java - Java 中的 Try-Catch 语句

Question

更新:感谢您提供有关保留代码 SSCCE 的建议。就像我说的，这是我第一次在这里发帖。下次我一定会花时间确保代码在我下次发布之前被充分精简。

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所以我正在为我的计算机科学类(class)编写一个程序，并且遇到了一个我无法弄清楚的奇怪问题。我试图通过使用 try/catch 语句来阻止无效的数据类型输入，以免它弄乱并结束整个程序，从而使我的程序健壮，但它似乎不起作用。当输入无效的数据类型时，程序结束时不会在窗口中显示任何 Java 的标准错误消息。我不知道这是为什么，但我假设我错误地使用了 try/catch 语句。这是我的程序的两个类:

/* The core class for the finance calculations to be done in the FinanceApp class. */

public class FinanceCore{
  // Instance variables
  int numYears;
  double principal;
  double interestRate;
  double balance;

  public FinanceCore(){
    numYears = 0;
    principal = 0;
    interestRate = 0;
    balance = 0;
  }

  // Mutator methods that return boolean values depending on whether the input was valid or not
  public boolean setYears(int y){
    if(y >= 0){
      numYears = y;
      return true;
    }
    else return false;
  }

  public boolean setPrincipal(double p){
    if(p >= 0){
      principal = p;
      balance = principal;
      return true;
    }
    else return false;
  }

  public boolean setInterestRate(double ir){
    if(ir >= 0 && ir <= 1){
      interestRate = ir;
      return true;
    }
    else return false;
  }

  // Two accessors
  public int getYears(){
    return numYears;
  }

  public double getPrincipal(){
    return principal;
  }

  // This method calculates and returns the balance at the end of each year
  public double plusYear(){
    balance = balance*(1+interestRate);
    return balance;
  }
}

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/* This program recieves three pieces of data (interest rate, principal amount, number of years) and generates an output
* table that shows how much money will be in a fund with the given parameters at the end of every year. */

import java.util.Scanner;

public class FinanceApp{

  public static void main(String[]args){
    // First, we will declare our global variables, and set them to default values
    Scanner reader = new Scanner(System.in);
    FinanceCore account = new FinanceCore();
    int menuItem = 0;

    // Now, we'll greet the user (because we're friendly like that)
    System.out.println("Welcome! Please select a menu option below.");

    while(true){
      /* Now, our first user interface: a menu system that displays four options to the user arranged in
       * columns for aesthetic effect. This is accomplished using the printf method.
       */

      System.out.printf("%n%-20s%-20s%n%-20s%-20s%n%-20s%n",
        "Set Principal[1]","Set Interest Rate[2]","Set Timespan[3]","Calculate[4]","Quit[5]");

      System.out.print(": ");

      // Now we get the user input until it is valid, and catch and errors in input type
      try {
        menuItem = reader.nextInt(); 
      }
      catch(Exception e){
        reader.nextLine(); // Clear the input stream to avoid an infinite loop
        System.out.println("Please a valid number 1-5.");
      }

      // The code for setting the principal amount
      if(menuItem == 1){
      while(true){
        System.out.print("Please enter the principal investment amount: ");

        try{
          if(account.setPrincipal(reader.nextDouble()));
          break;
        }
        catch(Exception e){
          reader.nextLine(); // Clear the input stream to avoid an infinite loop
          System.out.println("Please enter a valid dollar amount.");
        }
      }
    }
    // The code for setting the interest rate
    else if(menuItem == 2){
      while(true){
        System.out.print("Please enter the quarterly interest rate: ");
        try{
          if(account.setInterestRate(reader.nextDouble()));
          break;
         }
         catch(Exception e){
           reader.nextLine(); // Clear the input stream to avoid an infinite loop
           System.out.println("Please enter a valid decimal number between 0 and 1 (inclusive).");
         }
       }
     }

     // The code for setting the number of years
     else if(menuItem == 3){
       while(true){
         System.out.print("Please enter the number of years the account will exist: ");
         try{
           if(account.setYears(reader.nextInt()));
           break;
         }
         catch(Exception e){
           reader.nextLine(); // Clear the input stream to avoid an infinite loop
           System.out.println("Please enter a valid integer value.");
         }
       }
     }

     // This part actually executes the calculation
     else if(menuItem == 4){
       System.out.printf("%-10s%-10s%n%-10d%-10.2f%n","YEAR","BALANCE",0,account.getPrincipal());
       int count = 1;
       for(int c = account.getYears(); c > 0; c--){
         System.out.printf("%-10d%-10.2f%n",count,account.plusYear());
         count++;
       }
     }

     // If the user enters any other number, the program quits
     else
       break;
     }
   }
 }

请注意，此程序存在一个我似乎无法修复的顽固问题。由于某种原因，每次用户在菜单选择提示下输入无效数据类型，程序结束(尽管没有任何错误被抛出)。

Answer 1

当您使用 try-catch block 时，实际上是在告诉编译器您将负责显示错误消息——不会显示任何内置消息。相反，将显示您在 catch 语句中包含的任何错误提示。

当我运行你的程序时，我看到了你的错误信息，但没有内置的 Java 错误信息；这是应该的。实际上，错误正在被抛出——但您正在捕获它们，因此 Java 不会在控制台上显示默认消息。

关于您在节目结束时的评论:

如果用户在菜单提示下输入了错误的数据类型，看看会发生什么； menuItem 仍为零。因此，所有 if 语句的计算结果都是假的。因此，运行 else 语句，终止程序。