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Java, Java, Java Object-Oriented Problem Solving, 2024E

Ralph Morelli, Ralph Walde, Beryl Hoffman, David G. Cooper

Section 2.8 Chapter Summary

Subsection 2.8.1 Technical Terms

access modifier method call and return
class-level variable null pointer
default value null pointer exception
delimiter pointer
empty string reference
flow of control reference variable
interface static modifier
local variable user interface
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Subsection 2.8.2 Important Points

  • Dot notation is used to refer to an object’s public elements.
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  • Designing a class is a matter of deciding what role it will play and what information and actions it will have.
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  • Writing a Java program is a matter of defining one or more classes. A class definition serves as a template for creating instance of the class.
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  • Classes typically contain two kinds of elements, variables and methods. An object’s state is defined by its instance variables.
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  • Class elements that are declared public can be accessed by other objects. Elements that are declared private are hidden from other objects.
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  • A class’s instance variables are usually declared private so they cannot be accessed directly by other objects.
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  • An object’s public instance methods can be called by other objects. Thus, they make up the object’s interface with other objects.
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  • Object instantiation is the process of creating an object, using the new operator in conjunction with a constructor method.
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  • A class definition consists of a header and a body. The header gives the class a name, specifies its accessibility (public), and its place in the Java class hierarchy (extends Object). The class body contains declarations of the class’s variables and definitions of its methods.
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  • By default, a newly defined class is consider a subclass of Object.
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  • Class elements that are declared static, such as the main() method, are associated with the class(not with its instances).
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  • A Java application program must contain a main() method, which is where it begins execution.
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  • Methods that are used solely for the internal operations of the class should be declared private.
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  • An instance variable declaration reserves memory for the instance variable within the object, associates a name and a type with the location, and specifies its accessibility.
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  • A method definition consists of two parts: a header, which names the method and provides other general information about it, and a body, which contains its executable statements.
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  • Declaring a variable creates a name for an object but does not create the object itself. An object is created by using the new operator and a constructor method.
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Solutions 2.8.3 Solutions to Self-Study Exercises

2.4 Class Definition
2.4.5 Define, Create, Use

Self-Study Exercises
2.4.5.1. Class Name.
Solution.
The name of the class is Riddle.
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2.4.5.2. Riddle Instance Variables.
Solution.
The names of two instance variables: question, answer.
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2.4.5.3. Riddle Methods.
Solution.
The names of three methods: Riddle(), getQuestion(), getAnswer().
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2.4.5.4. Riddle Instances.
Solution.
The names of two Riddle instances: riddle1, riddle2.
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2.4.5.5. Riddle Method Calls.
Solution.
All six method calls of the Riddle objects in the program: 
Riddle("What is black and white and red all over?", "An embarrassed zebra.")
Riddle("What is black and white and read all over?","A newspaper.")
riddle1.getQuestion()
riddle1.getAnswer()
riddle2.getQuestion()
riddle2.getAnswer()
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2.4.5.6. Riddle Qualified Names.
Solution.
Qualified names: riddle1.getQuestion() and riddle1.getAnswer().
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2.5 CASE STUDY: Simulating a Two-Person Game
2.5.3 Testing the OneRowNim Class

Self-Study Exercises
2.5.3.1. Add Hint to Riddle Class.
Solution.
  1. Definition of new instance variable in the Riddle class: 
    private String hint;
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  2. The getHint() method of the Riddle class, which should be a public method, is: 
    public String getHint()
{
  return hint;
}
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  3. The setHint() method of the Riddle class, with the result type void. is: 
    public void setHint(String aHint)
{
  hint = aHint;
}
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2.5.3.2. Student Class.
Solution.
A possible definition of the Student class is given below. 
public class Student
{  private String firstName;
   private String lastName;
   private int studentID;
   public void setStudent(String fName, String lName,int anID)
   {
     firstName = fName;
     lastName = lName;
     studentID = anID;
   }
   public int getStudentID() { return studentID; }
   public String getStudentName() ( 
    return firstName + " " + lastName; 
  } 
}
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2.7 From the Java Library: java.util.Scanner
2.7.3 Exceptions

Self-Study Exercise
2.7.3.1. TestScanner for Decimals.
Solution.
A main method that reads and squares a real number is given below. 
public static void main(String[] args)
{               // Create Scanner object
  Scanner sc = Scanner.create(System.in);
  System.out.print("Input a real number:");    // Prompt
  double realNum= sc.nextDouble();      // Read a double
  System.out.println(num + " squared = " + realNum*realNum);
} //main()
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