Getting incompatible error for parent child classes java

问题

I have a HashMap defined:

        HashMap<String,ArrayList<Thing>> hashmap = new HashMap<>();

I do need an ArrayList since I want one key to have multiple values. This was the best solution I found for that problem. The Thing class is a parent class for a bunch of other classes. Problem is when I try to add I get an error.

 hashmap.put(b, world.ports);

This is the error:

no suitable method found for put(String,ArrayList<SeaPort>)
method Map.put(String,ArrayList<Thing>) is not applicable
  (argument mismatch; ArrayList<SeaPort> cannot be converted to ArrayList<Thing>)
method AbstractMap.put(String,ArrayList<Thing>) is not applicable
  (argument mismatch; ArrayList<SeaPort> cannot be converted to ArrayList<Thing>)
method HashMap.put(String,ArrayList<Thing>) is not applicable
  (argument mismatch; ArrayList<SeaPort> cannot be converted to ArrayList<Thing>)

` I dont understand this since SeaPort extends Thing isn't it supposed to be compatible? I have read a bunch of upcasting and downcasting threads but I don't see how they apply here. Here is the Thing class:

package seaportprogram;

import java.util.Scanner;


public class Thing implements Comparable<Thing>{
String name;
int index;
int parent;
World world;

public Thing(){
    name = null;
    index = 0;
    parent =0;
}

//Thing Scanner Constructor
public Thing (Scanner sc){
if(sc != null){
name = sc.next();
index = sc.nextInt();
parent = sc.nextInt();
    }
}

// Get Index
public int getIndex(){
    return index;
}
//Get Name
public String getName(){
    return name;
}
//Get Parent
public int getParent(){
return parent;
}
//Thing To String
public String toString(){
return name + " " + index;
}
//Auto-Gen Compare Method
@Override
public int compareTo(Thing o) {
    // TODO Auto-generated method stub
    return 0;

}

}//End - Class Thing

Here is the SeaPort class:

package seaportprogram;

import java.util.ArrayList;
import java.util.Scanner;

class SeaPort extends Thing {
ArrayList<Dock> docks;
ArrayList<Ship> que;
ArrayList<Ship> ships;
ArrayList<Person> persons;

//Sea Port Scanner Constructor
public SeaPort(Scanner sc){

    super(sc);
    docks = new ArrayList<>();
que = new ArrayList<>();
ships = new ArrayList<>();
persons = new ArrayList<>();
}
//Set Docks
public void setDocks(ArrayList<Dock> docks){
    this.docks = docks;
}
//Get Docks
public ArrayList<Dock> getDocks(){
    return docks;
}
//Set Ships
public void setShips(ArrayList<Ship> ships){
    this.ships = ships;
}
//Get Ships
public ArrayList<Ship> getShips(){
    return ships;
}
//Set Que
public void setQue(ArrayList<Ship> que){
    this.que = que;
}
//Get Que
public ArrayList<Ship> getQue(){
    return que;
}
//Sea Port To String
public String toString(){

String string = "\n\n Sea Port: " + super.toString() + "\n";

for(Dock md: docks){
        string += "\n" + md + "\n";
}

string += "\n\n --- List of all ships in Que: ";

for(Ship ms: que){
        string += "\n  > " + ms;
}

string += "\n\n --- List of all Ships:";

for(Ship ms: ships){
        string += "\n  > " + ms;
    }

string += "\n\n --- List of all Persons:";

for(Person mp: persons){
        string += "\n  > " + mp;
}
return string;

}//End 


}// End Sea Port Class

This is World class:

package seaportprogram;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.Scanner;
public class World extends Thing{

ArrayList<SeaPort> ports;
PortTime time;

//World Scanner Constructor
public World(Scanner sc){
    super (sc);
ports = new ArrayList<>();
}
//Set Ports
}

Any way to make this happen? TIA! update: I am trying to print the map and I get an null pointer exception. I think this is because the list is being initialized both in seaport and in the MyHashMap class. I cant seem to find a way to get around this and I'm not even sure this is the reason for the error. Here is the toString():

          public String toString(){
        if(map.isEmpty())
         {
             System.out.println("The hashMap is empty");
             return "empty";
         }
        String display=" ";
        Iterator<String> itr = map.keySet().iterator();
        while (itr.hasNext()) {
            display =display + itr.next();
            Iterator<ArrayList<T>> itr2 = map.values().iterator();
            while (itr2.hasNext()) {
                display +=itr2.next();
            }
        }

    return display;
    }

and this is the call to it from the gui:

 jta.setText(map.toString());

回答1:

ArrayList<Derived_Class> does not extend ArrayList<Base_Class>.

If you want to use an HashMap like this, one solution could be to write your own wrapper class using templates:

import java.util.ArrayList;
import java.util.HashMap;

public class MyHashMap<T extends Thing> {
    private HashMap<String, ArrayList<T>> map;

    public MyHashMap() {
        map = new HashMap<>();
    }

    public void add(String s, T element) {
        ArrayList<T> list = null;
        if ((list = map.get(s)) == null)
            map.put(s, list = new ArrayList<T>());
        list.add(element);
    }

    public ArrayList<T> get(String s) {
        return map.get(s);
    }
}

if you want an HashMap of SeaPorts, you can create a new HashMap like this,

MyHashMap<SeaPort> map = new MyHashMap<SeaPort>();

and use custom setters and getters to access the arraylists

回答2:

Seaport may extend Thing, but ArrayList<SeaPort> does not extend ArrayList<Thing>.

If you just want to store a map of Strings to Things, you don't need ArrayList. If you really want to map Strings to Lists of Things, then you can't do it like this.

来源：https://stackoverflow.com/questions/49736035/getting-incompatible-error-for-parent-child-classes-java

标签

java

inheritance

casting