Workbook
on Constructors
Video 1
The following four
questions are concerned with the following program.
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1. The
output of this program is _____________________.
2. Which
line represents the creation of the object inside the main method?
a.
9 b.
11 c. 12 d. 13
3. Which
line represents an object declaration inside the main method?
a.
9 b.
11 c. 12 d. 13
4. Lines
11 and 12 can be replaced by a single line as _____________________.
5. Object
creation in java utilizes the keyword.
a.
new
b. Car()
c.
SampleProgram
d. class
6. How
does object declaration differ from object creation?
a.
In object
declaration space is allocated in memory for the object, while in object
creation, the identifier of the object is set to refer to a space in the
memory.
b. In
object declaration, an identifier is created, while in object creation, the
actual object is created.
c.
In object
declaration, an identifier of an object is set to refer to a space in memory,
and in object creation, the space in memory is actually created for the object.
d. Other
than the fact that the syntax of the two operations differs, they perform the
same function of creating the object.
Video 2
7. Which
of the following shows the syntax for the object creation for a defined object?
a.
< object reference
name > = new <arguments>(<class name>);
b. <class
name> = new <object name>(<arguments>);
c.
<object
reference name> = <class name> <identifier>;
d. <
object reference name > = new <class name>(<arguments>);
8. Assume
getAge() is a method of the Person class and katie is a Person. Which of the
following represents a valid method call?
a.
Katie.getAge(Person);
b. katie
= Person.getAge();
c.
katie.getAge();
d. Person
= katie.getAge();
9. What
is the output?
class
Student{
String name;
public void Student(String
name){
name = name;
}
public
static void main(String args[]){
Student stu = new
Student(“Ram”);
System.out.println(stu.name);
}
}
Video 3
10. What
is the output?
class
employee{
int id;
public void disp(){
System.out.println(id);
}
public
static void main(String args[]){
Employee emp = new Employee();
emp.disp();
}
}
11. Create a
class Rectangle. The class has attributes x1,y1,x2,y2 where (x1,y1) is the
bottom-left corner and (x2,y2) is the top-right corner. Write two constructors,
one to take x1,y1,x2,y2 as arguments and the other to take width and height (in
this case, (0,0) will be the bottom-left corner.). It should have methods to: calculate the
perimeter and the area of the rectangle, move the rectangle by deltax and
deltay, find out if a point is inside the rectangle or not. It should also have
methods to get the union and intersection of two rectangles. Write a drive
program to test your class.
//Rectangle.java
/**
* This class represents a rectangle. Its fields represent the coordinates
* of the corners of the rectangle. Its methods define operations that can
* be performed on Rectangle objects.
**/
public class Rectangle {
// These are the data fields of the class
public int x1, y1, x2, y2;
/**
* The is the main constructor for the
class. It simply uses its arguments
* to initialize each of the fields of the
new object. Note that it has
* the same name as the class, and that it
has no return value declared in
* its signature.
**/
public Rectangle(int x1, int y1, int x2,
int y2) {
this.x1 = x1;
this.y1 = y1;
this.x2 = x2;
this.y2 = y2;
}
/**
* This is another constructor. It takes width and height (in
this case, (0,0) will be the bottom-left corner.)
**/
public Rectangle(int width, int height) {
this(0, 0, width, height);
}
/** This is yet another constructor. */
public Rectangle( ) { this(0, 0, 0, 0); }
/** Move the rectangle by the specified
amounts */
public void move(int deltax, int deltay) {
x1 += deltax; x2 += deltax;
y1 += deltay; y2 += deltay;
}
/** Test whether the specified point is
inside the rectangle */
public boolean isInside(int x, int y) {
return ((x >= x1)&& (x <=
x2)&& (y >= y1)&& (y <= y2));
}
/**
* Return the union of this rectangle with
another. I.e. return the
* smallest rectangle that includes them
both.
**/
public Rectangle union(Rectangle r) {
return new Rectangle((this.x1 <
r.x1) ? this.x1 : r.x1,
(this.y1 < r.y1) ?
this.y1 : r.y1,
(this.x2 > r.x2) ?
this.x2 : r.x2,
(this.y2 > r.y2) ?
this.y2 : r.y2);
}
/**
* Return the intersection of this
rectangle with another.
* I.e. return their overlap.
**/
public Rectangle intersection(Rectangle r)
{
Rectangle result = new Rectangle ((this.x1 > r.x1) ? this.x1
: r.x1,
(this.y1 >
r.y1) ? this.y1 : r.y1,
(this.x2 <
r.x2) ? this.x2 : r.x2,
(this.y2 <
r.y2) ? this.y2 : r.y2);
if (result.x1 > result.x2) {
result.x1 = result.x2 = 0; }
if (result.y1 > result.y2) {
result.y1 = result.y2 = 0; }
return result;
}
/**
* This is a method of our superclass,
Object. We override it so that
* Rectangle objects can be meaningfully
converted to strings, can be
* concatenated to strings with the +
operator, and can be passed to
* methods like System.out.println( )
**/
public String toString( ) {
return "[" + x1 +
"," + y1 + "; " + x2 + "," + y2 + "]";
}
}
// RectTest.java
/** This class demonstrates
how you might use the Rectangle class */
public class RectTest {
public static void main(String[ ] args) {
Rectangle rect = new Rectangle(1, 3, 6, 8); // Create Rectangle objects
int choice =
Integer.parseInt(args[0]);
if (choice == 1) {
int delX;
int delY;
// move the rectangle
rect by delx and dely
System.out.println(rect);
} else if (choice == 2) {
int x;
int y;
// check if x , y
lies inside the rectangle rect
} else if (choice == 3) {
// create a rectangle object named
rect3 and find its union with rect
int x1;
int y1;
int x2;
int y2;
System.out.println(rect3);
} else if (choice == 4) {
// create a rectangle object named
rect3 with height 5, and width 6 and
// find its intersection with rect
int width;
int height;
System.out.println(rect3);
} else {
System.out.println("Invalid
choice");
}
}
}
