Composition and Inheritance (of Classes)

Enhancing Class behavior

A Better Window Class

import javax.swing.*;

class JWindowUser1 {
	public static void main(String [] args) {
		JWindow window = new JWindow();
	}
}

Take 2

import javax.swing.*;

class JWindowUser2 {
	public static void main(String [] args) {
		JWindow window = new JWindow();
		window.setVisible(true);
	}
}

Third Attempt

import javax.swing.*;

class JWindowUser3 {
	public static void main(String [] args) {
		JWindow window = new JWindow();
		window.setSize(300, 300);
		window.setLocation(100, 100);
		window.setVisible(true);
	}
}

• Annoying
• Next user has to go through same process

	BetterWindow bw = new BetterWindow();

A Better Buffered Reader

BufferedReader br = 
	new BufferedReader(
		new InputStreamReader(
			new FileInputStream(filename)));

BetterBufferedReader br = new BetterBufferedReader(filename);

Our Collection Project

• We have a DynArray class that gives us the functionality of a resizeable array
• Would like to be able to enhance the functionality
  • List behavior - insertion/removal in middle; contains
  • Set behavior - no duplicates
• Could simply copy over logic into new classes
  • Burdensome
  • Dangerous-- error prone
  • Code redundancy
  • Have to retet all the code in the context of the new class

Class Composition

Involves having one class contain an instance variable of another class.

class NewClass
	...
	private ExistingClass ec;
	...
}

• Term is motivated by function composition
  • f(g(x)); f(g(x)+2); f(g(x), h(y))
  • The combination of functions in order to create more complex ones
  • f uses the result of invoking g
• Similarly NewClass's functionality is partly composed of using the functionality provided by ExistingClass

Revisting the BetterWindow Class

import javax.swing.*;

class ComposedJWindow {
	ComposedJWindow() {
		jwindow = new JWindow();
		jwindow.setSize(300, 300);
		jwindow.setLocation(100, 100);
		jwindow.setVisible(true);
	}

	private JWindow jwindow;
}

class ComposedJWindowUser {
	public static void main(String [] args) {
		ComposedJWindow window = new ComposedJWindow();
	}
}

import java.io.*;

class  {
	ComposedBufferedReader(String filename) {
		br = new BufferedReader(new InputStreamReader(new FileInputStream(filename)));
	}

	private BufferedReader br;
}

class ComposedBufferedReaderUser {
	public static void main(String [] args) {
		ComposedBufferedReader cbr = new ComposedBufferedReader("file.txt");
	}
}

/
class ArrList {
	ArrList() {
		da = new DynArr();
	}
	...
	DynArr da;	
} 

• Accessing behavior of the composed class via the composing class object
  • Accessing behavior of the ExistingClass class via the NewClass class object
  • ComposedJWindow
    • How do we access the move, setColor, etc methods of JWindow?
  • ComposedBufferedReader
    • How do we access the readLine method of BufferedReader?
  • ArrList
    • How do we access the get, length, etc methods of DynArr?
• Accessing composed object (instance variable) from composing class object
  • Accessing ExistingClass object (instance variable) from NewClass object
  • It's (in all likelihood) private
  • Should this be necessary?
• Using composing class object in context expecting composed class object
  • Using NewClass object in context expecting ExistingClass object
  • ComposedJWindow
    • The GUI framework expects to work with JWindows, not ComposedJWindow
  • ComposedBufferedReader
    • Our existing code was written for BufferedReader not ComposedBufferReader

Class Inheritance

Inherited (extended) object

• automatically inherits all behavior (methods) and state (instance variables) of the parent (super) class
• can add its own behavior and state
• can override its superclass' behavior
• can be used in any context requiring superclass' objects

import javax.swing.*;

class BetterJWindow extends JWindow {
	BetterJWindow() {
		setSize(300, 300);
		setLocation(100, 100);
		setVisible(true);
	}
}

class BetterJWindowUser {
	public static void main(String [] args) throws Exception {
		BetterJWindow window = new BetterJWindow();
		window.setSize(i * 100, i * 100);
	}
}

There's a difference between this class and the previous BetterJWindow

JWindow

• has a default constructor
• We were interested in subsequent modifications to the object (via) setSize, setLocation, etc)

import java.io.*;

class BetterBufferedReader extends BufferedReader {
	BetterBufferedReader(String filename) {
		super(new InputStreamReader(new FileInputStream(filename)));
	}
}

class BetterBufferedReaderUser {
	public static void main(String [] args) throws Exception {
		BetterBufferedReader window = new BetterBufferedReader("file.txt");
	}
}

class ArrList extends DynArr {
}