从映射中调用对象并直接强制转换

Question

private static final Map<Integer, GameObject> OBJECT = new ConcurrentHashMap<>();

我有一个地图，其中我存储GameObjects，这是由PlayerObject, NpcObject, ItemObject扩展。

我试图创建一个方法，我通过ID和类类型调用对象，并直接对其进行转换，如果它不存在或对象ID的类不匹配给定的返回null。例如

final PlayerObject object = getObject(<id>, PlayerObject);

有办法吗?

编辑:

I manage to do this:

public <T extends EventObject> T getObject(final int objectId)
{
final EventObject object = OBJECT.get(objectId);
return Objects.nonNull(object) && object.getClass() ==  ? T (object) : null;
}

但是我不想使用Class

Answer 1

您可以使用Class#isInstance检查对象的类型是否正确，Class#cast将对象转换为正确的类型。

public static <T extends GameObject> T getObject(Integer id, Class<T> clazz) {
GameObject obj = OBJECT.get(id);
if(!clazz.isInstance(obj)) return null;
return clazz.cast(obj);
}
// ...
final PlayerObject object = getObject(<id>, PlayerObject.class);

Answer 2

试试这个完整的泛型方法:

public static <T> T getObject(int id,  Class<T> c){
Object object = OBJECT.get(id);
return object != null && object.getClass() == c ? c.cast(object) : null;
}

程序的其他部分:

private static final Map<Integer, GameObject> OBJECT = new ConcurrentHashMap<>();
public static void main(String[] args) throws ParseException {
init();
PlayerObject p = getObject(3, PlayerObject.class);
ItemObject i = getObject(3, ItemObject.class);
PlayerObject p2 = getObject(4, PlayerObject.class);
System.out.println(p);
System.out.println(i);
System.out.println(p2);
}
private static void init() {
OBJECT.put(1, new PlayerObject(1, "SomePlayer1"));
OBJECT.put(2, new PlayerObject(2, "SomePlayer2"));
OBJECT.put(3, new ItemObject(3, 5));
OBJECT.put(4, new ItemObject(4, 7));
}

GameObject.class

public class GameObject {
protected int id;
GameObject(int id) {
this.id = id;
}
public int getId() {
return this.id;
}
public void setId(int id) {
this.id = id;
}
}

PlayerObject.class

public class PlayerObject extends GameObject {
private String playerName;
PlayerObject(int id, String playerName) {
super(id);
this.playerName = playerName;
}
public String getPlayerName() {
return this.playerName;
}
public void setPlayerName(String playerName) {
this.playerName = playerName;
}
@Override
public String toString() {
return "PlayerObject{"id": " +
this.id +
", "playerName": "" +
this.playerName +
""}";
}
}

ItemObject.class

public class ItemObject extends GameObject {
private int itemCount;
ItemObject(int id, int itemCount) {
super(id);
this.itemCount = itemCount;
}
public int getItemCount() {
return this.itemCount;
}
public void setItemCount(int itemCount) {
this.itemCount = itemCount;
}
@Override
public String toString() {
return "ItemObject{"id": " +
this.id +
", "itemCount": " +
this.itemCount +
"}";
}
}

和程序的输出:

PlayerObject{"id": 1, "playerName": "SomePlayer1"}
ItemObject{"id": 3, "itemCount": 5}
null

Answer 3

import java.util.Map;
import java.util.Objects;
import java.util.concurrent.ConcurrentHashMap;
public class Stackoverflow_68734414 {
private static Map<Integer, GameObject> OBJECT_MAP = new ConcurrentHashMap<>();
public static void main(String[] args) {
PlayerObject po = new PlayerObject();
ItemObject io = new ItemObject();
OBJECT_MAP.put(1, po);
OBJECT_MAP.put(2, io);
PlayerObject p1 = getObject(1, PlayerObject.class);
PlayerObject p2 = getObject(2, PlayerObject.class);
ItemObject i1 = getObject(1, ItemObject.class);
ItemObject i2 = getObject(2, ItemObject.class);
System.out.println(p1);
System.out.println(p2);
System.out.println(i1);
System.out.println(i2);
}
public static <T extends GameObject> T getObject(Integer id,  Class<T> klass){
GameObject object = OBJECT_MAP.get(id);
if(Objects.nonNull(object) && (object.getClass() ==  klass)) {
return klass.cast(object);
} else{
return null;
}
}
}
class GameObject{
}
class PlayerObject extends GameObject {
}
class ItemObject extends GameObject{
}

输出如预期:

PlayerObject@179d3b25
null
null
ItemObject@254989ff

Answer 4

您是否在寻找类似的内容:

public boolean isItemObject(int id){
GameObject obj = OBJECT.get(id)

if(obj instanceof ItemObject && obj != null){

return true;
}
return false;
}
public boolean isPlayerObject(int id){
GameObject obj = OBJECT.get(id)

if(obj instanceof PlayerObject && obj != null){

return true;
}
return false;
}
public boolean isNPCObject(int id){
GameObject obj = OBJECT.get(id)

if(obj instanceof NpcObject && obj != null){

return true;
}
return false;
}
//...
final PlayerObject pObject = isPlayerObject(objectID) ? OBJECT.get(id) : null;