package org.apache.cordova.firebase;

import com.google.firebase.messaging.RemoteMessage;

import java.util.ArrayList;
import java.util.List;

public class FirebasePluginMessageReceiverManager {
    private static List<FirebasePluginMessageReceiver> receivers = new ArrayList<>();

    public static void register(FirebasePluginMessageReceiver receiver){
        receivers.add(receiver);
    }

    public static boolean onMessageReceived(RemoteMessage remoteMessage){
        boolean handled = false;
        for(FirebasePluginMessageReceiver receiver : receivers){
            boolean wasHandled = receiver.onMessageReceived(remoteMessage);
            if(wasHandled){
                handled = true;
            }
        }
        return handled;
    }
}

package org.apache.cordova.firebase;

import com.google.firebase.messaging.RemoteMessage;

import java.util.ArrayList;
import java.util.List;

public class FirebasePluginMessageReceiverManager {
    private static List<FirebasePluginMessageReceiver> receivers = new ArrayList<FirebasePluginMessageReceiver>();

    public static void register(FirebasePluginMessageReceiver receiver){
        receivers.add(receiver);
    }

    public static boolean onMessageReceived(RemoteMessage remoteMessage){
        boolean handled = false;
        for(FirebasePluginMessageReceiver receiver : receivers){
            boolean wasHandled = receiver.onMessageReceived(remoteMessage);
            if(wasHandled){
                handled = true;
            }
        }
        return handled;
    }
}

Replace <> operator to make code compatible with JDK 1.6