To Lecture Notes

IT 313 -- Jan 14, 2020

Review Exercises

1. What does the IntelliJ shortcut psvm mean?
   Ans: public static void main
2. Use method calls needed to convert between these datatypes: String, int, double, char. A complete answer lists 4 * (4 - 1) = 12 method calls. You already know how to convert from String to int (Integer.parseInt) and from String to double (Double.parseDouble). Ans:
   

   int n = 46;
   double x = 2.18;
   char c = 'A';
   String s = "apple";
   String t = "102";
   String u = "B";
   		
   // Convert int to other datatypes.		
   System.out.println((double) n);
   System.out.println((char) n);
   System.out.println(String.valueOf(n));
           
   // Convert double to other datatypes.        
   System.out.println((int) x);
   System.out.println((char) x);
   System.out.println(String.valueOf(x));
           
   // Convert char to other datatypes.        
   System.out.println((int) c);
   System.out.println((double) c);
   System.out.println(String.valueOf(c));
           
   // Convert String to other datatypes.        
   System.out.println(Integer.parseInt(t));
   System.out.println(Double.parseDouble(t));
   System.out.println(u.charAt(0));
   

     You can use casts to convert one numeric type to another. For example:
   

   double x = 65.3782732;
   int n = (int) x;
   char c = (char) n;
   

   char counts as a numeric type because it contains the Unicode numeric code of a character.
   Ans: Here is a diagram of the methods needed to convert between the datatypes String, int, double, char. Here are the meanings of the arrow labels:
   1: Integer.parseInt
   2: Double.parseDouble
   c: Use cast, for example  int n = (int) x;
   v: Use String.valueOf
   *: char ch = s.charAt(0);
3. Define these object oriented programming terms:
       class   instance variable   local variable   instance method   constructor
   Ans:
   object:   a person, account, organization, ledger line, vehicle, or other entity that contains data and has methods for performing actions.
   class:   The blueprint or design from which objects are created (instantiated).
   local variable:   a variable that is only defined within the method where it is declared.
   instance variable:   a variable that is defined anywhere within the class where it is declared.
   An instance variable can be public (accessable anywhere) or private (only accessable in the class where it is declared). An instance variable is always called by an object.
   static variable:   a variable that is called from a class (not an object).
   constructor:   A special method that creates an object. A constructor is called using the new operator.
   startup method:   The first method that starts running when a Java class file is executed.
   Java startup methods are always called main.
4. Look up the String class in the Java 11 Documentation. Select and test five String methods. Ans:
   

    public class Main {
       public static void main(String[ ] args) {
   
           // Test these String methods:
           // indexOf isBlank isEmpty length toUpperCase
   
           String s = "umbrella";
           String t = " \n \t ";
           String u = "";
   
           System.out.println(s.indexOf("bre"));
           // Output: 2
   
           System.out.println(s.isBlank( ) + " " +
               t.isBlank( ) + " " + u.isBlank( ));
           // Output: false true true
   
           System.out.println(s.isEmpty( ) + " " +
               t.isEmpty( ) + " " + u.isEmpty( ));
           // Output: false false true
   
           System.out.println(s.length( ) + " " +
               t.length( ) + " " + u.length( ));
           // Output: 8 5 0
   
           System.out.println(s.toUpperCase( ));
           // Output: UMBRELLA
       }
   }

Built In Classes and Methods

• Know how to use and these these classes and methods on the midterm.
• Rewrite the

For Loops

• You know about while loops from your Python knowledge. Here is an example of a while loop that counts from 1 to 100:
  

  int i = 1;
  while(i <= 100) {
      System.out.println(i);
      i++;
  }
  

• for loops also exist in Python, but they are written differently in Java. Here is the general structure for a for loop:
  

  for(initialization; condition; iteration) {
     body
  }
  

• A for loop that counts to 100:
  

  for(int i = 1; i <= 100; i++) {
      System.out.println(i);
  }
  

• Look at the counting and dice1 modules in the Loops Example project.
• Modify the wordtonum module in the TerminalIO Example to use a switch statement. Ans:
  

  import java.util.Scanner;
  public class Main {
      public static void main(String[ ] args) {
          Scanner in = new Scanner(System.in);
          System.out.print("Enter word for a one digit integer: ");
          String word = in.nextLine( );
  
          // Declare and initialize num to 0 to avoid
          // an uninitialized variable warning.
          int num = 0;
          
          switch(num) {
              case "zero":
                  num = 0;
                  break;
              case "one":
                  num = 1;
                  break;
              case "two":
                  num = 2;
                  break;
              case "three":
                  num = 3;
                  break;
              case "four":
                  num = 4;
                  break;
              case "five":
                  num = 5;
                  break;
              case "six":
                  num = 6;
                  break;
              case "seven":
                  num = 7;
                  break;
              case "eight":
                  num = 8;
                  break;
              case "nine":
                  num = 9;
                  break;
              default:
                  num = -1;
                  break;
          }
      
          if (num == -1) {
              System.out.println("Illegal input.");
          }
          else {
              System.out.println("The integer is " + num + ".");
          }
          in.close( );
      }
  }
  

Project 2a

• Look at the Drawing Example. Check out the roles of the Color, Font, and Graphics classes.
• Look at the Project 2 Description.
• Additional Graphics methods are listed in the Classes and Methods Document.

Defining and Using Arrays

• An array is a list that can be accessed by index:
  

  34  56  72  91  15
  

  List Indices	0	1	2	3	4
  List Values	34	56	72	91	15

• Two methods to declare and initialize a list referenced by the variable a:
  

  // Method 1:
  int[ ] a = {34, 56, 72, 91, 15};
  
  // Method 2:
  int[ ] a = new int[5];
  // This method initializes the array to all zeros.
  
  // Method 3:
  int[ ] a = new int[5];
  a[0] = 34; a[1] = 56; a[2] = 72; 
  a[3] = 91; a[4] = 15;
  

• Loops that print the values of the array a:
  

  // Traditional loop
  for(int i = 0; i < a.length; i++) {
      System.out.println(a[i]);
  }
  
  // Modern for-each loop   
  for(int n : a) {
      System.out.println(n);
  }
  

Sequential Processing

• Look at these modules in the ComputeAverage Example:
       ave1   ave2   ave3
• Each of these examples shows a different method for terminating the input loop:
  ave1: Terminate input with 999.9 or any other string, even a blank line
  ave2: Enter number of entries before entering loop
  ave3: Terminate input with Control-D, which is the IntelliJ input stream terminator.
• Look up the nextLine and hasNextLine Scanner methods in the Java Class Library documentation. (See the link on the left of the class website.)