public interface Measurable
{
    double getMeasure();
}

public class DataSet
{
private double sum;
private Measurable maximum;
private int count;

public void add(Measurable x)
{
    sum = sum + x.getMeasure();
    if (count == 0 || maximum.getMeasure() < x.getMeasure())
        maximum = x;
    count++;
}

public Measurable getMaximum()
{
    return maximum;
}
}

public class BankAccount implements Measurable
{

private double balance;

public BankAccount(double balance)
{
    this.balance = balance;
}

public double getMeasure()
{
    return balance;
}
}

public class Coin implements Measurable
{
private double value;
private String name;

public Coin(double value, String name)
{
    this.value = value;
    this.name = name;
}

public String toString()
{
    return "value: " + value + " name: " + name;
}

public double getMeasure()
{
    return value;
}
}

import java.awt.Rectangle;

public class DataSetTester
{
public static void main(String[] args)
{

DataSet bankData = new DataSet();
bankData.add(new BankAccount(100));

BankAccount account = new BankAccount(1000);
// the line below is OK
// you can assign an object that implements an interface
// to a variable of that interface type
Measurable y = account;

// The following line is not OK because the Rectangle
// class does not implement the Measurable interface:
// Measurable x = new Rectangle(5, 10, 15, 20);

DataSet coinData = new DataSet();
coinData.add(new Coin(0.25, "quarter"));

Coin coin = new Coin(0.10, "dime");
Measurable z = coin;

DataSet coinData2 = new DataSet();
coinData2.add(new Coin(0.25, "quarter"));
coinData2.add(new Coin(0.1, "dime"));
coinData2.add(new Coin(0.05, "nickel"));
Measurable max = coinData2.getMaximum();
// The line below will cause max.toString() to run:
System.out.println("max coin: " + max);

// You can convert from an interface type
// (Measurable) back to the original class.
Coin maxCoin = (Coin) max;
System.out.println("max coin: " + maxCoin);

// Demonstrate polymorphism (section 9.3):
System.out.println("\n\nDemonstrate polymorphism: ");
Measurable ba = new BankAccount(1000);
System.out.println(ba.getMeasure());
ba = new Coin(0.5, "half dollar");
System.out.println(ba.getMeasure());

// If you use an interface type like Measurable,
// you can only use the methods listed in the
// interface.
Measurable f = new BankAccount(100);
f.newMethod(); // CAUSES AN ERROR
BankAccount g = new BankAccount(100);
g.newMethod(); // works fine

System.out.println("DataSetTester ran.");
}
}