$30
CS2030S-Problem Set 3 Solved
1. Given the following interfaces.
interface Shape { double getArea();
}
interface Printable { void print();
}
(a) Suppose class Circle implements both interfaces above. Given the following program fragment,
Circle c = new Circle(new Point(0,0), 10);
Shape s = c;
Printable p = c;
Are the following statements allowed? Why do you think Java does not allow some of the following statements?
i. s.print();
ii. p.print();
iii. s.getArea();iv. p.getArea();
(b) Someone proposes to re-implement Shape and Printable as abstract classes instead? Would statements (i) to (iv) be allowed?
(c) Now let’s define another interface PrintableShape as
public interface PrintableShape extends Printable, Shape {
}
and let class Circle implement PrintableShape instead. Would statements (i) to (iv) be allowed now? Can an interface inherit from multiple parent interfaces?
2. Suppose Java allows a class to inherit from multple parent classes. Give a concrete example why this could be problematic.
On the other hand, Java does allow classes to implement multiple interfaces. Explain why this isn’t problematic.
3. Consider the following classes: FormattedText that adds formatting information to the text. We call toggleUnderline() to add or remove underlines from the text. A PlainText is a FormattedText that is always NOT underlined.
class FormattedText { private final String text; private final boolean isUnderlined;
FormattedText(String text) { this.text = text; this.isUnderlined = false;
}
/*
* Overloaded constructor, but made private to prevent * clients from calling it directly.
*/ private FormattedText(String text, boolean isUnderlined) { this.text = text;
this.isUnderlined = isUnderlined;
}
FormattedText toggleUnderline() { return new FormattedText(this.text, !this.isUnderlined);
}
@Override public String toString() { if (this.isUnderlined) { return this.text + "(underlined)";
} else {
return this.text;
}
}
}
class PlainText extends FormattedText {
PlainText(String text) { super(text); // text is NOT underlined
}
@Override
PlainText toggleUnderline() { return this;
}
}
Does the above violate Liskov Substitution Principle? Explain.
4. Consider the following program.
class A { int x;
A(int x) { this.x = x;
}
A method() {return new A(x);
}
}
class B extends A {
B(int x) {
super(x);
}
@Override
B method() {return new B(x);
}
}
Does it compile? What happens if we swap the entire definitions of method() between class A and class B? Does it compile now? Give reasons for your observations.
