StudentGraderExercise (No interface)
======================================
public class Main {
	public static void main(String[] args) {
		Exercise ex = new Exercise("What is 4!", 24);
		Student m = new Student("Mary");
		Grader h = new Grader("Helena");	
		m.doExercise(ex,24);
		h.gradeExercise(ex);	
		ex.displayResult();
	}
}

class Exercise {
	private int studentAnswer;
	private char grade;
	private final String question;
	private final int modelAnswer;

	public Exercise(String q, int a) {question = q; modelAnswer=a;}

	public void writeAnswer(int anAnswer) {studentAnswer=anAnswer;}
	public void readAnswer() {System.out.println("Student's answer is "+studentAnswer);}
	public void grade() {if (studentAnswer==modelAnswer) grade='A';else grade='F';}

	public void displayResult() {System.out.println("Student's answer is "+studentAnswer+", grade is: "+grade);} 
}

class Student {
    private String name;
    public Student(String n) { name = n; }
    public void doExercise(Exercise x, int answer) { x.writeAnswer(answer); }
}

class Grader {
	private String name;
	public Grader(String n) {name=n;}
	public void gradeExercise(Exercise x){x.grade();}
}


StudentGraderExercise (With interface (Topic05 Page 6))
=======================================================
public class Main {
	public static void main(String[] args) {
		Exercise ex = new Exercise("What is 4!", 24);
		Student m = new Student("Mary");
		Grader h = new Grader("Helena");	
		m.doExercise(ex,24);
		h.gradeExercise(ex);	
		ex.displayResult();
	}
}

interface IGrade { void readAnswer(); void grade();  }

interface IReadWrite{ void readAnswer(); void writeAnswer(int anAnswer); }

class Exercise implements IGrade, IReadWrite{
	private int studentAnswer;
	private char grade;
	private final String question;
	private final int modelAnswer;

	public Exercise(String q, int a) {question = q; modelAnswer=a;}

	public void writeAnswer(int anAnswer) {studentAnswer=anAnswer;}
	public void readAnswer() {System.out.println("Student's answer is "+studentAnswer);}
	public void grade() {if (studentAnswer==modelAnswer) grade='A';else grade='F';}

	public void displayResult() {System.out.println("Student's answer is "+studentAnswer+", grade is: "+grade);} 
}

class Student {
    private String name;
    public Student(String n) { name = n; }
    public void doExercise(IReadWrite x, int answer) { x.writeAnswer(answer); }
}

class Grader {
	private String name;
	public Grader(String n) {name=n;}
	public void gradeExercise(IGrade x){x.grade();}
}


Another example (Multiple interfaces, Approximating Multiple Inheritance)
==========================================================================
	// Example: 
	// An employee is both a boss and a subordinate

	// Approximating Multiple Inheritance in Java
	// Using interface and/or composition
	// (Ref https://ioflood.com/blog/java-multiple-inheritance/)
	// (Ref https://en.wikipedia.org/wiki/Composition_over_inheritance)

	//-----------------------------
	
	interface IBoss {
		public void award();	
	}

	interface ISubordinate {
		public void work();	
	}

	class Boss implements IBoss {
		public void award() {System.out.print("I give award. ");}	
	}

	class Subordinate implements ISubordinate{
		public void work() {System.out.print("I work. ");}	
	}

	//-----------------------------
	class Employee implements IBoss, ISubordinate{
		private Boss b = new Boss();
		private Subordinate s = new Subordinate();
		
		public void award() {b.award();}
		public void work() {s.work();}
	}

	//-----------------------------
	class Main {
		public static void main(String[] args) {
			Employee p = new Employee();
			p.work();
			p.award();
		}
	}
	//-----------------------------


Innerclass : BankAccount and Money
==================================

class BankAccount {
	private class Money {
		private String currency;
		private double value;

		public Money(String c, double b) {
			currency = c;
			value = b;
		}

		public String getOwner() {
			return owner;
		}
	}

	private Money balance;
	private String owner;

	public BankAccount(String currency, String ow) {
		owner = ow;
		balance = new Money(currency, 0.00);
	}

	public void methodX() {
		BankAccount another = new BankAccount("RMB", "Tom");
		Money m1 = new Money("HKD", 88);
		Money m2 = another.new Money("JPY", 123);

		System.out.printf("%.2f(%s) %.2f(%s) %.2f(%s) %.2f(%s)", 
			this.balance.value, this.balance.getOwner(),
			another.balance.value, another.balance.getOwner(), 
			m1.value, m1.getOwner(), m2.value, m2.getOwner());
	}

	// public static void methodY() {
	// 	new Money("JPY", 0.00); // Compilation error
	// }
}

public class Main {
	public static void main(String[] args) {
		BankAccount account = new BankAccount("HKD", "Helena");
		account.methodX();
	}
}