枚举 NullPointerException Java

Question

我正在进行客户端服务器通信，其中一条消息是一个枚举。要处理服务器端并使用 .toString() 转换客户端的枚举值，并在服务器端根据接收到的值，创建的变量将获得枚举值。代码是服务器端的，错误是在服务器协议上：

if(x < -type.getLeftInset(rotation) || x + type.getDimension() - type.getRightInset(rotation) >= 10) {
            return false;
        }

服务器协议的代码是：

package Servidor;


import Servidor.TileType;



// Server side protocol

public class ServerProtocol {


    //HighScore HighScore = null;
    //private HighScore hs;
    DataPlayer dataPlayer;// = null;

    private TileType[][] tiles;

    private int lines, score, newLevel, level;

    private static TileType type;



    public ServerProtocol(){

        TileType type = TileType.TypeJ;


    }



    public boolean MsgValidate(String typeS, int x, int y, int rotation) {



        if(typeS == TileType.TypeI.toString()) {
            type = TileType.TypeI;
        } 

        if(typeS == TileType.TypeJ.toString()) {
            type = TileType.TypeJ;
        } 

        if(typeS == TileType.TypeL.toString()) {
            type = TileType.TypeL;
        } 

        if(typeS == TileType.TypeO.toString()) {
            type = TileType.TypeO;
        } 

        if(typeS == TileType.TypeS.toString()) {
            type = TileType.TypeS;
        } 

        if(typeS == TileType.TypeT.toString()) {
            type = TileType.TypeT;
        } 

        if(typeS == TileType.TypeZ.toString()) {
            type = TileType.TypeZ;
        }


        if(x < -type.getLeftInset(rotation) || x + type.getDimension() - type.getRightInset(rotation) >= 10) {
            return false;
        }

        if(y < -type.getTopInset(rotation) || y + type.getDimension() - type.getBottomInset(rotation) >= 22) {
            return false;
        }

        for(int col = 0; col < type.getDimension(); col++) {
            for(int row = 0; row < type.getDimension(); row++) {
                if(type.isTile(col, row, rotation) && isOccupied(x + col, y + row)) {
                    return false;
                }
            }
        }
        return true;

    }


    private boolean isOccupied(int x, int y) {
        return tiles[y][x] != null;
    }


}

调用服务器协议的服务器有代码：

package Servidor;

import Servidor.TileType;

public class Server implements Runnable {


    public static final String HighScore = null;
    public Socket server = null;

    private TileType[][] tiles;




    public Server(Socket socket){

        this.server = socket;

    }


    public static void main (String[] args) throws IOException, ClassNotFoundException{

        int port = 6666;

        try(ServerSocket serverSocket = new ServerSocket(port)) {
            System.out.println("Server is listening on port: " + port + "\n\n");

            while(true) {
                Socket socket = serverSocket.accept();
                System.out.println("----------------------------------------------------------------------------------------------------");
                System.out.println("Communication accepted: " + socket.getInetAddress().getHostAddress() /*+ "\n"*/);
                Server server = new Server(socket);
                Thread t = new Thread(server);
                t.start();

            }


        }catch(Exception e) {
            System.out.println(e.getMessage());
            System.exit(1);
        }


    }




    ServerProtocol Comunicao = new ServerProtocol();

    public void run() {


        ObjectInputStream input;

        try {
            input = new ObjectInputStream(server.getInputStream());

            String Msg = (String) input.readObject();
            System.out.println(Msg);

            if (Msg.equals("Start")) {

                System.out.println("Start");



                return;

            }



            if(Msg.equals("Validate")) {

                String typeS = (String) input.readObject();
                System.out.println("Type: " + typeS);

                int x = (int) input.readObject();
                System.out.println("X: " + x);

                int y = (int) input.readObject();
                System.out.println("Y: " + y);

                int rotation = (int) input.readObject();
                System.out.println("Rotation: " + rotation);


                boolean answer = Comunicao.MsgValidate(typeS, x, y, rotation);


                if(answer == false) {

                    System.out.println("Invalid move");
                    ObjectOutputStream saida = new ObjectOutputStream(server.getOutputStream());
                    saida.writeObject("Invalid move");

                } else if(answer == true) {

                    System.out.println("Valid move");
                    ObjectOutputStream saida = new ObjectOutputStream(server.getOutputStream());
                    saida.writeObject("Valid move");

                }


            }



        }catch (IOException | ClassNotFoundException e) {
            e.printStackTrace();
        }





    }



}

我认为这个错误可能是因为它没有被初始化但是在服务器协议代码的开头就被初始化了。错误堆栈跟踪是：

Exception in thread "Thread-1" java.lang.NullPointerException
    at Servidor.ServerProtocol.MsgValidate(ServerProtocol.java:224)
    at Servidor.Server.run(Server.java:138)
    at java.base/java.lang.Thread.run(Thread.java:830)

Answer 1

您遇到的问题是因为您正在使用 == 比较 java.lang.String，这会检查引用是否相等。因为你的条件 none 得到满足，你的方法 MsgValidate 中的变量 type 保持 null.

要解决这个具体问题，请将所有 == 检查替换为 typeS.equals(TileType.TypeI.toString())。但是，您显示的代码中还有其他几个缺陷。

陷阱

服务器协议：

您没有在构造函数中初始化成员变量 type。您只是创建了一个名为 type 的局部变量，该变量未被使用
成员变量 type 应该只是 MsgValidate 方法中的局部变量
您永远不会初始化您的 tiles 数组，也永远不会写入它。永不占用地块
您的 if 序列可以用简单的 for 循环替换

已应用建议的更改：

package Servidor;


import Servidor.TileType;



// Server side protocol

public class ServerProtocol {

    private static final TileType[] TILE_TYPES = new TileType[]{
        TileType.TypeI, TileType.TypeJ, TileType.TypeL, TileType.TypeO, TileType.TypeS, TileType.TypeT, TileType.TypeZ
    };

    //HighScore HighScore = null;
    //private HighScore hs;
    DataPlayer dataPlayer;// = null;

    private TileType[][] tiles;

    private int lines, score, newLevel, level;



    public ServerProtocol(){
    }



    public boolean MsgValidate(String typeS, int x, int y, int rotation) {
        TileType type = null;

        for (int i = 0; i < TILE_TYPES.length && type == null; i++) {
            if (typeS.equals(TILE_TYPES[i].toString())) {
                type = TILE_TYPES[i];
            }
        }

        if (type == null) {
            // or throw an IllegalArgumentException
            return false;
        }

        if(x < -type.getLeftInset(rotation) || x + type.getDimension() - type.getRightInset(rotation) >= 10) {
            return false;
        }

        if(y < -type.getTopInset(rotation) || y + type.getDimension() - type.getBottomInset(rotation) >= 22) {
            return false;
        }

        for(int col = 0; col < type.getDimension(); col++) {
            for(int row = 0; row < type.getDimension(); row++) {
                if(type.isTile(col, row, rotation) && isOccupied(x + col, y + row)) {
                    return false;
                }
            }
        }
        return true;

    }


    private boolean isOccupied(int x, int y) {
        if (tiles == null) {
            return false;
        } else if (y < 0 || y >= tiles.length) {
            return false;
        } else if (x < 0 || x >= tiles[y].length) {
            return false;
        }

        return tiles[y][x] != null;
    }
}

此外，您应该阅读一些常见的 Java 代码约定。例如，包应该完全小写。方法名应以小写字母开头

枚举 NullPointerException Java

Enum NullPointerException Java

java

communication

nullpointerexception

陷阱

服务器协议：