java - while 循环中缺少 return 语句

Question

我为“GPS蓝牙接收器”编写了一些代码，它向我发送连续的数据并且我接收它，但我无法在我的文本框中显示它。

建议我如何改进我的代码，以便在“while 循环”未结束时继续数据。

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Enumeration;
import java.util.TooManyListenersException;

public class communication {
  public static CommPortIdentifier portId = null;

  public String com() throws UnsupportedCommOperationException, IOException,
      TooManyListenersException
  {
    int c = 1;

    String wantedPortName = "COM16";

    Enumeration portIdentifiers = CommPortIdentifier.getPortIdentifiers();

    while (portIdentifiers.hasMoreElements()) {
      CommPortIdentifier pid = (CommPortIdentifier) portIdentifiers.nextElement();
      if (pid.getPortType() == CommPortIdentifier.PORT_SERIAL
          && pid.getName().equals(wantedPortName)) {
        portId = pid;
        break;
      }
    }
    if (portId == null) {
      System.err.println("Could not find serial port " + wantedPortName);
      System.exit(1);
    } else {
      System.out.println("system find gps reciever");
    }
    SerialPort port = null;
    try {
      port = (SerialPort) portId.open("RMC", 1);
      System.out.println("all are ok");
    } catch (PortInUseException e) {
      System.err.println("Port already in use: " + e);
      System.exit(1);
    }
    // int i=Integer.parseInt(new vps().baud_rate.getItemAt(new vps().baud_rate.getItemCount()));
    port.setSerialPortParams(4800, SerialPort.DATABITS_8, SerialPort.STOPBITS_1,
        SerialPort.PARITY_NONE);

    BufferedReader is = null;

    try {
      is = new BufferedReader(new InputStreamReader(port.getInputStream()));
      System.out.println("data is ok");
    } catch (IOException e) {
      System.err.println("Can't open input stream: write-only");
      is = null;
    }

    while (new test().bp)// this is variable is ouside of class and define by public it work
    {

      String st = is.readLine();

      // System.out.println(st);
      // st = st.replace(st, "");

      return st;

    }
    if (is != null)
      is.close();

    if (port != null)
      port.close();

  }
}

测试类代码...

import java.io.IOException;
import java.util.TooManyListenersException;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.comm.UnsupportedCommOperationException;

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */

/*
 * test.java
 *
 * Created on Mar 11, 2013, 9:08:52 AM
 */
/**
 *
 * @author DJ ROCKS
 */
public class test extends javax.swing.JFrame {
    public boolean bp=false;

    /** Creates new form test */
    public test() {
        initComponents();
    }

    /** This method is called from within the constructor to
     * initialize the form.
     * WARNING: Do NOT modify this code. The content of this method is
     * always regenerated by the Form Editor.
     */
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">
    private void initComponents() {

        ok = new javax.swing.JButton();
        a = new javax.swing.JTextField();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        ok.setText("ok");
        ok.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                okActionPerformed(evt);
            }
        });

        a.setText(" ");

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addGroup(layout.createSequentialGroup()
                        .addGap(62, 62, 62)
                        .addComponent(a, javax.swing.GroupLayout.PREFERRED_SIZE, 394, javax.swing.GroupLayout.PREFERRED_SIZE))
                    .addGroup(layout.createSequentialGroup()
                        .addGap(238, 238, 238)
                        .addComponent(ok)))
                .addContainerGap(124, Short.MAX_VALUE))
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(a, javax.swing.GroupLayout.PREFERRED_SIZE, 39, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addGap(67, 67, 67)
                .addComponent(ok)
                .addContainerGap(160, Short.MAX_VALUE))
        );

        pack();
    }// </editor-fold>

    private void okActionPerformed(java.awt.event.ActionEvent evt) {
        // TODO add your handling code here:
        bp=true;
        if(evt.getSource()==ok)
        {
            try {
                a.setText(new communication().com());
            } catch (UnsupportedCommOperationException ex) {
                Logger.getLogger(test.class.getName()).log(Level.SEVERE, null, ex);
            } catch (IOException ex) {
                Logger.getLogger(test.class.getName()).log(Level.SEVERE, null, ex);
            } catch (TooManyListenersException ex) {
                Logger.getLogger(test.class.getName()).log(Level.SEVERE, null, ex);
            }
        }
    }

    /**
     * @param args the command line arguments
     */
    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {

            public void run() {
                new test().setVisible(true);
            }
        });
    }
    // Variables declaration - do not modify
    private javax.swing.JTextField a;
    private javax.swing.JButton ok;
    // End of variables declaration
}

当我运行此代码时，编译器显示这样的错误......

Exception in thread "AWT-EventQueue-0" java.lang.RuntimeException: Uncompilable source code - missing return statement
at communication.com(communication.java:14)
at test.okActionPerformed(test.java:86)
at test.access$000(test.java:22)
at test$1.actionPerformed(test.java:47)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
at java.awt.Component.processMouseEvent(Component.java:6505)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
at java.awt.Component.processEvent(Component.java:6270)
at java.awt.Container.processEvent(Container.java:2229)
at java.awt.Component.dispatchEventImpl(Component.java:4861)
at java.awt.Container.dispatchEventImpl(Container.java:2287)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
at java.awt.Container.dispatchEventImpl(Container.java:2273)
at java.awt.Window.dispatchEventImpl(Window.java:2719)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:723)
at java.awt.EventQueue.access$200(EventQueue.java:103)
at java.awt.EventQueue$3.run(EventQueue.java:682)
at java.awt.EventQueue$3.run(EventQueue.java:680)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
at java.awt.EventQueue$4.run(EventQueue.java:696)
at java.awt.EventQueue$4.run(EventQueue.java:694)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:693)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:244)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:163)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:151)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:147)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:139)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:97)

Answer 1

这是你的函数的结尾:

while(new test().bp)//this is variable is ouside of class and define by public it work 
{

   st = is.readLine();
   return st;
}
if (is != null) is.close();
if (port != null) port.close();

如果 bp 为 false，则退出 while 循环而不返回。在这种情况下，您关闭 is 和 port，然后...没有 return 语句。因此编译器不知道如果发生这种情况应该返回什么。

顺便说一句，这个循环逻辑没有任何意义，因为它是一个美化的“if” - 如果你返回，你就不可能再次循环。