初學Java：如何讓它在「無效選擇」後再次顯示菜單

Question

如何在顯示「無效選擇」後再次顯示菜單？

現在，當輸入數字< 1或> 4時，它只顯示「無效選擇」。

理想我希望它顯示

"Invalid selection 
/nMETER CONVERSION 
1) Convert to Kilometers 
2) Convert to Inches 
3) Convert to Feet 
4) Quit the Program 
Please make a selection:" 

這裏是我的代碼

import java.util.Scanner; 
import java.text.DecimalFormat; 

public class Homework8bbb 
{ 
//create main 
public static void main(String[]args) 
{//open main method 
    int selection; 
    double meters; 

    Scanner keyboard = new Scanner(System.in); 

    DecimalFormat format = new DecimalFormat("##.00"); 

    //call showMenu method 
    //DISPLAY showMenu method 
    do 
    { 
    showMenu(); 
    //get user [selection] 
    selection = keyboard.nextInt(); 
    //validate selection 
    do { 
     if (selection < 1 || selection > 4) 
     { 
      System.out.print("Invalid selection\n"); 
      selection = keyboard.nextInt(); 
     } 
    }while(selection < 1 || selection > 4); 

    //get [meters] 
    System.out.print("How many meters?\n"); 
    meters = keyboard.nextInt(); 
     //validate meters: do not allow negative meters 
    while (meters < 0) 
    { 
     System.out.print("Invalid Selection. Please enter a positive number:\n"); 
     meters = keyboard.nextInt(); 
    } 

    switch (selection) 
    { 
     case 1: calcKilometers(meters); 
      break; 
     case 2: calcInches(meters); 
      break; 
     case 3: calcFeet(meters); 
      break; 
     case 4: 
      break; } 
    }while(selection != 4); 


} 


public static void showMenu() 
{ 

    System.out.print("\n" 
    + "METER CONVERSION\n" 
    + " 1) Convert to Kilometers\n" 
    + " 2) Convert to Inches\n" 
    + " 3) Convert to Feet\n" 
    + " 4) Quit the Program\n" 
    + " Please make a selection:\n"); 
} 

//calcKilometers method 
public static double calcKilometers(double meters) 
{ 

    double kilometers; 
    kilometers = meters * 0.001; 

    System.out.println(meters + " meters is " + 
    kilometers + " kilometers."); 
    return kilometers; 
} 


//calcInches method 
public static double calcInches(double meters) 
{ 
    double inches; 
    inches = meters * 39.37; 

    System.out.println(meters + " meters is " + 
    inches + " inches."); 
    return inches; 
} 

//calcFeet method 
public static double calcFeet(double meters) 
{ 
    double feet; 
    feet = meters * 3.281; 
    System.out.println(meters + " meters is " + 
    feet + " feet."); 

    return feet; 
} 



}//close class 

Answer 1

你不需要這個循環

do { 
      if (selection < 1 || selection > 4) 
      { 
       System.out.print("Invalid selection\n"); 
       continue; 
      } 
     }while(selection < 1 || selection > 4); 

這個

if (selection < 1 || selection > 4) { 
      System.out.print("Invalid selection\n"); 
      continue; 
    } 

取代它，如果你輸入如果statment繼續將帶您到循環