giáo trình Java By Example phần 7 docx

import java.awt.*; import java.applet.*; public class Applet16 extends Applet { TextField textField1, textField2, textField3; int avg1, avg2, avg3; public void init() { textField1 = new TextField(5); textField2 = new TextField(5); textField3 = new TextField(5); add(textField1); add(textField2); add(textField3); textField1.setText("0"); textField2.setText("0"); textField3.setText("0"); } public void paint(Graphics g) http://www.ngohaianh.info { g.drawString("Your bowlers' averages are: ", 50, 80); String s = textField1.getText(); g.drawString(s, 75, 110); avg1 = Integer.parseInt(s); s = textField2.getText(); g.drawString(s, 75, 125); avg2 = Integer.parseInt(s); s = textField3.getText(); g.drawString(s, 75, 140); avg3 = Integer.parseInt(s); } public boolean action(Event event, Object arg) { repaint(); return true; } } When you run Applet16, you can enter bowling scores into the three boxes at the top of the applet's display area. After you enter these averages, they're displayed on-screen as well as copied into the three variables avg1, avg2, and avg3. Nothing too tricky going on here, right? http://www.ngohaianh.info Now examine the listing. Remember in Chapter 10, "The while and do-while Loops," when you learned to keep an eye out for repetitive program code? How about all those calls to getText(), drawString(), and valueOf() in Listing 13.1? The only real difference between them is the specific bowler's score that's being manipulated. If you could find some way to make a loop out of this code, you could shorten the program significantly. How about a for loop that counts from 1 to 3? But how can you use a loop when you're stuck with three different variables? The answer is an array. An array is a variable that can hold more than one value. When you first studied variables, you learned that a variable is like a box in memory that holds a single value. Now, if you take a bunch of these boxes and put them together, what do you have? You have an array. For example, to store the bowling averages for your three bowlers, you'd need an array that can hold three values. You could call this array avg. You can even create an array for a set of objects like the TextField objects Applet16 uses to get bowling scores from the user. You could call this array textField. Now you have an array called avg that can hold three bowling averages and an array called textField that can hold three TextField objects. But how can you retrieve each individual average or object from the array? You do this by adding something called a subscript to the array's name. A subscript (also called an index) is a number that identifies the element of an array in which a value is stored. For example, to refer to the first average in your avg array, you'd write avg[0]. The subscript is the number in square brackets. In this case, you're referring to the first average in the array (array subscripts always start from zero.) To refer to the second average, you'd write avg[1]. The third average is avg[2]. If you're a little confused, look at Figure 13.2, which shows how the avg[] array might look in memory. In this case, the three bowling averages are 145, 192, and 160. The value of avg[0] is 145, the value of avg[1] is 192, and the value of avg[2] is 160. Figure 13.2 : An array can hold many values of the same type. Example: Creating an Array Suppose that you need an array that can hold 30 floating-point numbers. First, you'd declare the array like this: float numbers[]; Another way to declare the array is to move the square brackets to after the data type, like this: float[] numbers; After declaring the array, you need to create it in memory. Java lets you create arrays only using the new operator, like this: http://www.ngohaianh.info numbers = new float[30]; The last step is to initialize the array, a task that you might perform using a for loop: for (int x=0; x<30; ++x) numbers[x] = (float)x; These lines of Java source code initialize the numbers[] array to the numbers 0.0 to 29.0. Notice how the loop only goes up to 29. This is because, although there are 30 elements in the numbers[] array, those elements are indexed starting with 0, rather than 1. That is, the subscript is always one less than the number of the element you're accessing. The first element has a subscript of 0, the second a subscript of 1, the third a subscript of 2, and so on. Example: Using a Variable as a Subscript As you learned in a previous chapter, most numerical literals in a Java program can be replaced by numerical variables. Suppose you were to use the variable x as the subscript for the array avg[]. Then (based on the averages in Figure 13.2) if the value of x is 1, the value of avg[x] is 192. If the value of x is 3, the value of avg[x] is 160. Now take one last, gigantic, intuitive leap (c'mon, you can do it) and think about using your subscript variable x as both the control variable in a for loop and the subscript for the avg[] and textField arrays. If you use a for loop that counts from 0 to 2, you can handle all three averages with much less code than in the original program. Listing 13.2 shows how this is done. Listing 13.2 Applet17.java: Using Arrays. import java.awt.*; import java.applet.*; public class Applet17 extends Applet { TextField textField[]; int avg[]; http://www.ngohaianh.info public void init() { textField = new TextField[3]; avg = new int[3]; for (int x=0; x<3; ++x) { textField[x] = new TextField(5); add(textField[x]); textField[x].setText("0"); } } public void paint(Graphics g) { g.drawString("Your bowlers' averages are: ", 50, 80); for (int x=0; x<3; ++x) { String s = textField[x].getText(); g.drawString(s, 75, 110 + x*15); avg[x] = Integer.parseInt(s); http://www.ngohaianh.info } } public boolean action(Event event, Object arg) { repaint(); return true; } } Tell Java that the program uses classes in the awt package. Tell Java that the program uses classes in the applet package. Derive the Applet17 class from Java's Applet class. Declare TextField and int arrays. Override the Applet class's init() method. Create the textField and int arrays with three elements each. Loop from 0 to 2. Create a new TextField object and store it in the array. Add the new TextField object to the applet. Set the new TextField object's text. Override the Applet class's paint() method. Display a line of text. Loop from 0 to 2. Get the text from the currently indexed TextField object. Draw the retrieve text on the applet's display area. Convert the value and store it in the integer array. Override the Applet object's action() method. Force Java to redraw the applet's display area. Tell Java everything went okay. At the beginning of Listing 13.2, you'll see a couple of strange new variable declarations that look like this: TextField textField[]; http://www.ngohaianh.info int avg[]; These declarations are much like other declarations you've seen, except both of the variable names end with a set of square brackets. The square brackets tell Java that you're declaring arrays rather than conventional variables. Once you have the arrays declared, you must create them. In Applet17, this is done like this: textField = new TextField[3]; avg = new int[3]; Here you use the new operator to create the arrays. To tell Java the type of arrays to create, you follow new with the data type and the size of the array in square brackets. In other words, the first line above creates an array that can hold three TextField objects. The second line creates an array that can hold three integers. Once you have your arrays created, you can use a loop to reduce the amount of code needed to initialize the arrays. For example, the long way to initialize the arrays (without using a loop) would look something like Listing 13.3: Listing 13.3 LST13_3.TXT: Initializing an Array without Looping. textField[0] = new TextField(5); add(textField[0]); textField[0].setText("0"); textField[1] = new TextField(5); add(textField[1]); textField[1].setText("0"); textField[2] = new TextField(5); add(textField[2]); textField[2].setText("0"); http://www.ngohaianh.info As you learned, however, you can use a variable-specifically, a loop control variable-as the array subscript. That's what Applet17 does, which enables it to initialize the textField array as shown in Listing 13.4. Listing 13.4 LST13_4.TXT: Initializing an Array Using a Loop. for (int x=0; x<3; ++x) { textField[x] = new TextField(5); add(textField[x]); textField[x].setText("0"); } The first time through the loop, x is equal to 0, so that element 0 (the first element) of the textField array is being manipulated. The next time through the loop, x is 1, so that element 1 of the array is being manipulated in the body of the loop. Finally, when x is 2, the program takes care of the third array element. As you can see, using a loop with an array can greatly simplify handling a group of related values. Imagine how many lines of source code you'd save if the array had 1,000 elements instead of only three. To accommodate the larger array, you'd only have to change x<3 to x<1000 in the first line of the for loop. CAUTION Be careful not to try accessing a nonexistent array element. For example, in Listing 13.4, if you tried to access textField[3], you'd be beyond the boundaries of the array. Java will generate an exception when this happens, which means your applet may or may not perform the way you want it to. (You'll learn more about exceptions in Chapter 30, "Exceptions.") The init() method isn't the only place Applet17 takes advantage of a loop to handle the program's arrays. In the paint() method, you can see the loop shown in Listing 13.5. Listing 13.5 LST13_5.TXT: The for Loop from the paint( ) Method. for (int x=0; x<3; ++x) http://www.ngohaianh.info { String s = textField[x].getText(); g.drawString(s, 75, 110 + x*15); avg[x] = Integer.parseInt(s); } This loop simplifies the printing of the bowlers' scores and the loading of the avg[] array with the scores. Again, imagine how much time and space you'd save if the arrays in question had thousands of elements rather than only three. It's at times like those that you really learn to appreciate arrays. NOTE The memory locations that make up an array are called elements of the array. For example, in an array named numbers[], numbers[0] is the first element of the array, numbers[1] is the second element, and so on. The reason numbers[0] is the first element of the array is because of the number 0 inside the subscript.It is the number inside the subscript that defines which array location is being referred to. Multidimensional Arrays So far, you've looked at simple arrays that hold their data in a list. However, most programming languages also support multidimensional arrays, which are more like tables than lists. For example, take a look at Figure 13.3. The first array in the figure is a one-dimensional array, which is like the arrays you've used so far in this chapter. The next type of array in the figure is two-dimensional, which works like the typical spreadsheet type of table you're used to seeing. Figure 13.3 : Arrays can have more than one dimension. Although Java doesn't support multidimensional arrays in the conventional sense, it does enable you to create arrays of arrays, which amount to the same thing. For example, to create a two-dimensional array of integers like the second array in Figure 13.3, you might use a line of code like this: int table[][] = new int[4][4]; This line of Java code creates a table that can store 16 values-four across and four down. The first subscript selects the column and the second selects the row. To initialize such an array with values, you might use the lines shown in Listing 13.6, which would give you the array shown in Figure 13.4. http://www.ngohaianh.info Figure 13.4 : Here's the two-dimensional array as initialized in Listing 13.6. Listing 13.6 LST13_6.TXT: Initializing a Two-Dimensional Array. table[0][0] = 0; table[1][0] = 1; table[2][0] = 2; table[3][0] = 3; table[0][1] = 4; table[1][1] = 5; table[2][1] = 6; table[3][1] = 7; table[0][2] = 8; table[1][2] = 9; table[2][2] = 10; table[3][2] = 11; table[0][3] = 12; table[1][3] = 13; table[2][3] = 14; table[3][3] = 15; You refer to a value stored in a two-dimensional array by using subscripts for both the column and row in which the value you want is stored. For example, to retrieve the value 11 from the table[][] array shown in Figure 13.4, you use a line like this: int value = table[3][2]; http://www.ngohaianh.info [...]... Loop r r q Example: Using a while Loop Example: Using a while Loop in a Program The do-while Loop r Example: Using a do-while Loop r Example: Using a do-while Loop in a Program q Summary q Review Questions q Review Exercises A computer handles repetitive operations especially well-it never gets bored, and it can perform a task as well the 10,000th time as it did the first Consider, for example, a disk... CLASSES folder, naming the file Applet8 .java (You can copy the source code from the CD-ROM, if you like, and thus save on typing.) 2 Compile the source code by typing javac Applet8 .java at the MS-DOS prompt, which gives you the Applet8.class file 3 Type the HTML document shown in Listing 10.3, and save it to your CLASSES folder under the name APPLET8.htmL 4 Run the applet by typing, at the MS-DOS prompt,... accessed outside of that block Changing the Increment Value The previous example of a for loop increments the loop counter by 1 But suppose you want a for loop that counts from 5 to 50 by fives? This could be useful if you need to use the loop counter to display a value, which needs to be incremented by a different number You can do this by changing the sections of the for loop, like this for (x=5; x . Listing 13.2 shows how this is done. Listing 13.2 Applet 17 .java: Using Arrays. import java. awt.*; import java. applet.*; public class Applet 17 extends Applet { TextField textField[]; int avg[]; http://www.ngohaianh.info . Appletviewer. 7. http://www.ngohaianh.info Chapter 10 The while and do-while Loops CONTENTS The while Loop Example: Using a while Loop❍ Example: Using a while Loop in a Program❍ ● The do-while Loop Example: . naming the file Applet8 .java. (You can copy the source code from the CD-ROM, if you like, and thus save on typing.) 1. Compile the source code by typing javac Applet8 .java at the MS-DOS prompt,