# Java 程序：使用`for`，`while`和遞歸來反轉一個數字 > 原文： [https://beginnersbook.com/2014/01/java-program-to-reverse-a-number/](https://beginnersbook.com/2014/01/java-program-to-reverse-a-number/) 在 Java 中有三種方法可以反轉數字。 1）[使用`while`循環](#using-while) 2）[使用`for`循環](#using-for) 3）[使用遞歸](#using-recursion) 4）[用戶互動的反轉數字](#demo) ### 程序 1：使用`while`循環反轉一個數字 程序將提示用戶輸入數字，然后使用`while`循環反轉相同的數字。 ```java import java.util.Scanner; class ReverseNumberWhile { public static void main(String args[]) { int num=0; int reversenum =0; System.out.println("Input your number and press enter: "); //This statement will capture the user input Scanner in = new Scanner(System.in); //Captured input would be stored in number num num = in.nextInt(); //While Loop: Logic to find out the reverse number while( num != 0 ) { reversenum = reversenum * 10; reversenum = reversenum + num%10; num = num/10; } System.out.println("Reverse of input number is: "+reversenum); } } ``` 輸出： ```java Input your number and press enter: 145689 Reverse of input number is: 986541 ``` ### 程序 2：使用`for`循環反轉一個數字 ```java import java.util.Scanner; class ForLoopReverseDemo { public static void main(String args[]) { int num=0; int reversenum =0; System.out.println("Input your number and press enter: "); //This statement will capture the user input Scanner in = new Scanner(System.in); //Captured input would be stored in number num num = in.nextInt(); /* for loop: No initialization part as num is already * initialized and no increment/decrement part as logic * num = num/10 already decrements the value of num */ for( ;num != 0; ) { reversenum = reversenum * 10; reversenum = reversenum + num%10; num = num/10; } System.out.println("Reverse of specified number is: "+reversenum); } } ``` 輸出： ```java Input your number and press enter: 56789111 Reverse of specified number is: 11198765 ``` ### 程序 3：使用遞歸反轉數字 ```java import java.util.Scanner; class RecursionReverseDemo { //A method for reverse public static void reverseMethod(int number) { if (number < 10) { System.out.println(number); return; } else { System.out.print(number % 10); //Method is calling itself: recursion reverseMethod(number/10); } } public static void main(String args[]) { int num=0; System.out.println("Input your number and press enter: "); Scanner in = new Scanner(System.in); num = in.nextInt(); System.out.print("Reverse of the input number is:"); reverseMethod(num); System.out.println(); } } ``` 輸出： ```java Input your number and press enter: 5678901 Reverse of the input number is:1098765 ``` **示例：反轉已經初始化的數字** 在上述所有程序中，我們正在提示用戶輸入數字，但是如果不想要用戶交互部分并且想要反轉初始化數字那么這就是你能行的。 ```java class ReverseNumberDemo { public static void main(String args[]) { int num=123456789; int reversenum =0; while( num != 0 ) { reversenum = reversenum * 10; reversenum = reversenum + num%10; num = num/10; } System.out.println("Reverse of specified number is: "+reversenum); } } ``` 輸出： ```java Reverse of specified number is: 987654321 ```