Question

Please consider the following snippet:

public interface MyInterface {      public int getId(); }  public class MyPojo implements MyInterface {      private int id;      public MyPojo(int id) {         this.id = id;     }      public int getId() {         return id;     }  }  public ArrayList<MyInterface> getMyInterfaces() {      ArrayList<MyPojo> myPojos = new ArrayList<MyPojo>(0);     myPojos.add(new MyPojo(0));     myPojos.add(new MyPojo(1));      return (ArrayList<MyInterface>) myPojos; } 

The return statement does a casting that doesn’t compile. How can I convert the myPojos list to the more generic list, without having to go through each item of the list?

Thanks

Answer 1

Change your method to use a wildcard:

public ArrayList<? extends MyInterface> getMyInterfaces() {         ArrayList<MyPojo> myPojos = new ArrayList<MyPojo>(0);     myPojos.add(new MyPojo(0));     myPojos.add(new MyPojo(1));      return myPojos; } 

This will prevent the caller from trying to add other implementations of the interface to the list. Alternatively, you could just write:

public ArrayList<MyInterface> getMyInterfaces() {     // Note the change here     ArrayList<MyInterface> myPojos = new ArrayList<MyInterface>(0);     myPojos.add(new MyPojo(0));     myPojos.add(new MyPojo(1));      return myPojos; } 

As discussed in the comments:

• Returning wildcarded collections can be awkward for callers

• It’s usually better to use interfaces instead of concrete types for return types. So the suggested signature would probably be one of:

  public List<MyInterface> getMyInterfaces() public Collection<MyInterface> getMyInterfaces() public Iterable<MyInterface> getMyInterfaces()