Arraylist error

public class Aquarium{
public  ArrayList<Animal> animals;

public Aquarium(){
animals=new ArrayList<Animal>();
}
//String species,String origin,String dateAquired,String tank,String shortDescription,String behavior,boolean scales
public void makeFish(){
Fish a=new Fish("vargos","nai","oxi","12","a0a","1988",false);
animals.add(a);}
 //String species,String origin,String dateAquired,String tank,String shortDescription,String behavior,String name,float weight
public void makeSeaMammal(){
SeaMammal a=new SeaMammal("vargos","nai","oxi","12","a0a","1988","sasa",0.23);
animals.add(a);}

public void setAnimal(Animal a){
animals.add(a);}

public void showListAnimals(){
System.out.println(animals);}

public String toString(){ return "";
 	
}

public static void main(String [] args){
 Aquarium aa=new Aquarium();
 aa.makeFish();
 aa.makeFish();
 aa.makeSeaMammal();

 aa.showListAnimals();
 
}
}

public abstract class Animal {

protected String species="";
protected String origin="";
protected String dateAquired="";
protected String tank="";
protected String shortDescription="";
protected String behavior="";

public Animal(String species,String origin,
String dateAquired,String tank,String shortDescription,String behavior){
this.species=species;
this.origin=origin;
this.dateAquired=dateAquired;
this.tank=tank;
this.shortDescription=shortDescription;
this.behavior=behavior;
}

public String getbehavior(){
return behavior;
}

public void setbehavior(String behavior){
this.behavior=behavior;
}

public String getSpecies(){
return species;
}

public String getOrigin(){
return origin;}

public String getDateAquired(){
return dateAquired;
}

public String getTank(){
return tank;
}

public String getShortDescription(){
return shortDescription;
}

    @Override
    public String toString(){ return super.toString()+species+"species"+origin +"origin"+dateAquired+"dateAquired"+tank+"tank"+shortDescription+"shortDescription"+behavior+"behavior";
 	
}


abstract public void search();
abstract public boolean isEqual();
abstract public Animal addAnimal();
abstract public Animal deleteAnimal();
abstract public int sizeArray();
abstract public void demarcationArray();

    
} 

public class Fish extends Animal{

private boolean scales;

public Fish(String species,String origin,
String dateAquired,String tank,String shortDescription,
String behavior,boolean scales) {
super(species,origin,dateAquired,tank,shortDescription,behavior);
this.scales=scales;
}


public boolean getScales(){
return scales;
}

    @Override
    public String toString(){ return super.toString()+"scales"+scales;
	
}

public void search() {
	// TODO Auto-generated method stub
	
}



public boolean isEqual() {
	// TODO Auto-generated method stub
	return false;
}



public Animal addAnimal() {
	// TODO Auto-generated method stub
	return null;
}



public Animal deleteAnimal() {
	// TODO Auto-generated method stub
	return null;
}



public int sizeArray() {
	// TODO Auto-generated method stub
	return 0;
}



public void demarcationArray() {
	// TODO Auto-generated method stub
	
}

}