农场有头大母牛，每年生一小母牛，母牛5岁生母牛，20年后多少牛

public class Cow {
private int age = 1;

public Cow() {
// TODO Auto-generated constructor stub
}
public Cow(int age){
this.age = age;
}
public int getAge() {
return age;
}

public void setAge(int age) {
this.age = age;
}
public void grow(){
this.age++;
}
public Cow bear(){
if(age >= 5){
return new Cow();
}
return null;
}
}

public class Farm {
private List<Cow> cows = new ArrayList<Cow>();
private List<Cow> childrens = new ArrayList<Cow>();

public List<Cow> getCows() {
return cows;
}
public void setCows(List<Cow> cows) {
this.cows = cows;
}
public List<Cow> getChildrens() {
return childrens;
}
public void setChildrens(List<Cow> childrens) {
this.childrens = childrens;
}
public void addCow(Cow c){
cows.add(c);
}
public void reduceCow(Cow c){
cows.remove(c);
}
public void enLarge(int year){
for(int i = 1;i <= year;i++){
for(Cow c:cows){
c.grow();
Cow child = c.bear();
if(child != null){
childrens.add(child);
}
}
cows.addAll(childrens);
System.out.println("第"+i+"年，出生"+childrens.size()+"头牛  农场总共"+cows.size()+"头牛！");
childrens.clear();
}
}
}

public class CowTest {
public static void main(String[] args) {
Farm farm = new Farm();
farm.addCow(new Cow(5));
farm.enLarge(20);
}

}