Kaninchen.java

public class Kaninchen implements Nager {

	private int ohren = 0;
	private String name = "";

	public Kaninchen(int ohren, String name) {
		this.ohren = ohren;
		this.name = name;
	}

	@Override
	public void nagen() {
		System.out.println("Das Kaninchen mit Ohrenlänge " + ohren + " nagt!");
	}

	public int getOhren() {
		return ohren;
	}

	public String getName() {
		return this.name;
	}

	@Override
	public boolean equals(Object o) {
		if (o instanceof Kaninchen) {
			Kaninchen k = (Kaninchen) o;
			if (k.getOhren() == this.getOhren()) {
				return true;
			}
		}
		return false;
	}

}

KaninchenComparator.java

import java.util.*;

class KaninchenComparator implements Comparator<Kaninchen> {

	@Override
	public int compare(Kaninchen a, Kaninchen b) {

		int ret = 0;

		ret = a.getName().compareTo(b.getName());

		if (ret == 0) {
			ret = a.getOhren() - b.getOhren();
		}

		return ret;

	}

}

Meerschweinchen.java

public class Meerschweinchen implements Nager {

	@Override
	public void nagen() {
		System.out.println("Das Meerschweinchen nagt!");
	}
	
}

Nager.java

public interface Nager {
	public void nagen();
}

NagerListe.java

import java.util.*;

public class NagerListe {

	public static void main(String[] args) {

		Kaninchen k1 = new Kaninchen(12,"Klaus");
		Kaninchen k2 = new Kaninchen(13,"Sabine");
		Kaninchen k3 = new Kaninchen(14,"Eva");
		Meerschweinchen m1 = new Meerschweinchen();
		Meerschweinchen m2 = new Meerschweinchen();

		List<Nager> l1 = new ArrayList<Nager>();
		l1.add(k1);
		l1.add(k2);
		l1.add(m1);

		List<Nager> l2 = new ArrayList<Nager>();
		l2.add(k3);
		l2.add(m2);

		Map<String,List<Nager>> nager = new HashMap<String,List<Nager>>();
		nager.put("Max",l1);
		nager.put("Jule",l2);

	}

}