Answer Posted / chitij mehrotra

1 A class may inplement several interfaces. A class may
extend only one abstract class.
2 An interface can have only abstract methods. An abstract
class can have both abstract methods and concrete methods.
At one abstract method in the abstract class.
3 Interfaces cannot have constructors. Abstract class can
have constructors.
4 Only static final variables are allowed to be decelared.
Both static and instance variables are allowed.
5 All decelared methods are implicitly public. Can have any
kind of methods.
6 We should initialize variables in an interface. Not
necessary to initialize variables in an abstract class.
7 In the case of interface you can make any class in the
hierarchy. In the case of abstract class you know the class
hierarchy in advance.
8 An interface implementation may be added to any existing
third party class. A third party class must be rewritten to
extend only from the abstract class.
9 Interfaces can be implemented by classes that are not
related to one another Abstract classes are used only when
there is a “is-a” type of relationship between the classes.

Question 5 [15] Consider the following classes, illustrating the Strategy design pattern: import java.awt.*; abstract class Text { protected TextApplet tA; protected Text(TextApplet tApplet) { tA = tApplet; } abstract public void draw(Graphics g); } class PlainText extends Text { protected PlainText(TextApplet tApplet) { super(tApplet); } public void draw(Graphics g) { g.setColor(tA.getColor()); g.setFont(new Font("Sans-serif", Font.PLAIN, 12)); g.drawString(tA.getText(), 20, 20); } } class CodeText extends Text { protected CodeText(TextApplet tApplet) { super(tApplet); } public void draw(Graphics g) { g.setColor(tA.getColor()); g.setFont(new Font("Monospaced", Font.PLAIN, 12)); g.drawString(tA.getText(), 20, 20); } } public class TextApplet extends java.applet.Applet { protected Text text; protected String textVal; protected Color color; public String getText() { return textVal; } public Color getColor() { return color; } public void init() { textVal = getParameter("text"); String textStyle = getParameter("style"); String textColor = getParameter("color"); if (textStyle == "code") text = new CodeText(this); else text = new PlainText(this); if (textColor == "red") color = Color.RED; else if (textColor == "blue") color = Color.BLUE; else color = Color.BLACK; } public void paint(Graphics g) { text.draw(g); 10 } } The Text class is more complicated than it should be (there is too much coupling between the Text and TextApplet classes). By getting rid of the reference to a TextApplet object in the Text class and setting the colour in the paint() method, one could turn the Text class into an interface and simplify the strategy classes considerably. 5.1 Rewrite the Text and PlainText classes to do what is described above. (6) 5.2 Explain the consequent changes that are necessary to the TextApplet class. (4) 5.3 Write an additional strategy class called FancyText (to go with your simplified strategy classes) to allow fancy text to be displayed for the value "fancy" provided for the style parameter. It should use the font Font ("Serif", Font.ITALIC, 12). (3) 5.4 Explain what changes are necessary to the TextApplet class for this. (2)

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