父子类java出现不兼容错误 [英] Getting incompatible error for parent child classes java

问题描述

我定义了一个HashMap:

I have a HashMap defined:

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

我确实需要一个ArrayList，因为我希望一个键具有多个值.这是我针对该问题找到的最佳解决方案.Thing类是其他许多类的父类.问题是当我尝试添加时出现错误.

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);

这是错误:

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>)

`我不明白这是因为SeaPort扩展了Thing，它是否应该兼容?我读过很多上下游的线程，但是在这里看不到它们如何应用.这是Thing类:

` 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

这是SeaPort类:

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

这是世界一流的课程:

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
}

有什么办法可以做到这一点?TIA！更新:我正在尝试打印地图，但出现了空指针异常.我认为这是因为该列表在海港和MyHashMap类中都已初始化.我似乎找不到解决此问题的方法，我甚至不确定这是错误的原因.这是toString():

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;
    }

这是gui对它的调用:

and this is the call to it from the gui:

 jta.setText(map.toString());

推荐答案

ArrayList< Derived_Class> 不扩展 ArrayList< Base_Class> .

如果要使用这样的HashMap，一种解决方案可能是使用模板编写自己的包装器类:

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);
    }
}

如果要使用SeaPorts的HashMap，则可以像这样创建新的HashMap，

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

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

并使用自定义的setter和getter来访问数组列表

and use custom setters and getters to access the arraylists

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