Java OOP

SHORT ANSWER QUESTIONS

CONSTRUCTORS

QUESTIONS – PART 1

a) The special method that is used to create an object is called a _____________.

b) A constructor’s name is always the same as the _____________ name.

c) What is a constructor’s return-type? _______________

d) What is the purpose of a constructor? ________________

e) For the following class, write the statement(s) needed to create the point (2, 7).

public class Point

{

public double x;

public double y;

}

f) For the following class, write the statement(s) needed to create the point (6, 4).

public class Point

{

public double x;

public double y;

public Point(double tx, double ty)

{

x = tx;

y = ty;

}

}

g) Assume the following code creates Point objects with the Point class above. What will be outputted to screen?

public class PointTester

{

public static void main(String[] args)

{

Point p1 = new Point(2, 3);

Point p2 = new Point(4, 8);

p1.x = p2.y * 2;

System.out.println(p1.x);

}

}

h) Assume the following code creates Point objects with the Point class above. What will be outputted to screen?

public class PointTester

{

public static void main(String[] args)

{

Point a = new Point(2, 3);

Point b = a;

System.out.println(b.y);

}

}

i) Assume the following code creates Point objects with the Point class above. What will be outputted to screen?

public class PointTester

{

public static void main(String[] args)

{

Point a = new Point(2, 3);

Point b = a;

b.x = 0;

b.y = 4;

System.out.println(a.x);

}

}

j) Assume the following code creates Point objects with the Point class above. What will be outputted to screen?

public class PointTester

{

public static void main(String[] args)

{

Point coco = new Point(5, 5);

Point dodo = new Point(1, 1);

dodo = coco;

coco = dodo;

System.out.println(coco.x);

}

}

k) Assume the following code creates Point objects with the Point class above. What will be outputted to screen?

public class PointTester

{

public static void main(String[] args)

{

Point p = new Point(1, 5);

Point q = new Point(4, 7);

q = p;

q.x = 2;

q = new Point(9, 8);

System.out.println(p.x);

}

}

l) Write the missing statements to complete the class below:

public class Name

{

public String fullName;

public Name(String firstName, String lastName)

{

//missing code

}

}

SOLUTIONS

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QUESTIONS – PART 2

Fill the blanks and short answer questions. Solutions are at the bottom of this document.

m) One can have multiple constructors in a class. True or false? ______________

n) Constructor _______________ is when we have several constructors inside a class. Each one gives us a different way of creating an object. Java requires that each constructor have a different parameter list so that it can tell which one is being used at a given time.

o) How do we refer to the instance that we are inside?

p) Complete the constructor by providing the missing code. No, you may not change any of the other code.

public class Point

{

public double x;

public double y;

public Point(double x, double y)

{

//missing code

}

}

q) How many instance variables and how many constructors does the following Person class have?

public class Person

{

public String name;

public int age;

public Person(String tname, int tage)

{

name = tname;

age = tage;

}

public Person()

{

name = "Jimmy";

age = 4;

}

public Person(int tage)

{

name = "Karen";

age = tage;

}

}

r) Assuming we are using the Person class from above, what will the following code output?

public class PersonTester

{

public static void main(String[] args)

{

Person p = new Person("Pat", 25);

System.out.println(p.age);

}

}

s) Assuming we are using the Person class from above, what will the following code output?

public class PersonTester

{

public static void main(String[] args)

{

Person p = new Person(9);

System.out.println(p.name);

}

}

t) Assuming we are using the Person class from above, what will the following code output?

public class PersonTester

{

public static void main(String[] args)

{

Person p = new Person();

System.out.println(p.name);

}

}

u) If the class Coconut contains only instance variables and no constructor, then why can we still do something like Coconut c = new Coconut(); ?

v) Does the default constructor ever have parameters?

w) What values do the instance variables get when the default constructor is used?

x) Is it good practice to create classes that make use of a default constructor?

y) When you create an object of a class that has no constructor, Java uses the ____________ constructor.

z) When a constructor calls a different constructor of the same class, this is called _______________ ______________.

aa) Constructor chaining has to occur on the ______________ line of a constructor.

bb) We can call another constructor by using the _____________ keyword following by a parameter list that matches another constructor’s needed parameter list.

cc) In the Point class below, what statement(s) is needed to make the 2^nd constructor call the first constructor and make it create a Point at (0,0)?

public class Point

{

private double x;

private double y;

public Point (double tx, double ty)

{

x = tx;

y = ty;

}

public Point()

{

//what goes here?

}

}

SOLUTIONS

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