Diamond operator ‘<>’ was introduced in Java 7 to make instantiation of generic classes easier.

 Thus instead of having:

List<Integer> list = new ArrayList<Integer>();

We could have:

List<Integer> list = new ArrayList<>();

But this is not the case for anonymous classes. Let’s say we have Diamond interface:

public interface Diamond<T> {
    void run();
}

In Java 8 we can write the code like this:

Diamond<Integer> diamond = new Diamond<Integer>() {
    @Override
    public void run() {
    //run
    }
};

But if we want to use here the diamond operator, we will get compile time error

Diamond<Integer> diamond = new Diamond<>() {
    @Override
    public void run() {
    // run
    }
};

cannot use '<>' with anonymous inner classes

With Java 9 changes it is possible to use diamond operator in anonymous inner classes:

Diamond<Integer> diamond = new Diamond<>() {
  @Override
  public void run() {
    // run
  }
};