gt;Applet Test Page&lt;/h1&gt;

&lt;applet

    code=&quot;Applet15.class&quot;

    width=250

    height=250

    name=&quot;Applet15&quot;&gt;

&lt;/applet&gt;

</PRE>

</BLOCKQUOTE>

<HR>

<P>

When you run Applet15, the program selects a random numeral from

1 to 100. Your task is to guess the numeral in the least number

of tries. You type your guesses into the box at the top of the

applet. When the program starts, this box contains the number

50, and the hint at the bottom of the screen tells you whether

the number is high or low. Each time you enter a new number, the

hint tells you whether the guess is high or low.

<P>

Yes, it's true that Applet15 is much larger than the example applets

you've used so far in this book, but the program needs to be long

in order to accommodate several different functions. By analyzing

the program's flow, you can determine whether or not you understand

how functions work.

<P>

Start with the <TT>paint()</TT> method. By examining the function

calls it contains, you can get a good idea of what the program

does. Specifically, the program displays the game's instructions,

gets a guess from the user, and then displays a message to the

user. If you were only interested in examining the <TT>paint()</TT>

method, you wouldn't have to go any further; the details of how

these other functions work are tucked out of your way.

<P>

If you wanted to see exactly how the program prints the game's

instructions, however, you could find the <TT>DrawInstructions()</TT>

method in the source code. The same is true for the other functions

called in the <TT>paint()</TT> method.

<P>

You can see that some of the functions return values and others

don't. Similarly, some functions require arguments and others

don't. How the function is constructed depends on whether it must

calculate and return a value to the program (such as in <TT>CreateNumber()</TT>)

and whether the function requires values from the program in order

to perform its task (such as <TT>ShowMessage()</TT>, which needs

the <TT>Graphics</TT> object and the player's latest guess).

<P>

There are probably several lines of Java source code in Listing

12.5 that don't make sense to you right now. The pseudocode section

following the listing describes, in general, what the program

is doing. You'll learn about many of the details in other chapters.

At this point, you only need to worry about being able to follow

the function calls.

<H2><A NAME="Summary"><FONT SIZE=5 COLOR=#Ff0000>

Summary</FONT></A></H2>

<P>

Functions enable you to break complicated programs down into easy-to-handle

chunks. Moreover, by using a top-down structure, your programs

can organize code with functions that start off being general

in nature but get more detailed as you work your way deeper into

the hierarchy. When a function doesn't return a value (has a return

type of <TT>void</TT>), it is used as a subroutine, which simply

performs some task before program execution jumps back to the

line after the line that called the function. A function with

a return type other than <TT>void</TT> enables your programs to

calculate a value and return the value to the calling function.

Either type of function can have one or more arguments, which

enable the program to pass values into the function.

<H2><A NAME="ReviewQuestions"><FONT SIZE=5 COLOR=#Ff0000>

Review Questions</FONT></A></H2>

<OL>

<LI>What is top-down programming?

<LI>How do functions make top-down programming possible?

<LI>Do all functions return values?

<LI>What are arguments and how are they used?

<LI>What is meant by &quot;defining a function&quot;?

<LI>How do you return a value from a function?

<LI>How do the arguments given in a function call relate to the

arguments accepted by the function?

<LI>How do you determine how to break a program up into functions?

</OL>

<H2><A NAME="ReviewExercises"><FONT SIZE=5 COLOR=#Ff0000>

Review Exercises</FONT></A></H2>

<OL>

<LI>Write a function that prints a two-line message to the user.

<LI>Write a function that returns the number 1 as an integer.

<LI>Write a function that accepts two integer arguments and returns

the sum of the arguments.

<LI>Modify the function from exercise 3 so that it sums the two

arguments, calls yet another function that multiplies the two

arguments, sums the product and the original sum, and returns

the result. Write the function that performs the multiplication.

<LI>Modify Applet15 by adding a function that starts the game

over each time the user guesses the number. That is, after the

user guesses the number, the program should select a new random

number and continue the game. Name the program <TT>GuessApplet.java</TT>.

(You can find the solution to this programming problem in the

CHAP12 folder of this book's CD-ROM.)

</OL>

<HR>











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