To Notes

CSC 212 -- 2/5/08

Review Questions

1. What is the output?

   int i = 3;
   int[ ] a = { 45, 21, 97, 38, 90, 77, 62 };
   System.out.println(++a[++i]);
   System.out.println(a[i--]--);
   System.out.println(a[--i]++);
   System.out.println(--a[i++]);
   System.out.println(a[i] += 2);
   

   Ans:

   91
   91
   97
   97
   40
   

2. Explain the difference between InsertionSort and SelectionSort.

   Ans: Insertionsort moves up array elements one by one until a slot for the next item to insert into the array is found. SelectionSort swaps the item immediately to the right of the sorted segment with the smallest element of the remaining unsorted elements.

3. Explain how to read data from a file.

   Ans: (1) Import java.io.*, (2) declare the main method header to throw FileNotFoundException, (3) declare and instantiate a Scanner object s from a FileReader object, (3) while s.hasMoreXX (you choose XX) is true use the s.nextXX methods to read data from the file.

4. Rewrite using only one line:

   FileReader fr = new FileReader("persons.txt");
   Scanner s = new Scanner(fr);
   

   Ans:

   Scanner s = new Scanner(new FileReader("persons.txt"));
   

5. Create an array of the 52 playing cards (poker cards without jokers). Use a nested double for loop. The constructor for the Card class has this header:

   public Card(int theRank, int theSuit, boolean faceShowing)
   

   Save this class file to the folder where you are creating your Java file:   Card.class

   Ans:

   public static void main(String[ ] args)
   {
       Card[ ] deck = new Card[52];
       int index = 0;
       for(int suit = 1; suit <= 4; suit++)
           for(int rank = 2; rank <= 14; rank++)
               deck[index++] = new Card(rank, suit, false);
   
       for(index = 0; index <= 51; index++)
           System.out.println(deck[index]);
   }
   

Writing to a File

• To write to an output file:
  1. Import the java.io package:

     import java.io.*;
     

  2. Set up the main method to throw the FileNotFoundException:

     public static void main(String[ ] args)
                        throws FileNotFoundException
     

  3. Declare and instantiate a PrintWriter object:

     PrintWriter pw = new PrintWriter("out.txt");
     

  4. Use the println, print or printf methods to write data to the output file.

  5. Close the PrintWriter object when finished to flush the output buffer:

     pw.close( );
     

     If PrintWriter object is not closed, output will not be written to the output file.

  6. Look at the WriteToFile Example:   WriteToFile.java.

Command Line Arguments

• The array args defined as a parameter for the main method is called the array of command line arguments:

  public static void main(String[ ] args)
  

• Individual command line arguments are accessed as args[0], args[1], args[2], etc.

• The number of command line arguments is args.length.

• See the GetArgs Example:   GetArgs.java

• Practice Problem:   Write a Java program that gets two doubles as command line arguments, adds them, and outputs the result to the console.

  Ans: Only the main method is given. Be sure to include java.io.* and java.util.Scanner.

  public static void main(String[ ] args)
                     throws FileNotFoundException
  {
      int x = Double.parseDouble(args[0]);
      int y = Double.parseDouble(args[1]);
      System.out.println("The sum is " + (x + y));
  }
  

• Practice problem:   Modify the previous problem to get the input and output files as command line arguments. The program should get two doubles from the input file, add them, and output the result to the output file.

  Ans: Only the main method is given. Be sure to include java.io.* and java.util.Scanner.

  public static void main(String[ ] args)
                     throws FileNotFoundException
  {
      String inputFileName = args[0];
      String outputFileName = args[1];
      Scanner s = new Scanner(new FileReader(inputFileName));    
      PrintWriter pw = new PrintWriter(outputFileName);
  
      int x = Integer.parseInt(s.nextInt( ));
      int y = Integer.parseInt(s.nextInt( ));
      pw.println("The sum is " + (x + y));
      pw.close( );
  }
  

• See the ToUpper Example:   ToUpper.java

• Use the example input file raven.txt and run the application with command line arguments like this:

  > java ToUpper raven.txt raven-upper.txt
  

An Object that Contains an Array

• A stack is an object in which items are inserted into the top of the stack and also removed from its top.

• Uses for stacks:
  Keep track of tasks in progress with the possiblity of interruptions.

  Keep track of pending fonts on a webpage.

  Keep track of pending methods in a Java program.

  Evaluation of postfix expressions.

• Items inserted into the stack are said to be pushed onto the stack.

• Items removed from the stack are said to be popped from the stack.

• Here is the UML Diagram for the Stack class.

• Here are summaries of the Stack methods:
  isEmpty returns true if the stack contains no Person objects and false otherwise.

  isFull returns true if the stack is full (contains 200 Person objects) and false otherwise.

  getTop returns a reference to the Person object on the top of the stack. It returns a null reference if the stack is empty.

  push pushes a Person object onto the top of the stack if the stack is not full. If the stack is full, push does nothing.

  pop removes the Person object from the top of the stack if the stack is not empty. If the stack is empty, pop does nothing.

  toString returns the contents of the stack as a string.

• See the PersonStack Example:   Stack.java   Person.java   TestStack.java.

• Practice Problem   Create a stack that contains Card objects. Push the 52 cards of a poker deck (without jokers) onto the stack.