如何摆脱main方法中的showUsage（）方法？

Question

Every time I run my program, it goes straight into the showUsage method in the if statement. Why is that? How can I continue with the program without getting stuck in the showUsage method? I want to be able to run this program so I can input the data into txt file but unfortunately, I'm getting stuck in the showUsage() method.

public class DataGenerator 
{
    public static void main(String[] args) 
    {

        if ( args.length != 1)
       {
           showUsage();
           System.exit(0);
       }


       String target = args[0];  
       switch (target)
       {
          case "trans" :
             generateTransactionRecords();
             break; 
          case "master" :
             generateAccountRecords(); 
             break;
          default : 
              showUsage();     
       }



    }

    private static void showUsage()
    {
    System.out.println("Usage: DataGenerator trans|master");
    System.out.println(" trans to generate trans.txt representing transcations data");
    System.out.println(" master to generate oldmast.txt representing account recorddata");      
    }

    /**
     * In a loop, prompts the user to enter transaction record one at time. 
     * Figure out a way to enable user to exit the loop
     * Write out all the transaction records in a text file called trans.txt  
     * 
     */
    private static void generateTransactionRecords()
    {
        System.out.println("generateTransactionRecords() called");
        Scanner keyboard= new Scanner (System.in);
        String [] args = new String [1];
        args[0] = "trans.txt";
        //String fileName = arg[0];//"trans.txt";
        //String fileName = "";
        PrintWriter outputStream = null;

        try
        {
        outputStream = new PrintWriter(args[0]);//fileName);

        }
        catch(FileNotFoundException e)
        {
            System.out.println("Error opening the file" + args[0]);
            System.exit(0);
        }

        outputStream.println ("Transaction file" + "        " + "Transaction");
        outputStream.println ("account number" +   "        " + "    amount");
        boolean keepGoing = true;
        while (keepGoing)
        {

            System.out.println("What is the account number?");
            int accountNum = keyboard.nextInt();
            System.out.println("What is the transaction amount?");
            double TransAction = keyboard.nextDouble();
            outputStream.println(accountNum + "                        " + TransAction);

            System.out.println("Do you want to enter more information?");
            String answer;
            answer = keyboard.next();
            if(answer.equalsIgnoreCase("no"))
            {
                keepGoing = false;
            }


        }
        outputStream.close();       
    }


    /**
     * In a loop, prompts the user to enter customer account record one at time. 
     * Figure out a way to enable user to exit the loop
     * Write out all the account records in a text file called oldmast.txt  
     * 
     */
    private static void generateAccountRecords()
    {
        System.out.println("generateAccountRecords() called");
        Scanner keyboard= new Scanner (System.in);

        String [] args = new String [1];
        args[0] = "trans.txt";
        PrintWriter outputStream = null;

        try
        {
        outputStream = new PrintWriter(args[0]);

        }
        catch(FileNotFoundException e)
        {
            System.out.println("Error opening the file" + args[0]);
            System.exit(0);
        }

        outputStream.println ("Master file");
        outputStream.println  ("account number" +   "       Name" + "      Balance");
        boolean keepGoing = true;
        while (keepGoing)
        {
            System.out.println("What is the account number?");
            int accountNum = keyboard.nextInt();
            System.out.println("What is the name?");
            String name = keyboard.nextLine();
            System.out.println("What is the name?");
            int Balance = keyboard.nextInt();
            outputStream.println(accountNum +      "                   " +name+"          " +        Balance);

            System.out.println("Do you want to enter more information?");
            String answer;
            answer = keyboard.nextLine();
            if(answer.equalsIgnoreCase("no"))
            {
                keepGoing = false;
            }


        }
        outputStream.close();
}


}

Answer 1

Run the program as:

java DataGenerator someArgument

Coming back to your code:

public static void main(String[] args) 
{
   // here you are checking for the length of the args array not equal to 1
   // If its not equal to one then, enter the if block
   // To avoid the if block, you need to run the program with exactly 1
   // command line argument
   if ( args.length != 1)
   {
       showUsage();
       System.exit(0);
   }
 ...............

}

args is an array of String which are called Command line arguments . So, if you run your program from command line as java DataGenerator someX someY , then args will contain [someX,someY] and args.length will be 2.

Answer 2

If you are entering the showUsage method, it means you haven't supplied exactly one argument.

Try printing a copy of the arguments to see what was supplied:

for (String s : args) {
  System.out.println(s);
}

On the command line, you need to run java DataGenerator foo (where foo is your argument). If you are using an IDE, check the help to understand how to specify runtime arguments.

Answer 3

I'm going to go ahead and guess it's because you aren't passing any command line arguments when you run the program. String[] args is filled with the command line arguments you pass to the program at runtime.

If there aren't any arguments the if statement will be evaluated to true and showUsage() will run followed by System.exit(0) which exits the program.