To Lecture Notes

IT 313 -- Jan 16, 2020

Review Exercises

1. What does the IntelliJ shortcut key combination Alt-Enter do?
2. What is the output?
   

   System.out.println(1 + 2 + "3" + 4 + 5);
   System.out.println(6 / 4 * 4);
   

   Ans: The order of precedance is the same for + when it means addition and + when it means concatenation. Remember that
   

   number + number --> number
   string + string --> string
   number + string --> string
   string + number --> string
   Output for first println statement:
   3345
   
   For the second println statement:
   int / int --> int
   double / int --> double
   int / double --> double
   double / double --> double
   Therefore, 6 / 4 * 4 == 1 * 4 == 4.
   

3. What is the output?
   

   double[ ] s = new double[5];
   a[2] = 5.1;  a[0] = 3.8; a[4] = a[2];
   for(double x : a) {
       System.out.printf("%4.2f ", x);
   }
   System.out.println( )
   

4. Explain the output:
   

   String s = "dog", t = "dog";
   String u = new String("dog");
   System.out.println(s.equals(t) + " " + s.equals(u));
   System.out.println((s == t) + " " + (s == u)); 
   // Output:
   true true
   true false
   

5. Look at the ave4 module in the ComputeAverage Project. This example shows how to process multiple fields on each line of an input file.
6. Convert the wordtonum Example module in the TerminalIO Example project so that it uses a String array instead of if..if else statements. Use this String array:
   

   String[ ] words = {"zero", "one", "two", "three", 
     "four", "five", "six", "seven", "eight", "nine"}
   
   // Ans:
   import java.util.Scanner;
   public class Main {
   
       public static void main(String[] args) {
           String[ ] words = {"zero", "one", "two", "three", 
               "four", "five", "six", "seven", "eight", "nine"};
           Scanner s = new Scanner(System.in);
           System.out.print(
               "Enter the word for a number from 0 to 9: ");
           String word = s.nextLine( );
           int i;
           for(i = 0; i < words.length; i++) {
               if(words[i].equals(word)) {
                   System.out.println(i);
               }
           }
           System.out.println("Unknown number");
           }
       }
   }
   

7. Modify the Drawing Example to draw a picture like this. Look up the Color object in the Java class library to see how to create a custom color.
   Here is a Table of Extended HTML Colors if you want to use hex color codes.
   Ans: Here is the modified source code for the paintComponent method of the MyPanel class.

Writing your Own Classes

• Look at the person module in the Classes Example project.
• Identify these items in the Person Example:
  

  class  object  public  private  
  instance variable  local variable
  getter  setter  instance method
  argument  parameter  constructor
  

  Ans: Items are identified in the Person and Test1 classes.
  class: Person  Test1
  object: In Test1: p
  public: In Person: constructor  all getters and setters  haveBirthday  toString
  private: name  gender  age
  instance variable: name  gender  age
  local variables: In Test1: p
  getter: In Person: getName  getGender  getAge
  setter: In Person: setAge
  instance method: In Person: getName  getGender  getAge  haveBirthday  toString
  argument: In Test1: "Alice"  'F'  12  "Name: %s; Gender: %c; Age: %d\n"  10
  parameter: In Person: theName  theGender  theAge
  constructor: Person
• A local variable is only defined within a specific method.
• An instance variable is defined anywhere in the code for a class object.
• An instance variable or method is said to have private accessibility if it can only be called from within the class where the item is defined.
• An instance variable or method is said to have public accessibility if it can be called anywhere in the entire Java project.
• Traditional best software engineering practice is to declare instance variables with private accessibility, for example, if age is defined as
  

  private int age;
  

  and methods with public accessability, for example,
  

  public int getAge( ) {
      return age;
  }
  

  and

  public void setAge(theAge) {
      age = theAge;
  }
  

• However, some authors are questioning the use of traditional getters and setters. They argue that public instance variables without getters and setters are simpler than getters and setters. However, public getters and setters with private instance variables are well established in Java. We need to learn how to write and use them.
• Practice Problems:
  
  1. Start with this version of the Person class:
     

     public class Person {
         private String name;
         private char gender;
         private int age;
     }
     

     Let IntelliJ write the class constructor, getters, setter for age, and the toString method. Use
          Code >> Generate ...

Project 3

• Look at the Project 3 Description.

Traditional Tests

• Testing methods by printing the output of their return values and verifying that they are correct is called traditional testing.
• Practice Problem:  Check that the Test1 class in the person module in the Classes Example tests all of the public methods in the Person class.

Unit Tests

• Steps for Creating a JUnit5 Test Class in an IntelliJ Project:
• Here is a JUnit test file for the Person class:
  

  import static org.junit.jupiter.api.Assertions.*;
  import org.junit.jupiter.api.BeforeEach;
  import org.junit.jupiter.api.Test;
  class PersonUnitTest {
  
      private Person p;
  	
      @BeforeEach
      void setUp() {
          p = new Person("Alice", 'F', 12);	
      }
  
      @Test
      void test() {
          assertEquals(12, p.getAge( ));
          p.setAge(10);
          assertEquals(10, p.getAge( ));
          p.haveBirthday( );
          assertEquals(11, p.getAge( ));		
      }
  }