Java | Adding items to List<?>

Question

I googled around and I couldn't really understand how to do this. I'm trying to add items to a list like this: List<?>. My code looks something like this:

public class Test {
    private List<?> list;

    public Test(List<?> useThisList) {
        this.list = useThisList;
    }

    public void add(Object add) {
        this.list.add(add); // this won't compile
    }
}

However, as commented, that code won't compile. I've tried to change it to something like:

public void add(? add) {
    this.list.add(add);
}

But that won't compile for more obvious reasons. Does anyone know what I need to change this to to make it function properly? Thanks in advance!

By the way, when it does work, you should be able to do this:

List<String> list = new ArrayList<String>();
new Test(list).add("hello");

Well if you don't know the kind of list you've got, how do you know you can add add to it? Perhaps your Test class should be generic... — Jon Skeet
– Jon Skeet, Commented Jan 17, 2016 at 14:53
I would encourage you to read effective java 2nd edition By Joshua Blochs and refer to the item about generics. It gives a handful of insights on how to do a good use of generics / wildcarss — Louis F.
– Louis F., Commented Jan 17, 2016 at 15:15

Adil Shaikh · Accepted Answer · 2016-01-17 15:10:16Z

5

Make your Test class generic

public class Test<E> {
    private List<E> list;

    public Test(List<E> useThisList) {
        this.list = useThisList;
    }

    public void add(E add) {
        this.list.add(add); // this will compile
    }
}

Instantiate your test class like this

Test<String> t = new Test<String>(new ArrayList<String>());
t.add("Something");

edited Jan 17, 2016 at 15:10

answered Jan 17, 2016 at 14:53

Adil Shaikh

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