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CS1101X:
Programming Methodology
Recitation 10
Inheritance and
Polymorphism
Qn 1: Class F (1/2)
Suppose class F has the following definition:
public class F {
private int value;
public F(int x) {
value = x;
System.out.println("Made F: " + value);
}
}
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Qn 1: Class F (2/2)
What is the output of the following program?
public class G extends F {
private F value;
public G(int x, int y) {
super(x);
value = new F(y);
}
public static void main(String[] args) {
G g = new G(11, 28);
}
}
Made F: 11
Made F: 28
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Qn 2: Class Y (1/5)
Given this class X:
public class X {
// default constructor
public X() {
// no body needed
}
// isX(): class method
public static boolean isX(Object v) {
return (v instanceof X);
}
// isObject(): class method
public static boolean isObject(X v) {
return (v instanceof Object);
}
}
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Qn 2: Class Y (2/5)
And this class Y:
public class Y extends X {
// Y(): default constructor
public Y() {
// no body needed
}
// isY(): class method
public static boolean isY(Object v) {
return (v instanceof Y);
}
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Qn 2: Class Y (3/5)
Class Y (continued):
public static void main(String[] args) {
X x = new X();
Y y = new Y();
X z = y;
System.out.println("x is an Object: " X.isObject(x));
System.out.println("x is an X: " + X.isX(x));
System.out.println("x is a Y: " + Y.isY(x));
System.out.println();
System.out.println("y is an Object: " + X.isObject(y));
System.out.println("y is an X: " + X.isX(y));
System.out.println("y is a Y: " + Y.isY(y));
System.out.println();
System.out.println("z is an Object: " + X.isObject(z));
System.out.println("z is an X: " + X.isX(z));
System.out.println("z is a Y: " + Y.isY(z));
}
}
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Qn 2: Class Y (4/5)
Which of the following statements could be
the fourth statement in method main()?
What would be the output? Explain.
public static void main(String[] args) {
X x = new X();
Y y = new Y();
X z = y;
// fourth statement here
. . .
}
a)
b)
c)
d)
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x
x
y
y
=
=
=
=
y;
(X) y;
x;
(Y) x;
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Qn 2: Class Y (5/5)
Answers:
a)
x = y;
This state is legal. It assigns a subclass object to a superclass variable.
x is an Object: true
x is an X: true
x is a Y: true
b)
x = (X) y;
This state is legal. It casts a subclass object to an object of its superclass,
then assigns that value to a superclass variable.
y is an Object: true
y is an X: true
y is a Y: true
c)
y = x; This statement is illegal. It attempts to assign a superclass object
to a subclass variables, which is not permitted.
y = (Y) x; This statement is illegal. It attempts to cast a superclass
object to an object of its subclass, which is not permitted.
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d)
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Qn 3: Colored3DPoint (1/2)
Suppose the following method main() was added to
class Colored3DPoint. What would the output be?
public static void main(String[] args) {
Colored3DPoint c = new Colored3DPoint();
Colored3DPoint d =
new Colored3DPoint(1, 2, 3, color.BLACK);
Colored3DPoint e = (Colored3DPoint) d.clone();
System.out.println(c);
System.out.println(d);
System.out.println(e);
System.out.println(d.equals(c));
System.out.println(d.equals(e));
}
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Qn 3: Colored3DPoint (2/2)
Output:
class Colored3DPoint[0, 0, 0, java.awt.Color[r=0,g=0,b=255]]
class Colored3DPoint[1, 2, 3, java.awt.Color[r=0,g=0,b=0]]
class Colored3DPoint[1, 2, 3, java.awt.Color[r=0,g=0,b=0]]
false
true
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Qn 4: Sum of two elements (1/5)

Given this problem:


A sorted list of integers list and a value is given. Write
an algorithm to find the subscripts of (any) two distinct
elements in the list whose sum is equal to the given
value.
Example:
list: 2, 3, 8, 12, 15, 19, 22, 24
value: 23
answer: elements 8 (at subscript 2) and 15 (at subscript 4)

Write an efficient code for this problem. What is
the running-time of your algorithm?
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Qn 4: Sum of two elements (2/5)

Sample run:
Enter number of elements: 8
Enter elements (in non-decreasing order):
2 3 8 12 15 19 22 24
Enter sum: 23
Answer: 2, 4
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Qn 4: Sum of two elements (3/5)
import java.util.*;
class SumTwoElements {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
int[] intArray = createArray(scanner);
// printArray(intArray); // for checking
System.out.print("Enter sum: ");
int sum = scanner.nextInt();
search(intArray, sum);
}
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Qn 4: Sum of two elements (4/5)
public static int[] createArray(Scanner scan) {
System.out.print("Enter number of elements: ");
int n = scan.nextInt();
System.out.println(
"Enter elements (in non-decreasing order): ");
int arr[] = new int[n];
for (int i = 0; i < n; ++i) {
arr[i] = scan.nextInt();
}
return arr;
}
public static void printArray(int[] arr) {
for (int i = 0; i < arr.length; ++i)
System.out.print(arr[i] + " " );
System.out.println();
}
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Qn 4: Sum of two elements (5/5)
public static void search(int[] arr, int sum) {
if (arr.length >= 2) {
int left = 0;
int right = arr.length - 1;
while (left < right) {
if (arr[left] + arr[right] == sum) {
System.out.println("Answer: " + left + ", " + right);
return;
}
else if (arr[left] + arr[right] < sum)
left++;
else // arr[left] + arr[right] > sum
right--;
}
}
System.out.println("No solution.");
}
}
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End of Recitation #10
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