java - SWT 空指针异常

Question

我有一个客户端和服务器。服务器向客户端发送一些问题。在客户端，我想在 SWT 应用程序窗口上显示这些问题。

我使用一个线程来显示窗口和主线程与服务器的联系。客户端收到问题后，文本小部件中会抛出 NullPointerException。 (下面的 writeQuestions 方法)。我认为“q1Txt”不为空，因为它是在“createContets”方法中启动的。此外，有问题的数组列表也不为空，因为我检查了它。

你能帮我解决这个问题吗？

我的代码如下。

public class Client {

protected static Shell shell;
private static Text q1txt;
private static Text q2txt;
private static Text q3txt;
private static Text q4txt;
private static Text q5txt;
private static Text ans1txt;
private static Text ans2txt;
private static Text ans3txt;
private static Button ans5YesBtn;
private static Button ans4NoBtn;
private static Button ans4YesBtn;
private static Button ans5NoBtn;
private static ArrayList<Integer> ansList = new ArrayList<Integer>();
private static boolean isAnswered = false;
static String name;
static int socketNum;
static Socket socket;
public static int l = 4;


public static void main(String[] args) throws InterruptedException {

    new Screen().start();

    try {
        socket = new Socket("", 9898);
        ObjectOutputStream objOut = new ObjectOutputStream(socket.getOutputStream());
        ObjectInputStream objIn = new ObjectInputStream(socket.getInputStream());
        objOut.writeObject("8080");

        ArrayList<String> questions = (ArrayList<String>) objIn.readObject();

        writeQuestions(questions); 
        while(!isAnswered)
            Thread.sleep(2000);

        socket.close();
    } catch (UnknownHostException e) {
        System.err.println("There is not like this company!");
    } catch (IOException e) {
        System.err.println("Company does not answer!");
    } catch (ClassNotFoundException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}


public static void writeQuestions(ArrayList<String> questions){
    q1txt.setText(questions.get(0)); // The exception point is here
    q2txt.setText(questions.get(1));
    q3txt.setText(questions.get(2));
    q4txt.setText(questions.get(3));
    q5txt.setText(questions.get(4));
}

private static class Screen extends Thread {
    public void run() {
        open();     
    }

}


public static void open(){
    Display display = Display.getDefault();
    createContents();
    shell.open();
    shell.layout();
    while (!shell.isDisposed()) {
        if (!display.readAndDispatch()) {
            display.sleep();
        }

    }
}


protected static void createContents() {
    shell = new Shell();
    shell.setSize(499, 369);
    shell.setText("SWT Application");
    shell.setLayout(null);

    Composite questionsComp = new Composite(shell, SWT.NONE);
    questionsComp.setBounds(10, 10, 373, 298);

    q1txt = new Text(questionsComp, SWT.BORDER | SWT.READ_ONLY);
    q1txt.setBounds(0, 0, 259, 21);

    q2txt = new Text(questionsComp, SWT.BORDER | SWT.READ_ONLY);
    q2txt.setBounds(0, 51, 259, 21);

    q3txt = new Text(questionsComp, SWT.BORDER | SWT.READ_ONLY);
    q3txt.setBounds(0, 95, 259, 21);
    q4txt = new Text(questionsComp, SWT.BORDER | SWT.READ_ONLY);
    q4txt.setBounds(0, 140, 259, 21);

    q5txt = new Text(questionsComp, SWT.BORDER | SWT.READ_ONLY);
    q5txt.setBounds(0, 189, 259, 21);

    ans1txt = new Text(questionsComp, SWT.BORDER);
    ans1txt.setBounds(272, 0, 58, 21);

    ans2txt = new Text(questionsComp, SWT.BORDER);
    ans2txt.setBounds(272, 51, 58, 21);

    ans3txt = new Text(questionsComp, SWT.BORDER);
    ans3txt.setBounds(272, 95, 58, 21);

    Group ans4group = new Group(questionsComp, SWT.NONE);
    ans4group.setBounds(265, 121, 97, 47);
    ans4group.setLayout(null);

    ans4YesBtn = new Button(ans4group, SWT.RADIO);
    ans4YesBtn.setBounds(10, 20, 39, 16);
    ans4YesBtn.addSelectionListener(new SelectionAdapter() {
        @Override
        public void widgetSelected(SelectionEvent e) {
        }
    });
    ans4YesBtn.setText("Yes");

    ans4NoBtn = new Button(ans4group, SWT.RADIO);
    ans4NoBtn.setBounds(55, 20, 39, 16);
    ans4NoBtn.addSelectionListener(new SelectionAdapter() {
        @Override
        public void widgetSelected(SelectionEvent e) {
        }
    });
    ans4NoBtn.setText("No");

    Group ans5Group = new Group(questionsComp, SWT.NONE);
    ans5Group.setBounds(265, 176, 97, 47);

    ans5NoBtn = new Button(ans5Group, SWT.RADIO);
    ans5NoBtn.setBounds(55, 21, 39, 16);
    ans5NoBtn.setText("No");

    ans5YesBtn = new Button(ans5Group, SWT.RADIO);
    ans5YesBtn.setBounds(10, 21, 39, 16);
    ans5YesBtn.setText("Yes");

    Button btnGainCalculation = new Button(questionsComp, SWT.NONE);
    btnGainCalculation.addSelectionListener(new SelectionAdapter() {
        @Override
        public void widgetSelected(SelectionEvent e) {
            //controls all answers
            if(ans1txt.getText().equals("") || ans2txt.getText().equals("") || ans3txt.getText().equals("")){
                MessageDialog.openWarning(shell, "Warning", "Please answer all questions.");
            }else if((!ans4NoBtn.getSelection() && !ans4YesBtn.getSelection()) || (!ans5NoBtn.getSelection() && !ans5YesBtn.getSelection())){
                MessageDialog.openWarning(shell, "Warning", "Please answer all questions.");
            }else if(!isNumeric(ans1txt.getText()) || !isNumeric(ans2txt.getText()) || !isNumeric(ans3txt.getText()) ){
                MessageDialog.openWarning(shell, "Warning", "Please answer correctly.");
            }else
                isAnswered = true;

        }
    }
            );
    btnGainCalculation.setBounds(0, 236, 97, 25);
    btnGainCalculation.setText("Gain Calculation");

}
public static boolean isNumeric(String str)
{
    return str.matches("-?\\d+(\\.\\d+)?");  //match a number with optional '-' and decimal.
}

}

Answer 1

我很确定问题出在“createContent”的稍后执行上。 我们需要确保组件在创建后会被使用

试试这个:

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.Socket;
import java.net.UnknownHostException;
import java.util.ArrayList;

import org.eclipse.jface.dialogs.MessageDialog;
import org.eclipse.swt.SWT;
import org.eclipse.swt.events.SelectionAdapter;
import org.eclipse.swt.events.SelectionEvent;
import org.eclipse.swt.widgets.Button;
import org.eclipse.swt.widgets.Composite;
import org.eclipse.swt.widgets.Display;
import org.eclipse.swt.widgets.Group;
import org.eclipse.swt.widgets.Shell;
import org.eclipse.swt.widgets.Text;

public class Client{
    protected static Shell              shell;
    private static Text                 q1txt;
    private static Text                 q2txt;
    private static Text                 q3txt;
    private static Text                 q4txt;
    private static Text                 q5txt;
    private static Text                 ans1txt;
    private static Text                 ans2txt;
    private static Text                 ans3txt;
    private static Button               ans5YesBtn;
    private static Button               ans4NoBtn;
    private static Button               ans4YesBtn;
    private static Button               ans5NoBtn;
    private static ArrayList<Integer>   ansList     = new ArrayList<Integer>();
    private static boolean              isAnswered  = false;
    static String                       name;
    static int                          socketNum;
    static Socket                       socket;
    public static int                   l           = 4;

    public static void main(String[] args) throws InterruptedException {
        Screen screen = new Screen();
        screen.setCreationComplete(new CreationCompleteAction());
    }


    private static class Screen extends Thread {
        Runnable creationCompleteAction; 

        public void setCreationComplete(Runnable action){
            this.creationCompleteAction = action;
        }

        public void run() {
            Display display = Display.getDefault();
            createContents();

            if(creationCompleteAction != null){
                creationCompleteAction.run();
            }

            shell.open();
            shell.layout();
            while (!shell.isDisposed()) {
                if (!display.readAndDispatch()) {
                    display.sleep();
                }

            }
        }

    }

    public static class CreationCompleteAction implements Runnable {
        @Override
        public void run() {
            try {
                socket = new Socket("", 9898);
                ObjectOutputStream objOut = new ObjectOutputStream(socket.getOutputStream());
                ObjectInputStream objIn = new ObjectInputStream(socket.getInputStream());
                objOut.writeObject("8080");

                ArrayList<String> questions = (ArrayList<String>) objIn.readObject();

                writeQuestions(questions);
                while (!isAnswered)
                    Thread.sleep(2000);

                socket.close();
            } catch (UnknownHostException e) {
                System.err.println("There is not like this company!");
            } catch (IOException e) {
                System.err.println("Company does not answer!");
            } catch (ClassNotFoundException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }

    public static void writeQuestions(ArrayList<String> questions) {
        q1txt.setText(questions.get(0)); // The exception point is here
        q2txt.setText(questions.get(1));
        q3txt.setText(questions.get(2));
        q4txt.setText(questions.get(3));
        q5txt.setText(questions.get(4));
    }

    private static class Screen extends Thread {
        public void run() {
            Display display = Display.getDefault();
            createContents();
            shell.open();
            shell.layout();
            while (!shell.isDisposed()) {
                if (!display.readAndDispatch()) {
                    display.sleep();
                }

            }
        }

    }

    protected static void createContents() {
        shell = new Shell();
        shell.setSize(499, 369);
        shell.setText("SWT Application");
        shell.setLayout(null);

        Composite questionsComp = new Composite(shell, SWT.NONE);
        questionsComp.setBounds(10, 10, 373, 298);

        q1txt = new Text(questionsComp, SWT.BORDER | SWT.READ_ONLY);
        q1txt.setBounds(0, 0, 259, 21);

        q2txt = new Text(questionsComp, SWT.BORDER | SWT.READ_ONLY);
        q2txt.setBounds(0, 51, 259, 21);

        q3txt = new Text(questionsComp, SWT.BORDER | SWT.READ_ONLY);
        q3txt.setBounds(0, 95, 259, 21);
        q4txt = new Text(questionsComp, SWT.BORDER | SWT.READ_ONLY);
        q4txt.setBounds(0, 140, 259, 21);

        q5txt = new Text(questionsComp, SWT.BORDER | SWT.READ_ONLY);
        q5txt.setBounds(0, 189, 259, 21);

        ans1txt = new Text(questionsComp, SWT.BORDER);
        ans1txt.setBounds(272, 0, 58, 21);

        ans2txt = new Text(questionsComp, SWT.BORDER);
        ans2txt.setBounds(272, 51, 58, 21);

        ans3txt = new Text(questionsComp, SWT.BORDER);
        ans3txt.setBounds(272, 95, 58, 21);

        Group ans4group = new Group(questionsComp, SWT.NONE);
        ans4group.setBounds(265, 121, 97, 47);
        ans4group.setLayout(null);

        ans4YesBtn = new Button(ans4group, SWT.RADIO);
        ans4YesBtn.setBounds(10, 20, 39, 16);
        ans4YesBtn.addSelectionListener(new SelectionAdapter() {
            @Override
            public void widgetSelected(SelectionEvent e) {
            }
        });
        ans4YesBtn.setText("Yes");

        ans4NoBtn = new Button(ans4group, SWT.RADIO);
        ans4NoBtn.setBounds(55, 20, 39, 16);
        ans4NoBtn.addSelectionListener(new SelectionAdapter() {
            @Override
            public void widgetSelected(SelectionEvent e) {
            }
        });
        ans4NoBtn.setText("No");

        Group ans5Group = new Group(questionsComp, SWT.NONE);
        ans5Group.setBounds(265, 176, 97, 47);

        ans5NoBtn = new Button(ans5Group, SWT.RADIO);
        ans5NoBtn.setBounds(55, 21, 39, 16);
        ans5NoBtn.setText("No");

        ans5YesBtn = new Button(ans5Group, SWT.RADIO);
        ans5YesBtn.setBounds(10, 21, 39, 16);
        ans5YesBtn.setText("Yes");

        Button btnGainCalculation = new Button(questionsComp, SWT.NONE);
        btnGainCalculation.addSelectionListener(new SelectionAdapter() {
            @Override
            public void widgetSelected(SelectionEvent e) {
                //controls all answers
                if (ans1txt.getText().equals("") || ans2txt.getText().equals("") || ans3txt.getText().equals("")) {
                    MessageDialog.openWarning(shell, "Warning", "Please answer all questions.");
                } else if ((!ans4NoBtn.getSelection() && !ans4YesBtn.getSelection()) || (!ans5NoBtn.getSelection() && !ans5YesBtn.getSelection())) {
                    MessageDialog.openWarning(shell, "Warning", "Please answer all questions.");
                } else if (!isNumeric(ans1txt.getText()) || !isNumeric(ans2txt.getText()) || !isNumeric(ans3txt.getText())) {
                    MessageDialog.openWarning(shell, "Warning", "Please answer correctly.");
                } else
                    isAnswered = true;

            }
        });
        btnGainCalculation.setBounds(0, 236, 97, 25);
        btnGainCalculation.setText("Gain Calculation");

    }

    public static boolean isNumeric(String str) {
        return str.matches("-?\\d+(\\.\\d+)?"); //match a number with optional '-' and decimal.
    }
}