Questions

• Reading?
• Quiz?
• Homework?

Concept - Command line

• To execute a Java program from the command line shell (terminal window)

  java ProgramName parameter-list

• The parameters in the list are provided as parameters to the main method in the String[] args
• The code can retrieve the arguments and do something with them.
• We will use the command line arguments to supply file names
• You only want to try to access the array elements if they exist
• If there is one command line argument

  if (args.length < 1)
  {
        System.out.println("Usage: java Classname input_file.txt ");
        return; //can't do anything without the file
  }

• Eclipse can simulate command line arguments
  • Run -> Run Configuration...
  • Select Arguments tab
  • Enter the command line arguments under Program arguments: - one to a line

 

Activity - Command line arguments

Record your participation in Piazza clicker question Lesson21 Q1

Last lesson you had an activity where you read lines containing height of presidents from a hard-coded file name. Here was the problem statement

There is a theory that the being taller is an advantage in a U.S. presidential election. Download presidents.zip. It contains a text file with a list of all presidents and their heights, and also a starter program. The entries in the presidents_by_height.txt file have this format

Abraham Lincoln 6 ft 4 in 193  cm
Lyndon B. Johnson 6 ft 3.5 in 192 cm

Create a project with the files and complete the PresidentAverageHeight class to find the average height in centimeters of all US presidents to date (or 0 if the file is empty). Print with 2 decimal places. You will use nextLine, split, and Integer.parseInt()

Now modify the program to get the file name as a command line argument. Be sure there was an argument before you try to access it. Print a Usage message if there is no parameter.

 

Concept - Exception handling

• Throwable
  • Error - Generally can not recover from these
    • OutOfMemoryError
    • StackOverflowError
  • Exception
    • RuntimeException - Good programming prevents these
      • ArrayIndexOutOfBoundsException
      • NullPointerException
    • All others are called checked exceptions
• Checked Exceptions - IO exceptions and others
  • Can happen in perfect code
  • The compiler insists you handle a checked exception
  • Either catch it
  • Or declare that the method can throw it

Concept - Purpose of exception handling

• To pass the exception to a competent handler
• Do not catch an exception unless you can do something useful with it.
• Pass it back to the caller to handle

Concept - try / catch

• You can handle an exception with a try/catch block
• Put the code that can cause an exception in a try block
• Add a catch block to catch specific exceptions
• Start with the most specific.
• Example

      try
      {
         //most of your code will go in here. 
         //put the code that throws the exception and any code that depends on it in the try block
      
      }      
      catch(FileNotFoundException e )
      {
          System.out.println(e.getMessage());
      }
      
      catch(Exception e )  //some other exeption
      {
          System.out.println(e.getMessage());
      }

Activity - try/catch

Record your participation in Piazza clicker question Lesson21 Q2

Now in your PresidentAverageHeight.java class

• Remove the throws clause from the main method
• Add a try/catch for the exception that could be thrown

Activity - try/catch

Record your participation in Piazza clicker question Lesson21 Q3

Below is the specificaion for the ReadAlice program

There is a file at http://www.laughton.com/obrien/sjsu/cs49j/lessons/lesson20/alice30.txt You are to read the file a character at a time and determine what percentage of the letters are letters, numbers, and punctuation (anything else except whitespace).

Do not copy the file into your project. Read it from the URL.

Use the static methods in the Character class to classify the characters. (page 526, section 11.2.3)

Approximately what percent are letters? (round to a whole number)

1. 51
2. 71
3. 83
4. something else

Here is the completed version: ReadAlice.java.

Modify the program to use try catch rather than declaring that the method throws exceptions.

• If a MalformedURLException is thrown, print a message that gives the URL and indicates it is not a valid URL
• If an IOException is thrown, print a message saying the URL could not be opened.

 

Concept - When not to throw an exception

Do not throw an exception when a simple if statement will prevent the error

• Instead of throwing an ArithmeticException caused by dividing by integer 0
  • test if the divisor is 0 before dividing

Activity - When not to throw an exception

Record your participation in Piazza clicker question Lesson21 Q4

Which of these exceptions should you generally not place in a try/catch block? Select all that apply

1. FileNotFoundException
2. ArrayIndexOutOfBoundsException
3. NullPointerException
4. MalformedURLException
5. InputMismatchException

Concept - try-with-resource

• This is fairly new addition to Java
• Using it prevents unclosed resources
• You have used code like this to write to a file

        PrintWriter out = new PrintWriter(new File("output.txt"), "UTF-8");
        //other statements might throw an exception
        out.println("Number of even numbters: " + count);
        //other statements might throw an exception
        out.close(); //may not execute

• If an exception happens before the close, data could be lost
• Solution: use the try-with-resource statement

  try (PrintWriter out = new PrintWriter(new File("output.txt"), "UTF-8"))
  {
        //other statements might throw an exception
        out.println("Number of even numbters: " + count);
        //other statements might throw an exception
        out.close();
  }
        

• Even if the an excepion occurs, the file is closed

 

Activity - try-with-resource

Record your participation in Piazza clicker question Lesson21 Q5

Now in your ReadFile.java class

• Use try-with-resource when opening the PrintWriter

Concept - Creating custom exceptions

In your own classes

• you can create an exception that gives information specific to you application
• and then throw that exception
• Do not use a throw statement and catch the exception in the same method
• Create a class that is a subclass of RuntimeExcption (or one of its subclasses)

  public class InvalidDepositException extends RuntimeException
  {
      public InvalidDepositException() 
      {
          super("The deposit amount is invalid");
      }
  
      public InvalidDepositException(String message) 
      {
          super(message);
      }
  }    

• In the BankAccount deposit method

  public void deposit(double amount)
  {
      if (amount < 0)
      {
         throw new InvalidDepositException("Deposit amount can not be negative: " + amount);
      }
  }
  

 

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