CODEWITHSUNDEEP
javagenericsinterface
Richard Nichols
Richard Nichols

Reputation: 1940

Is there a way with Java Generics to take Generic parameter that requires implementation of 2 interfaces?

Say I have this code - 

public interface ParentInterface1 {
    public List<? extends ChildInterface1> getChildren();
    public void setChildren(List<? extends ChildInterface1> children);
}
public interface ParentInterface2 {
    public List<? extends ChildInterface2> getChildren();
    public void setChildren(List<? extends ChildInterface2> children);
}
public interface ChildInterface1 {
    public String getField();
    public void setField(String field);
}
public interface ChildInterface2 {
    public String getField();
    public void setField(String field);
}
public class LParentImpl implements ParentInterface1, ParentInterface2 {
    private List<ChildImpl> list;
    public List<ChildImpl> getChildren() {
        return list;
    }
    public void setChildren(List<... wants to accept ChildImpl, which 
                                   implements ChildInterface1 & ChildInterface2> children) {
        throw new UnsupportedOperationException("Not supported yet.");
    }
}
public class ChildImpl implements ChildInterface1, ChildInterface2 {
    private String field;
    public String getField() {
        return field;
    }
    public void setField(String field) {
        this.field = field;
    }
}

Is there a way to make the setChildren() in the ParentImpl class work, without removing the Generic typing completely from the interfaces and implementation?

I'd like to do something like - 

public void setChildren(List<? extends ChildInterface1 & ChildInterface2> children)

This sort of interface/implementation structure is valid for non Generic types, but it seems some aspect of the run-time erasure of Generics might make this impossible? Or am I missing some magic syntax?

Edit: Using the List<? extends ChildInterface1 & ChildInterface2> yields this compile error - 

...\ParentImpl.java:20: > expected
    public void setChildren(List<? extends ChildInterface1 & ChildInterface2> children) {

Upvotes: 4

Views: 4694

Answers (4)

cletus
cletus

Reputation: 625057

You can do this:

public class MyClass<T extends ChildInterface1 & ChildInterface2> { ... }

Take a look at Java Generics Wildcarding With Multiple Classes

Upvotes: 4

Hendy Irawan
Hendy Irawan

Reputation: 21384

This isn't an answer, but I have to thank Jorn for his answer!

I stumbled upon this thread when trying to do exactly what your answer gave me (although it doesn't answer the original questioner). I tried to do this:

public static interface Displayable
    extends Identifiable, Nameable { }

public void print(Displayable obj) {
    System.out.println("[" + obj.getIdentity() + "] " + obj.getName());
}

which doesn't work because the object I am passing doesn't implement Displayable, but not Nameable and Identifiable individually.

Using Jorn's technique, I do this:

public <T extends Identifiable & Nameable> void print(T obj) {
    System.out.println("[" + obj.getIdentity() + "] " + obj.getName());
}

and it works flawless!

I hope this will be helpful to others that having the similar problem as me (and stumbled to this thread).

Upvotes: 0

user8681
user8681

Reputation:

You can specify a method that takes an object that implements those two interfaces like this:

public <T extends IFirst & ISecond> void doSomething(T obj) {}

However, it won't matter much in your example, since both your child interfaces specify the same methods.

Upvotes: 11

Tom Hawtin - tackline
Tom Hawtin - tackline

Reputation: 147154

Your problem doesn't makes sense.

ParentInterface1.setChildren accepts List<ChildInterface1>. Therefore so much LParentImpl.setChildern, but you are trying to constrain it so that it does not.

You might want to say, parameterise ParentInterface1/2, but I'd suggest avoiding multiple inheritance of interface wherever possible (not just where generics are involved).

Upvotes: 4

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