JAVA语言程序设计基础篇（Chapter 6）课后习题参考答案

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1. (简答题)(Sum the digits in an integer) Write a method that computes the sum of the digits in an integer. Use the following method header: public static int sumDigits(long n) For example, sumDigits(234) returns 9 (2 + 3 + 4). (Hint: Use the % operator to extract digits, and the / operator to remove the extracted digit. For instance, to extract 4 from 234, use 234 % 10 (= 4). To remove 4 from 234, use 234 / 10 (= 23). Use a loop to repeatedly extract and remove the digit until all the digits are extracted. Write a test program that prompts the user to enter an integer and displays the sum of all its digits.

import java.util.Scanner;
public class Demo1 {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
System.out.println("Enter a number:");
int n = sc.nextInt();
int sum=sumDigits(n);
System.out.println("The sum is :"+sum);
}
public static int sumDigits(long n){
int sum = 0;
while ((int)(n%10)!=0){
sum+=n%10;
n=n/10;
}
return sum;
}
}

2. (简答题)(Display an integer reversed) Write a method with the following header to display an integer in reverse order: public static void reverse(int number) For example, reverse(3456) displays 6543. Write a test program that prompts the user to enter an integer and displays its reversal.

import java.util.Scanner;
public class Demo2 {
public static long reversal = 0;
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
System.out.println("Enter a number:");
int number = sc.nextInt();
reverse(number);
System.out.println("The reversal is "+reversal);
}
public static void reverse(int number){
while (true){
reversal =reversal * 10 + number%10;
number/=10;
if (number==0){
break;
}
}
}
}

3. (简答题)(Palindrome integer) Write the methods with the following headers

// Return the reversal of an integer, i.e., reverse(456) returns 654

public static int reverse(int number)

// Return true if number is a palindrome

public static boolean isPalindrome(int number)

Use the reverse method to implement isPalindrome. A number is a palindrome if its reversal is the same as itself. Write a test program that prompts the user to enter an integer and reports whether the integer is a palindrome.

import java.util.Scanner;
public class Demo3 {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
System.out.println("Enter a number: ");
int number =sc.nextInt();
boolean flag = isPalindrome(number);
System.out.println("The number is a palindrome :"+flag);
}
public static int reverse(int number){
int result = 0;
while (true){
result=result*10+number%10;
number/=10;
if (number==0){
break;
}
}
return result;
}
public static boolean isPalindrome(int number){
if(number==reverse(number)){
return true;
}else {
return false;
}
}
}

4. (简答题)

(Palindromic prime) A palindromic prime is a prime number and also palindromic. For example, 131 is a prime and also a palindromic prime, as are 313 and 757. Write a program that displays the first 100 palindromic prime numbers. Display 10 numbers per line, separated by exactly one space, as follows:

2 3 5 7 11 101 131 151 181 191

313 353 373 383 727 757 787 797 919 929 ...

public class Demo4 {
public static void main(String[] args) {
boolean flag;
int line = 0;
int i = 2;
while (line<=100){
flag = true;
for (int j = 2; j <=Math.sqrt(i); j++) {
if (i % j == 0) {
flag = false;
break;
}
}
if (flag) {
if(isPalindrome(i)){
System.out.print(i + "t");
line++;
}else {
flag =false;
}
}
if (line != 0 && flag && line % 10 == 0 ) {
System.out.println();
}
i++;
}
}
public static int reverse(int number){
int result = 0;
while (true){
result=result*10+number%10;
number/=10;
if (number==0){
break;
}
}
return result;
}
public static boolean isPalindrome(int number){
if(number==reverse(number)){
return true;
}else {
return false;
}
}
}

5. (简答题)(Emirp) An emirp (prime spelled backward) is a nonpalindromic prime number whose reversal is also a prime. For example, 17 is a prime and 71 is a prime, so 17 and 71 are emirps. Write a program that displays the first 100 emirps. Display 10 numbers per line, separated by exactly one space, as follows:

13 17 31 37 71 73 79 97 107 113

149 157 167 179 199 311 337 347 359 389 ...

public class Demo5 {
public static void main(String[] args) {
isEmirp();
}
public static void isEmirp() {
int i = 2;
boolean flag;
int line = 0;
while (line <= 100) {
flag = true;
for (int j = 2; j <= Math.sqrt(i); j++) {
if (i % j == 0) {
flag = false;
break;
}
}
if (flag) {
if (isPrime(reverse(i))&&isNonPalindrome(i)){
System.out.print(i + "t");
line++;
}else {
flag=false;
}
}
if (line != 0 && flag && line % 10 == 0) {
System.out.println();
}
i++;
}
}
public static boolean isPrime(int number) {
int i = number;
boolean flag = true;
for (int j = 2; j <= Math.sqrt(i); j++) {
if (i % j == 0) {
flag = false;
break;
}
}
return flag;
}
public static int reverse(int number) {
int result = 0;
while (true) {
result = result * 10 + number % 10;
number /= 10;
if (number == 0) {
break;
}
}
return result;
}
public static boolean isNonPalindrome(int number){
if(number==reverse(number)){
return false;
}else {
return true;
}
}
}

6. (简答题)(Check password ) Some websites impose certain rules for passwords. Write a method that checks whether a string is a valid password. Suppose the password rules are as follows:

■ A password must have at least eight characters.

■ A password consists of only letters and digits.

■ A password must contain at least two digits.

Write a program that prompts the user to enter a password and displays Valid Password if the rules are followed or Invalid Password otherwise.

public class Demo6 {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
System.out.println("Enter a password :");
String password = sc.next();
switch (checkPassword(password)) {
case 0:
System.out.println("A password must have at least eight characters!");
break;
case 1:
System.out.println("A password consists of only letters and digits!");
break;
case 2:
System.out.println("A password must contain at least two digits!");
break;
default:
System.out.println("Valid Password!");
}
}
public static int checkPassword(String pass) {
if(pass.length()>=8){
int numDigit = 0;
int numLetter = 0;
for (int i = 0; i < pass.length(); i++) {
if(Character.isDigit(pass.charAt(i))){
numDigit++;
}
if (Character.isLetter(pass.charAt(i))){
numLetter++;
}
}
if (numDigit==0||numLetter==0){
return 1;
}else if (numDigit<2&&numDigit!=0){
return 2;
}else {
return 3;
}
}else {
return 0;
}
}
}

7. (简答题)(The MyTriangle class) Create a class named MyTriangle that contains the following two methods:

/** Return true if the sum of any two sides is

* greater than the third side. */

public static boolean isValid( double side1, double side2, double side3)

/** Return the area of the triangle. */

public static double area( double side1, double side2, double side3)

Write a test program that reads three sides for a triangle and computes the area if the input is valid. Otherwise, it displays that the input is invalid.

import java.util.Scanner;
public class Demo7 {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
System.out.println("Enter the value of side1:");
double side1 = sc.nextDouble();
System.out.println("Enter the value of side2:");
double side2 = sc.nextDouble();
System.out.println("Enter the value of side3:");
double side3 = sc.nextDouble();
if(isValid(side1,side2,side3)){
System.out.println("The area of the Triangle is "+area(side1,side2,side3));
}else {
System.out.println("The input is invalid!");
}
}
public static boolean isValid(double side1,double side2,double side3){
double [] arr = new double[3];
arr[0]=side1;
arr[1]=side2;
arr[2]=side3;
for (int i = 0; i < arr.length-1; i++) {
for(int j =0;j<arr.length-1-i;j++){
if(arr[j]>arr[j+1]){
double temp = arr[j];
arr[j]=arr[j+1];
arr[j+1]=temp;
}
}
}
if((arr[0]+arr[1])>arr[2]){
return true;
}else {
return false;
}
}
public static double area(double side1,double side2,double side3){
double p = (side1 + side2 + side3)/2;
double x1 = p-side1;
double x2 = p-side2;
double x3 = p-side3;
double s1 = p*x1*x2*x3;
double area = Math.sqrt(s1);
return area;
}
}