java - 如何修复无法启动 Activity ComponentInfo？

Question

我的问题是当我运行应用程序并单击注册按钮时突然显示弹出窗口:“强制关闭”

这是我的代码:

Main.java

package com.example.server;
import java.util.Timer;
import java.util.TimerTask;
import android.app.Activity;
import android.app.ProgressDialog;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

public class Main extends Activity {
    private Button login,register,exit;
    private EditText usertext,passtext;
    public static String res="";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        login=(Button) findViewById(R.id.login);
        register=(Button) findViewById(R.id.register);
        exit=(Button) findViewById(R.id.exit);
        usertext=(EditText) findViewById(R.id.usertext);
        passtext=(EditText) findViewById(R.id.passtext);
        login.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View arg0) {
                login(usertext.getText().toString(),passtext.getText().toString());
            }
        });
        register.setOnClickListener(new OnClickListener() {
            public void onClick(View arg0) {
                Intent re=new Intent(Main.this,register.class); 
                startActivity(re);
            }
        });
        exit.setOnClickListener(new OnClickListener() {
            public void onClick(View arg0) {
                finish();
            }
        });
    }
    public void login(String user,String pass) {
        new loginserver("http://ujo.ir/login,php",user,pass).execute();
        final ProgressDialog pd=new ProgressDialog(Main.this);
        pd.setMessage("Loading...");
        pd.show();
        final Timer tm=new Timer();
        tm.schedule(new TimerTask() {
            public void run() {
                runOnUiThread(new Runnable() {
                    public void run() {
                        if(!res.equals("")){
                            pd.cancel();
                            Toast.makeText(getApplicationContext(), res, Toast.LENGTH_LONG).show();
                            res="";
                            tm.cancel();
                        }
                    }
                });
            }
        }, 1, 1000);
    }
}

注册.java

import java.io.ObjectOutputStream.PutField;
import java.util.Timer;
import java.util.TimerTask;
import com.example.server.R.layout;
import android.app.Activity;
import android.app.ProgressDialog;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;
public class register extends Activity {    
    public static String res="";
    private Button register,exit;
    private EditText name,family,user,pass,email;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.register);
        exit.setOnClickListener(new OnClickListener() {
            public void onClick(View arg0) {
                finish();
            }
        });
        register.setOnClickListener(new OnClickListener() {
            public void onClick(View arg0) {
                register1(name.getText().toString(),family.getText().toString(),user.getText().toString(),pass.getText().toString(),email.getText().toString());
            }
        });
    }
    public void register1(String name,String family,String user,String pass,String email) {         
        new registerserver("http://ujo.ir/register.php", name, family, user, pass, email).execute();
        final ProgressDialog pd=new ProgressDialog(this);
        pd.setMessage("Loading...");
        pd.show();
        final Timer tm=new Timer();
        tm.scheduleAtFixedRate(new TimerTask() {
            public void run() {
                runOnUiThread(new Runnable() {
                    public void run() {
                        if(!res.equals("")) {
                            pd.cancel();
                            Toast.makeText(getApplicationContext(), res, Toast.LENGTH_LONG).show();
                            res="";
                            tm.cancel();
                        }
                    }
                });
            }
        }, 1, 1000);            
    }
}

registerserver.java

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.net.URL;
import java.net.URLConnection;
import java.net.URLEncoder;
import android.os.AsyncTask;
public class registerserver extends AsyncTask {
    private String Name="";
    private String Family="";
    private String Link="";
    private String User="";
    private String Pass="";
    private String Email="";
    public registerserver(String link,String name,String family,String user,String pass,String email) {
        name=Name;
        family=Family;
        link=Link;
        user=User;
        pass=Pass;
        email=Email;
    }
    @Override
    protected String doInBackground(Object... arg0) {
        try {
            String data=URLEncoder.encode("name","UTF8")+"="+URLEncoder.encode(Name,"UTF8");
            data+="&"+URLEncoder.encode("family","UTF8")+"="+URLEncoder.encode(Family,"UTF8");
            data+="&"+URLEncoder.encode("username","UTF8")+"="+URLEncoder.encode(User,"UTF8");
            data+="&"+URLEncoder.encode("password","UTF8")+"="+URLEncoder.encode(Pass,"UTF8");
            data+="&"+URLEncoder.encode("email","UTF8")+"="+URLEncoder.encode(Email,"UTF8");
            data+="&"+URLEncoder.encode("status","UTF8")+"="+URLEncoder.encode("a","UTF8");         
            URL mylink=new URL(Link);
            URLConnection connect=mylink.openConnection();
            connect.setDoOutput(true);
            OutputStreamWriter wr=new OutputStreamWriter(connect.getOutputStream());
            wr.write(data);
            wr.flush();
            BufferedReader reader=new BufferedReader(new InputStreamReader(connect.getInputStream()));
            StringBuilder sb=new StringBuilder();
            String line=null;
            while((line=reader.readLine()) != null) {
                sb.append(line);
            }
            register.res=sb.toString(); 
        } catch (Exception e) {}
        return "";
    }
}

最后，LogCat

08-04 21:19:11.704: D/AndroidRuntime(2066): Shutting down VM
08-04 21:19:11.704: W/dalvikvm(2066): threadid=1: thread exiting with uncaught exception (group=0xa000f180)
08-04 21:19:11.704: E/AndroidRuntime(2066): FATAL EXCEPTION: main
08-04 21:19:11.704: E/AndroidRuntime(2066): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.server/com.example.server.register}: java.lang.NullPointerException
08-04 21:19:11.704: E/AndroidRuntime(2066): 	at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1956)
08-04 21:19:11.704: E/AndroidRuntime(2066): 	at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1981)
08-04 21:19:11.704: E/AndroidRuntime(2066): 	at android.app.ActivityThread.access$600(ActivityThread.java:123)
08-04 21:19:11.704: E/AndroidRuntime(2066): 	at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1147)
08-04 21:19:11.704: E/AndroidRuntime(2066): 	at android.os.Handler.dispatchMessage(Handler.java:99)
08-04 21:19:11.704: E/AndroidRuntime(2066): 	at android.os.Looper.loop(Looper.java:137)
08-04 21:19:11.704: E/AndroidRuntime(2066): 	at android.app.ActivityThread.main(ActivityThread.java:4424)
08-04 21:19:11.704: E/AndroidRuntime(2066): 	at java.lang.reflect.Method.invokeNative(Native Method)
08-04 21:19:11.704: E/AndroidRuntime(2066): 	at java.lang.reflect.Method.invoke(Method.java:511)
08-04 21:19:11.704: E/AndroidRuntime(2066): 	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
08-04 21:19:11.704: E/AndroidRuntime(2066): 	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
08-04 21:19:11.704: E/AndroidRuntime(2066): 	at dalvik.system.NativeStart.main(Native Method)
08-04 21:19:11.704: E/AndroidRuntime(2066): Caused by: java.lang.NullPointerException
08-04 21:19:11.704: E/AndroidRuntime(2066): 	at com.example.server.register.onCreate(register.java:30)
08-04 21:19:11.704: E/AndroidRuntime(2066): 	at android.app.Activity.performCreate(Activity.java:4465)
08-04 21:19:11.704: E/AndroidRuntime(2066): 	at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049)
08-04 21:19:11.704: E/AndroidRuntime(2066): 	at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1920)
08-04 21:19:11.704: E/AndroidRuntime(2066): 	... 11 more
08-04 21:19:11.724: D/dalvikvm(2066): GC_CONCURRENT freed 147K, 3% free 6716K/6919K, paused 0ms+0ms
08-04 21:19:12.174: I/dalvikvm(2066): threadid=3: reacting to signal 3
08-04 21:19:12.174: I/dalvikvm(2066): Wrote stack traces to '/data/anr/traces.txt'
08-04 21:19:12.234: I/dalvikvm(2066): threadid=3: reacting to signal 3
08-04 21:19:12.234: I/dalvikvm(2066): Wrote stack traces to '/data/anr/traces.txt'

Answer 1

您忘记在注册类 Activity 中实例化按钮“退出”和“注册”。这就是为什么你会得到空指针异常。通过阅读日志，很容易得出这个结论;)

在继续之前，我建议实例化所有其他元素!