习 题 4.23 编写一个 applet ，读取一个矩形的边长，然后 用在 paint 方法中使用 drawString 方法画出用星组成 的空心矩形。程序应能画出边长从 1 到 20 的任何矩 形。

// Hollow.java // Program prints a hollow square import javax.swing.*; import java.awt.Graphics; public class Hollow extends JApplet { int stars; public void init() { String input; // for user input // read number from user as a string input = JOptionPane.showInputDialog( "Enter length of side:" ); // convert numbers from type String to type int stars = Integer.parseInt( input );

if ( stars < 1 ) { stars = 1; JOptionPane.showMessageDialog( null, "Using default value 1", "Error", JOptionPane.ERROR_MESSAGE ); } else if ( stars > 20 ) { stars = 20; JOptionPane.showMessageDialog( null, "Using default value 20", "Error", JOptionPane.ERROR_MESSAGE ); }

public void paint( Graphics g ) { int x = 5, y = 70, i = 1, j = 1; while ( i <= stars ) { while ( j <= stars ) { if ( i == 1 ) g.drawString( "*", x += 5, y ); else if ( i == stars ) g.drawString( "*", x += 5, y ); else if ( j == 1 ) g.drawString( "*", x += 5, y ); else if ( j == stars ) g.drawString( "*", x += 5, y ); else g.drawString( " ", x += 5, y ); j++; } j = 1; i++; y += 5; x = 5; } //end of while }//end of paint }//end of Hollow

4.31 一家公司想通过电话传输数据（数据以 4 位的 整数形式传输） …… 程序必须能从文本区中读取输 入的 4 位数字，并用以下方式加密：每位数字加 7 后 模 10 ，然后将第 1 位数与第 3 位数交换，第 2 位数与 第 4 位数交换。最后打印出加密后的数据。编写一个 独立的应用程序，接受一个加密的 4 位数，并把它解 密成原来的数字。

// Exercise 4.31 Part A Solution Encrypt.java // Program encrypts a four digit number import java.awt.*; import javax.swing.JOptionPane; public class Encrypt { public static void main( String args[] ) { int digit1, digit2, digit3, digit4; String inputString; // Enter four digit number to be encrypted inputString = JOptionPane.showInputDialog( "Enter a four digit number: " ); int number = Integer.parseInt( inputString );

digit1 = ( number / ) % 10; digit2 = ( number % 1000 / ) % 10; digit3 = ( number % 1000 % 100 / ) % 10; digit4 = ( number % 1000 % 100 % ) % 10; int temp = digit1; digit1 = digit3 * 1000; digit3 = temp * 10; temp = digit2; digit2 = digit4 * 100; digit4 = temp; int encryptedNumber; encryptedNumber = digit1 + digit2 + digit3+ digit4; JOptionPane.showMessageDialog( null, "Encrypted number is " + encryptedNumber, "Encrytor", JOptionPane.INFORMATION_MESSAGE ); System.exit( 0 ); }

// Exercise 4.31 Part B Solution Decrypt.java // Program decrypts a four digit number import java.awt.*; import javax.swing.JOptionPane; public class Decrypt { public static void main( String args[] ) { String inputString; // Enter four digit number to be decrypted inputString = JOptionPane.showInputDialog( "Enter a four digit number: " ); int number = Integer.parseInt( inputString ); int digit1, digit2, digit3, digit4;

digit1 = number / 1000; digit2 = number % 1000 / 100; digit3 = number % 1000 % 100 / 10; digit4 = number % 1000 % 100 % 10; int temp = ( digit1 + 3 ) % 10; digit1 = ( digit3 + 3 ) % 10; digit3 = temp; temp = ( digit2 + 3 ) % 10; digit2 = ( digit4 + 3 ) % 10; digit4 = temp; int decryptedNumber = digit1 * digit2 * digit3 * 10 + digit4; JOptionPane.showMessageDialog( null, "Decrypted number is " + decryptedNumber, "Encrytor", JOptionPane.INFORMATION_MESSAGE ); System.exit( 0 ); } }

LAB1 // Exercise 4.12: Credit.java // Program monitors accounts. import java.awt.*; import javax.swing.JOptionPane; public class Credit { // main method begins execution of Java application public static void main( String args[] ) { String inputString, // user input resultsString, // result String creditReport; // credit status int account, // account number charges, // total charges credits, // total credits creditLimit, // allowed credit limit balance; // coutomer's balance

inputString = JOptionPane.showInputDialog( "Enter account number: " ); /* write code to convert the input string to an integer and store it in account */ /* write code to input the rest of the customer information and convert the inputs to integers. Use inputString to input each value. */ /* write code to compute the new balance */ /* write the code that will check if the new balance is greater than the credit limit, and set the proper value for creditStatusString */ resultsString = creditReport + "\nCurrent balance is " + balance + "."; JOptionPane.showMessageDialog( null, resultsString, creditReport, OptionPane.INFORMATION_MESSAGE ); System.exit( 0 ); } // end method main

// Exercise 4.24: Palindrome.java Program tests for a palindrome import java.awt.*; import javax.swing.JOptionPane; public class Palindrome { // main method begins execution of Java application public static void main( String args[] ) { String resultString; // result String int number, // user input number originalNumber, // stores original value in number for output digit1, // first digit digit2, // second digit digit4, // fourth digit digit5, // fifth digit digits; // number of digits in input number = 0; digits = 0; LAB2

/* Write code here that inputs a five-digit number. Display an error message if the number is not five digits. Loop until a valid input is received. */

/* Write code that separates the digits in the five digit number. Use division to isolate the left-most digit in the number, use modulus to remove that digit from the number and repeat. Store the original value of number in variable originalNumber before performing calculations. */

/* Write code that determines whether the first and last digits are identical and the second and fourth digits are identical. Assign resultString a string indicating whether or not the original string is a palindrome. */

/* Display whether or not the given number is a palindrome. */ System.exit( 0 ); } // end method main } // end class Palindrome