To Lecture Notes

IT 212 -- Nov 9 , 2020

Review Exercises

1. Translate this Python script into Java:
   

   score = float(input('Input a course score:'))
   if course_score >= 70.0:
       print('Pass')
   else:
       print('Fail')
   
   // Java Translation:
   public class CourseGrade 
   {
       public static main(String[ ] args)
       {
           Scanner s = new Scanner(System.in);
           System.out.print("Input a course score: ");
           double courseScore = Double.parseDouble(
               s.nextLine( ));
           if (courseScore >= 70.0)
           {
               System.out.println("Pass");
           }
           else
           {
              System.out.println("Fail");
           }
       }
   }
   

2. List ways that Java is different than Python.
   
   1. All Java code must be contained in a class.
   2. A main method must be present to be the startup method.
   3. Every line (except those followed by a block) must end with a semicolon.
   4. // marks the beginning of a source code comment line.
   5. A Scanner object is need to read from the keyboard.
   6. Use { } to delimit a code block.
   7. n++ and n-- are short versions of n += 1 and n -= 1.
3. System.out.println(output); and System.out.print(output); are used instead of
   print(output) and print(output, end="")

Java Control Structures

• Look at these examples in the Java Control Stuctures section:
       CountTo100   CountDownBy7s
• A Java for loop has this form:
  

            (1)           (2)        (3)
             |             |          |
             V             V          V
  for(initialization; condition; iteration)
  {
      action;
  }
  next statement;
  

  (1) The initialization is performed once at the beginning of the loop.
  (2) The condition is checked every time before the action is performed.
        If the condition is true, the action is performed;
        if false, the loop terminates and the next statement after the end of the loop body is executed.
  (3) The iteration is performed at the end of the action but before the condition is checked.
  
  Example:
  

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

  This for loop prints the numbers from 1 to 100, one number per line.

Java Methods

1. What do each of the Java words mean in the main method header?
   

   public  static  void  main  args
   

   Ans:
   public: the method can be called from anywhere
   static: the method is not called from an object
   void: the method has no return value
   String[ ] args: args is the name of the command line arguments array.
2. Translate these Python methods fragment into Java, then test them. Here is the form of a Java standalone method:
   

   public static returnType methodName(
       parameterDatatype1 param1, 
       parameterDatatype2 param2) {
   
       body of method;
       return output;
   }
   
   # h.
   # Use a for loop in Java.
   # Python Statements:
   def countDown(n):
       while n >= 0:
           print(n)
           n -= 1
       print("\nBlastoff")
   
   # Standalone script.
   countDown(10)
   
   // Java Statements:
   public class CountDown
   {
       public static void countDown(int n)
       {
           for(int i = n; i >= 0; i--)
           {
               System.out.println(i);
           }
           System.out.println("\nBlastoff");
       }
   
       public static void main(String[ ] args)
       {
           countDown(10);
       }
   }
   
   # i.
   # Python Statements:
   def celToFahr(cel):
       fahr = 9.0 * cel / 5.0 + 32.0
       return fahr
   
   # Standalone script:
   c = 20.0
   print(celToFahr(c))
   
   // Java Statements:
   public class Cel2Fahr
   {
       public static celToFahr(double cel)
       {
           double fahr = 9.0 * cel / 5.0 + 32.0;
           return fahr;
       }
   
       public static void main(String[ ] args)
       {
           System.out.println(celToFahr(20.0));
       }
   }
   

3. Create a Java array that contains three strings and print it using the two methods to access array items "access by value" and "access by index".
   

   String[ ] a = {"abc", "def", "hij"};
   
   # Method 1: access by value:
   for(String s : a)
   {
       System.out.print(s + " ");
   }
   
   # Method 2: access by index:
   for(int i = 0; i < a.length; i++)
   {
       System.out.print(a[i] + " ");
   }
   

Random Number Generators

• Translate this Python script that generates 20 random die rolls.
  

  # Python Statements:
  import random
  for i in range(0, 20):
      roll = random.randrange(1, 7)
      print(roll, end=" ")
  print( )
  
  // Java Statements:
  public class DieRolls
  {
      Random r = new Random( );
      for(int i = 0; i <= 19; i++)
      {
          int roll = r.nextInt(6) + 1;
          System.out.print(roll + " "); 
      }
      System.out.println( );
  }
  

• Look at the Project 4 Description.

Review Exercise

• We did not do this exercise in class.
• Rewrite the LongSong Example so that a method named playSong prints the song. Pass in the number of verses as a parameter. Ans:
  

  public class LongSong
  {
      public static void main(String[ ] args)
      {
          // Print 99 verses of the song.
          playSong(99);
      }
  
      public static void playSong(int numVerses)
      {    
          for (int n = numVerses; n >= 1; n--)
          {
              System.out.println(n + 
                  " bottles of beer on the wall,");
              System.out.println(n + 
                  " bottles of beer.");
              System.out.println(
                  "Take one down and pass it around,");
              System.out.println((n - 1) + 
                  " bottles of beer on the wall.\n");
          }
      }
  }
  

Java Arrays

• Creating a Java array is similar to a creating a Python list. The difference is that the array must be declared:
  

  # Python List:
  a = [18, 22, 37, 40]
  
  // Java Array:
  int[ ] a = {18, 22, 37, 40};
  
  # Python List
  a = [0, 0, 0, 0] // or a = list([0] * 4)
  a[0] = 18
  a[1] = 22
  a[2] = 37
  a[3] = 40
  
  // Java array
  // In Java, the size of array must be
  // specified in advance.
  int[ ] a = int[4];
  a[0] = 18; 
  a[1] = 22; 
  a[2] = 37; 
  a[3] = 40;
  

• Recall that in Python, there are two methods to print the contents of a list:
  

  # Method 1: by value
  for n in a:
      print(n)
  
  # Method 2: by index
  for i in range(0, len(a)):
      print(a[i])
  

  Java uses these same two methods to print an array:
  

  // Method 1: by value
  for(int n : a)
  {
      System.out.print(n + " ");
  }
  
  // Method 2: by index
  for(int i = 0; i < a.length; i++)
  {
      System.out.print(a[i] + " ");
  }
  

• Create a Java array that contains three strings and print it using the two preceding methods.

Java Classes

• Java classes are similar to Python classes. Here are some key syntax comparisons:
   
  Python __init__ Method Definition:
  

  def __init__(self, the_name, the_gender, the_age):
      self.name = the_name
      self.gender = the_gender
      self.age = the_age
  

  Java Constructor Definition:
  

  public String name;
  public char gender;
  public int age
  
  public Person(String theName, char theGender, int theAge)
  {
      name = theName;
      gender = theGender;
      age = theAge;
  }
  

  Alternatively, the instance variables can be assigned like this:
  

  this.name = theName;
  this.gender = theGender;
  this.age = theAge;
  

  The instance variable this refers to the current object in Java like self refers to the current object in Python.
   
  Python Constructor Invocation:
  

  p = Person("Alice", "F", 11)
  

  Java Constructor Invocation:
  

  Person p = new Person("Alice", 'F', 11);
  

  Python __str__ Method Definition:
  

  def __str__(self):
      return self.name + " " + self.gender + " " + str(self.age)
  

  or

  def __str__(self):
      return f"{self.name} {self.gender} {self.age}"
  

  or

  def __str__(self):
      return "{0} {1} {2}".format(self.name, self.gender, self.age)
  

  Java toString Definition:
  

  // Java toString method using concatenation.
  public String toString( )
  {
      return name + " " + gender + " " + age;
  }
  
  // Java toString method using a format string
  String toString( )
  {
    return String.format("%s %c %d", name, gender, age);
  }

  Python str Invocation:
  

  print(str(p))
  

  or

  print(p)
  

  Java toString Invocation:
  

  System.out.println(p.toString( ));
  

  or
  

  System.out.println(p);
  

• Translate the Person Example into Java using public instance variables.
  Ans: See the Person2 Example