切換活動Android Studio

Question

我為嘗試在Android Studio中進行切換活動而感到非常疲倦，我知道也許這個問題太重復了，但是在我啟動項目后，請幫我，很多錯誤

，這是我的MainActivity

 package com.example.satan.custommm; import android.support.v7.app.AppCompatActivity; import android.os.Bundle; import android.view.Menu; import android.content.Intent; import android.view.MenuItem; import android.view.View; import android.widget.Button; import android.widget.EditText; import android.widget.TextView; public class MainActivity extends AppCompatActivity { private Button btt1; private EditText et1; private EditText et2; private TextView tv1; private TextView tv2; private TextView tv4; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); run(); btt1 = (Button) findViewById(R.id.button); et1 = (EditText) findViewById(R.id.editText); et2 = (EditText) findViewById(R.id.editText2); tv1 = (TextView) findViewById(R.id.textView); tv2 = (TextView) findViewById(R.id.textView2); tv4 = (TextView) findViewById(R.id.textView4); } public void run() { btt1.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { if (et1.getText().toString().equals("admin") && et2.getText().toString().equals("admin")) { Intent i = new Intent(MainActivity.this, Main2Activity.class); startActivity(i); } else { tv4.setText("Wrong Username/Password!"); } } }); } @Override public boolean onCreateOptionsMenu(Menu menu) { // Inflate the menu; this adds items to the action bar if it is present. getMenuInflater().inflate(R.menu.menu_main, menu); return true; } @Override public boolean onOptionsItemSelected(MenuItem item) { // Handle action bar item clicks here. The action bar will // automatically handle clicks on the Home/Up button, so long // as you specify a parent activity in AndroidManifest.xml. int id = item.getItemId(); //noinspection SimplifiableIfStatement if (id == R.id.action_settings) { return true; } return super.onOptionsItemSelected(item); } } 

這是我的manifest.xml

 <?xml version="1.0" encoding="utf-8"?> <manifest xmlns:android="http://schemas.android.com/apk/res/android" package="com.example.satan.custommm" > <application android:allowBackup="true" android:icon="@mipmap/ic_launcher" android:label="@string/app_name" android:theme="@style/AppTheme" > <activity android:name=".MainActivity" android:label="@string/app_name" > <intent-filter> <action android:name="android.intent.action.MAIN" /> <category android:name="android.intent.category.LAUNCHER" /> </intent-filter> </activity> <activity android:name=".Main2Activity" android:label="@string/app_name" > </activity> </application> </manifest> 

在啟動項目后，出現以下錯誤：

 09-02 21:09:05.998 26875-26875/? I/art﹕ Not late-enabling -Xcheck:jni (already on) 09-02 21:09:06.049 26875-26882/? E/art﹕ Failed sending reply to debugger: Broken pipe 09-02 21:09:06.049 26875-26882/? I/art﹕ Debugger is no longer active 09-02 21:09:06.205 26875-26887/? I/art﹕ Background sticky concurrent mark sweep GC freed 2736(226KB) AllocSpace objects, 0(0B) LOS objects, 0% free, 13MB/13MB, paused 11.436ms total 32.643ms 09-02 21:09:06.339 26875-26875/? D/AndroidRuntime﹕ Shutting down VM 09-02 21:09:06.339 26875-26875/? E/AndroidRuntime﹕ FATAL EXCEPTION: main Process: com.example.satan.custommm, PID: 26875 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.satan.custommm/com.example.satan.custommm.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390) at android.app.ActivityThread.access$800(ActivityThread.java:151) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:135) at android.app.ActivityThread.main(ActivityThread.java:5257) at java.lang.reflect.Method.invoke(Native Method) at java.lang.reflect.Method.invoke(Method.java:372) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference at com.example.satan.custommm.MainActivity.run(MainActivity.java:42) at com.example.satan.custommm.MainActivity.onCreate(MainActivity.java:28) at android.app.Activity.performCreate(Activity.java:5990) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)            at android.app.ActivityThread.access$800(ActivityThread.java:151)            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)            at android.os.Handler.dispatchMessage(Handler.java:102)            at android.os.Looper.loop(Looper.java:135)            at android.app.ActivityThread.main(ActivityThread.java:5257)            at java.lang.reflect.Method.invoke(Native Method)            at java.lang.reflect.Method.invoke(Method.java:372)            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698) 

對不起，我英語不好

Answer 1

您有一個java.lang.NullPointerException因為您正在調用

btt1.setOnClickListener

在初始化btt1對象之前。 然后，您在null obj（btt1 = null）上調用方法。

更改您的onCreate方法

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        btt1 = (Button) findViewById(R.id.button);
        et1 = (EditText) findViewById(R.id.editText);
        et2 = (EditText) findViewById(R.id.editText2);
        tv1 = (TextView) findViewById(R.id.textView);

        tv2 = (TextView) findViewById(R.id.textView2);
        tv4 = (TextView) findViewById(R.id.textView4);


        // move this line
        run();
    }

Answer 2

在調用run（）方法之前初始化您的視圖