convert Collection to Collection

Question

I have a class with the following constructor:

public UniqueField(Collection<Object> items) {
      this.items=items;
}

The idea behind the Collection<Object> is that I would be able to use Collection<OtherType>.

When doing:

Collection<OtherType> collection=... 
new UniqueField(collection);

I getting a compile error of invalid argument. How can I fix this?

score 4 · Accepted Answer · edited May 23 '17 at 12:11

4

You have to use this instead

public UniqueField(Collection<? extends Object> items) {
      this.items=items;
}

or ? because it is equal to "? extends Object"

public UniqueField(Collection<?> items) {
      this.items=items;
}

You can see here for the reason

edited May 23 '17 at 12:11

Community

answered Feb 08 '12 at 20:22

Simon LG

score 0 · Answer 2 · answered Feb 08 '12 at 20:26

you can use either:

public UniqueField(Collection<?> items) {
      this.items=items;
}

or:

public UniqueField(Collection<? super OtherType> items) {
      this.items=items;
}

or simply:

public UniqueField(Collection<OtherType> items) {
  this.items=items;
}

2 Answers2