Question

It's been too long since I have done any Java programming. Can anyone help me with this one? Thank you

6.5 Write the declaration for a class named C that declares: (1) a private int instance variable named mX; (2) a private int class variable named mY initialized to 0; (3) a private int class constant named A which is equivalent to 100; (4) a public int class constant named B which is equivalent to 200; (5) public accessor and mutator methods for mX named getX() and setX(); (6) public accessor and mutator methods for mY named getY() and setY(); (7) a constructor that has one int input parameter named pX which calls setX() to initialize mX to pX; (8) a default constructor that calls C(int) to initialize mX to -1.

6.6 Continuing with the previous exercise, write the declaration for a class named Main that implements the main() method. Within main() suppose we wish to instantiate a C object named cObj1 calling the default constructor. Write the code to do this.

6.7 Continuing, write the code to instantiate another C object named cObj2 calling the second constructor to initialize the mX instance variable to 10.

6.8 Continuing, within main(), are the following statements legal, i.e., do they compile? If so, explain what happens when

the statement is executed. If not, explain why the statement is illegal.

(a) int a1 = C.mX;

(b) int a2 = C.mY;

(c) int a3 = C.A;

(d) int a4 = C.B;

(e) cObj1.C(20);

(f) int a5 = cObj1.getX();

(g) cObj1.setX(20);

(h) cObj2.setX(cObj1.getX());

(i) int a6 = C.getX();

(j) C.setX(20);

(k) int a7 = cObj1.getY();

(l) cObj1.setY(20);

(m) int a8 = C.getY();

(n) C.setY(20);

6.9 Continuing, suppose we add these two methods to the declaration of class C. For each assignment statement, if it is legal (compiles) explain what happens when the statement is executed. For each assignment statement that is illegal (syntax error) explain why the code is illegal.

     public void f() {
         mX = 0;

mY = 0; }

     public static void g() {
         mX = 0;

mY = 0; }

Answer 1

Complete Solution:

// 6.5
// File: C.java
public class C
{
private int mX;
private static int mY;
private static final int A = 100;
public static final int B = 200;

public C()
{
  this(-1);
}

public C(int px)
{
  setX(px);
}

public int getX()
{
  return mX;
}

public void setX(int pX)
{
  mX = pX;
}

public int getY()
{
  return mY;
}

public void setY(int newY)
{
  mY = newY;
}

// 6.9
   public void f()
    {
       mX = 0;
        mY = 0;
    }
    // It is a legal method declaration.
    // It sets mX to 0 and mY to 0 when this method is executed.
    // A static variable can be accessed in both static and non-static methods.
   

    public static void g()
    {
        //mX = 0;
        mY = 0;
    }
    // It is an illegal method declaration.
    // It sets mY to 0 but cannot set mX to 0.
    // The variable mX is a non-static variable and the method g() is a static method.
    // A non-static variable cannot be accessed in a static method.
}

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// File: Main.java
public class Main
{
public static void main(String[] args)
{
  // 6.6
  C cObj1 = new C();
  
  // 6.7
  C cObj2 = new C(10);
  
  // 6.8
  // (a)
  // int a1 = C.mX;
  // It is an illegal statement.
  // The mX is a private variable in class C.
  // A private variable cannot be accessed using its class name in another class.
  
  // (b)
  // int a2 = C.mY;
  // It is an illegal statement.
  // The mY is a private variable in class C.
  // A private variable cannot be accessed using its class name in another class.
  
  // (c)
  // int a3 = C.A;
  // It is an illegal statement.
  // The A is a private and static constant variable in class C.
  // A private variable cannot be accessed using its class name in another class.
  
  // (d)
  int a4 = C.B;
  // It is a legal statement.
  // The B is a public and static constant variable in class C.
  // A public and static variable can be accessed using its class name in another class.
  // It stores the value of B into the variable a4.
  System.out.println("(d): a4 = " + a4);
  
  // (e)
  // cObj1.C(20);
  // It will be legal statement when you create a method in class C with the following heading
  // public void C(int i)
  // But it is not a good practice to write method name as same as the class name.
  
  // (f)
  int a5 = cObj1.getX();
  // It is a legal statement.
  // It stores the current value of cObj1's mX into the variable a5.
  System.out.println("(f): a5 = " + a5);
  
  // (g)
  cObj1.setX(20);
  // It is a legal statement.
  // It sets cObj1's mX to 20.
  System.out.println("(g): cObj1's mX = " + cObj1.getX());
  
  // (h)
  cObj2.setX(cObj1.getX());
  // It is a legal statement.
  // It sets cObj2's mX to cObj1's mX
  System.out.println("(h): cObj2's mX = " + cObj2.getX());
  
  // (i)
  // int a6 = C.getX();
  // It is an illegal statement.
  // The getX method is not a static method.
  // A non-static method cannot be accessed using its class name in another class.
  
  // (j)
  // C.setX(20);
  // It is an illegal statement.
  // The setX method is not a static method.
  // A non-static method cannot be accessed using its class name in another class.
  
  // (k)
  int a7 = cObj1.getY();
  // It is a legal statement.
  // It stores the current value of cObj1's mY into the variable a7.
  System.out.println("(k): a7 = " + a7);
  
  // (l)
  cObj1.setY(20);
  // It is a legal statement.
  // It sets cObj1's mY to 20.
  System.out.println("(l): cObj1's mY = " + cObj1.getY());
  
  // (m)
  // int a8 = C.getY();
  // It is an illegal statement.
  // The getY method is not a static method.
  // A non-static method cannot be accessed using its class name in another class.
  
  // (n)
  // C.setY(20);
  // It is an illegal statement.
  // The setY method is not a static method.
  // A non-static method cannot be accessed using its class name in another class.
}
}

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Sample Output: