Question

我是android studio的新手。我正在创建一个应用程序，它将使用套接字在PC和移动设备之间传递数据。我已将基于命令的程序与android相结合。在这种情况下，客户端将号码发送到服务器，服务器将检查它是否为素数，根据它，服务器将回复发送给客户端。但是我在启动主要活动时遇到致命异常。

Logcat是：

08-18 11:59:26.977 1728-1728/com.example.admin.test_purpose W/dalvikvm: threadid=1: thread exiting with uncaught exception (group=0xb4e68288)
08-18 11:59:26.977 1728-1728/com.example.admin.test_purpose E/AndroidRuntime: 
FATAL EXCEPTION: main java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.admin.test_purpose/com.example.admin.test_purpose.MainActivity}: java.lang.NullPointerException

 at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1983)
   at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
 at android.app.ActivityThread.access$600(ActivityThread.java:130)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:4745)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
at android.app.Activity.findViewById(Activity.java:1825)
at com.example.admin.test_purpose.MainActivity.<init>(MainActivity.java:15)
at java.lang.Class.newInstanceImpl(Native Method)
at java.lang.Class.newInstance(Class.java:1319)
at android.app.Instrumentation.newActivity(Instrumentation.java:1053)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1974)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084) 
at android.app.ActivityThread.access$600(ActivityThread.java:130) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195) 
at android.os.Handler.dispatchMessage(Handler.java:99) 
at android.os.Looper.loop(Looper.java:137) 
at android.app.ActivityThread.main(ActivityThread.java:4745) 
at java.lang.reflect.Method.invokeNative(Native Method)
 at java.lang.reflect.Method.invoke(Method.java:511) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553) 
at dalvik.system.NativeStart.main(Native Method)

这是我的源文件

基于命令的服务器程序

import java.net.*; 
import java.io.*;

class tcpServerPrime {
public static void main(String args[]) {
    try {

        ServerSocket ss = new ServerSocket(8001); 
        System.out.println("Server Started..............."); 
        Socket s = ss.accept();
        DataInputStream in = new DataInputStream(s.getInputStream()); 
        int x= in.readInt();
        DataOutputStream otc = new DataOutputStream(s.getOutputStream()); 
        int y = x/2;

          if(x ==1 || x ==2 || x ==3) {
            otc.writeUTF(x + "is Prime"); 
            System.exit(0);
            }
          for(int i=2; i<=y; i++) {
            if(x%i != 0) {
            otc.writeUTF(x + " is Prime");
            } else {
            otc.writeUTF(x + " is not Prime");
            }

          }
    }catch(Exception e) {
        System.out.println(e.toString());
      }
 }

}

主要活动

package com.example.admin.test_purpose;

import android.os.Bundle;
import android.app.Activity;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;
import java.net.*;
import java.io.*;


public class MainActivity extends Activity {
    EditText user_num = (EditText)findViewById(R.id.editText);
    TextView reply  = (TextView)findViewById(R.id.textView);
    Button btnsbm = (Button)findViewById(R.id.button);
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        onclicklistener();
    }
    public void onclicklistener(){
        btnsbm.setOnClickListener(
                new View.OnClickListener() {
                    @Override
                    public void onClick(View v) {
                             try {
                                Socket cs = new Socket("192.168.1.101", 8001);
                                int a = Integer.parseInt(user_num.getText().toString());
                                DataOutputStream out = new DataOutputStream(cs.getOutputStream());
                                out.writeInt(a);
                                DataInputStream in = new DataInputStream(cs.getInputStream());
                                reply.setText(in.readUTF());
                                cs.close();
                            } catch (Exception e) {
                                Toast.makeText(MainActivity.this, e.toString(), Toast.LENGTH_SHORT).show();
                            }
                    }
                }
        );
    }

}

activity_main.xml中

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    >

    <EditText
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:id="@+id/editText"
        android:layout_gravity="center_horizontal"
        android:hint="@string/user_value"
        android:layout_marginTop="15dp" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/submit"
        android:id="@+id/button"
        android:layout_gravity="center_horizontal"
        android:layout_marginTop="15dp" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:id="@+id/textView"
        android:layout_gravity="center_horizontal"
        android:layout_marginTop="20dp" />
</LinearLayout>

的AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.admin.test_purpose">
    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>
    <uses-permission android:name="android.permission.ACCESS_WIFI_STATE"/>
    <uses-permission android:name="android.permission.INTERNET"/>
</manifest>

Answer 1

问题在于这三行：

EditText user_num = (EditText)findViewById(R.id.editText);
TextView reply  = (TextView)findViewById(R.id.textView);
Button btnsbm = (Button)findViewById(R.id.button);

问题是，您在视图甚至初始化之前调用findViewById() 。

将findViewById - 组件移至onCreate()，然后它应该有效：

EditText user_num; TextView reply; Button btnsbm; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); user_num = (EditText)findViewById(R.id.editText); reply = (TextView)findViewById(R.id.textView); btnsbm = (Button)findViewById(R.id.button); onclicklistener(); }

Answer 2

更新主要活动

EditText user_num;
TextView reply;
Button btnsbm;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    user_num = (EditText)findViewById(R.id.editText);
    reply  = (TextView)findViewById(R.id.textView);
    btnsbm = (Button)findViewById(R.id.button);
    onclicklistener();
}

启动应用程序时出现致命异常

2 个答案: