Compile error with generic types with circular generic dependencies
Consider the following code:
public abstract class Animal<A extends Cage<? extends Animal<A>>>
{
private A cage;
public A getCage()
{
return this.cage;
}
public void setCage(final A cage)
{
this.cage = cage;
}
}
public abstract class Cage<A extends Animal<? extends Cage<A>>>
{
private List<A> animals;
public void addAnimal(final A animal)
{
if (animal != null)
{
animal.setCage(this);
if (this.animals == null)
{
this.animals = new ArrayList<A>();
}
this.animals.add(animal);
}
}
}
These classes are further extended with other classes.
This code will not compile in line
animal.setCage(this);
Any thoughts on why this code won't compile and how you can fix it? We are using Java 1.6.
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3 answers
This should suit your needs:
public abstract class Animal<A extends Cage<C, A>, C extends Animal<A, C>> {
private A cage;
public A getCage() {
return this.cage;
}
public void setCage(final A cage) {
this.cage = cage;
}
}
public abstract class Cage<A extends Animal<C, A>, C extends Cage<A, C>> {
private List<A> animals;
@SuppressWarnings("unchecked")
public void addAnimal(final A animal) {
if (animal != null) {
animal.setCage((C) this);
if (this.animals == null) {
this.animals = new ArrayList<A>();
}
this.animals.add(animal);
}
}
}
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For the methods you showed, this seems to be sufficient:
public abstract class Animal<C> {
private C cage;
public C getCage() {
return this.cage;
}
public void setCage(final C cage) {
this.cage = cage;
}
}
public abstract class Cage<A extends Animal<C>, C> {
private List<A> animals;
public abstract C getThis();
public void addAnimal(final A animal) {
if (animal != null) {
animal.setCage(getThis());
if (this.animals == null) {
this.animals = new ArrayList<A>();
}
this.animals.add(animal);
}
}
}
public class CatCage extends Cage<Cat, CatCage> {
@Override
public CatCage getThis() { return this; }
}
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