Any java collection is just a collection of objects be it string or other. The type argument is just sugar. Depending on situation, such as dealing with very large lists, you may just want to convert it - obviously risking mixing two different types of objects in the same list.

List<Object> objectList = (List)stringList;

And put a @SuppressWarning to get rid of nasties...

Personally, while both of the currently top rated answers are right in a way, I do not think any of them solves the problem in an elegant, reusable way, especially if you have to do this very often.

Suppose you have some old legacy code / dependency that you cannot change in any way (so that it would at least accept List<? extends Object> as @ReverendGonzo suggested in his comment. Suppose also, that you need to talk to this legacy module a lot.

I do not think either casting / copying all the time would be bearable on the long run. It makes your code either vulnerable to insidious bugs and hard to follow or slightly (or drastically) inefficient and hard-to-read.

To have readable and efficient production code, it is better to encapsulate the dirty part in a separate module which deals with the otherwise harmless but ugly cast.

class ProductionCode {
    public void doJob() {
        List<String> strings = Arrays.asList("pear", "apple", "peach");
        StringMagicUtils.dealWithStrings(strings);
    }
}

class StringMagicUtils {
    @SuppressWarnings("unchecked")
    public static void dealWithStrings(List<String> strings) {
        ExternalStringMagic.dealWithStringsAsObjects((List) strings);
    }
}

// Legacy - cannot edit this wonderful code below ˇˇ
class ExternalStringMagic {
    public static void dealWithStringsAsObjects(List<Object> stringsAsObjects) {
        // deal with them as they please
    }
}

If you are willing to convert to an unmodifiable List<Object>, you can simply wrap your list with Collections.unmodifiableList. This works because this static method has a proper wildcard type ? extends T for the element type of the wrapped list (where T is the type of the result list).

Note that, in most cases, creating an unmodifiable view is what you should do, otherwise objects of different types (other than String) may be added in the original list (which should only hold Strings).

This is pretty inefficient, but at least you don't have to write a lot of code~

List<String> stringList = new ArrayList<String>();
List<Object> objectList = Arrays.asList(stringList.toArray());

List<Object> ofObjects = new ArrayList<Object>(ofStrings);

as in:

import java.util.*;
class C { 
  public static void main( String[] args ) { 
     List<String> s = new ArrayList<String>();
     s.add("S");
     List<Object> o = new ArrayList<Object>(s);
     o.add( new Object() );
     System.out.println(  o );

  }
}

As an alternative you can try the addAll method, if the list of objects is an existing list.

model.class

public class Model {

private List<String> stringList = new ArrayList<>();

public List<String> getStringList() {
    return stringList;
}

public void setStringList(List<String> stringList) {
    this.stringList = stringList;
}

}

MainActivity

public class MainActivity extends AppCompatActivity {

Model model = new Model();
Spinner spinner;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    spinner=findViewById(R.id.spinner);

    List<String> itemList = new ArrayList<String>();
    itemList.add("item1");
    itemList.add("item2");
    itemList.add("item3");


   model.setStringList(itemList);


    ArrayAdapter<String> dataAdapter = new ArrayAdapter<String>(this, android.R.layout.simple_spinner_item, model.getStringList());
    dataAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);

    spinner.setAdapter(dataAdapter);

}

}