Android Studio - java.lang.NullPointerException：嘗試調用虛擬方法'void android.widget.Button.setOnClickListener

Question

我在我的android studio應用程序中創建了一個新的活動，並且我有一個按鈕，可以在單擊時執行操作。我不確定是什麼原因導致了錯誤，因爲這發生在我實際運行該活動時。如果我刪除了onClick代碼，則該活動將運行。謝謝你的幫助。我得到這個錯誤：

E/AndroidRuntime: FATAL EXCEPTION: main 
        Process: com.masesk.kalkulator, PID: 4102 
        java.lang.RuntimeException: Unable to start activity ComponentInfo{com.masesk.kalkulator/com.masesk.kalkulator.Main2Activity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference 
         at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416) 
         at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
         at android.app.ActivityThread.-wrap11(ActivityThread.java) 
         at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
         at android.os.Handler.dispatchMessage(Handler.java:102) 
         at android.os.Looper.loop(Looper.java:148) 
         at android.app.ActivityThread.main(ActivityThread.java:5417) 
         at java.lang.reflect.Method.invoke(Native Method) 
         at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 
        Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference 
         at com.masesk.kalkulator.Main2Activity.onCreate(Main2Activity.java:29) 
         at android.app.Activity.performCreate(Activity.java:6237) 
         at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107) 
         at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369) 
         at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)  
         at android.app.ActivityThread.-wrap11(ActivityThread.java)  
         at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)  
         at android.os.Handler.dispatchMessage(Handler.java:102)  
         at android.os.Looper.loop(Looper.java:148)  
         at android.app.ActivityThread.main(ActivityThread.java:5417)  
         at java.lang.reflect.Method.invoke(Native Method)  
         at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)  
         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 

這是我的Main2Activity.java代碼：

package com.masesk.kalkulator; 

import android.os.Bundle; 
import android.support.design.widget.FloatingActionButton; 
import android.support.design.widget.Snackbar; 
import android.support.v7.app.AppCompatActivity; 
import android.support.v7.widget.Toolbar; 
import android.view.View; 
import android.widget.Button; 
import android.widget.EditText; 
import android.widget.TextView; 
import android.widget.ToggleButton; 

public class Main2Activity extends AppCompatActivity { 
    private ToggleButton textToBinary; 
    private Button trace; 
    private EditText text; 
    private EditText binary; 
    private String textInput; 
    private String binaryInput; 
    StringBuilder binaryText = new StringBuilder(); 
    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     this.textToBinary = (ToggleButton)findViewById(R.id.textToBinary); 
     this.trace = (Button)findViewById(R.id.trace); 
     this.text = (EditText)findViewById(R.id.text); 
     this.binary = (EditText)findViewById(R.id.binary); 

     trace.setOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View view) { 
       if(textToBinary.isActivated() == false){ 
        textInput = text.getText().toString(); 
        byte[] bytes = textInput.getBytes(); 
        StringBuilder binary = new StringBuilder(); 
        for (byte b : bytes) 
        { 
         int val = b; 
         for (int i = 0; i < 8; i++) 
         { 
          binaryText.append((val & 128) == 0 ? 0 : 1); 
          val <<= 1; 
         } 
         binaryText.append(' '); 
        } 
       } 
      } 
     }); 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main2); 
     Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar); 
     setSupportActionBar(toolbar); 

    } 
    @Override 
    public void onResume(){ 
     this.binary = (EditText)findViewById(R.id.binary); 
     binary.setText(binaryText.toString(), TextView.BufferType.EDITABLE); 
    } 

} 

這裏是我的XML代碼

<?xml version="1.0" encoding="utf-8"?> 
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" 
    xmlns:app="http://schemas.android.com/apk/res-auto" 
    xmlns:tools="http://schemas.android.com/tools" 
    android:id="@+id/content_main2" 
    android:layout_width="match_parent" 
    android:layout_height="match_parent" 
    android:paddingBottom="@dimen/activity_vertical_margin" 
    android:paddingLeft="@dimen/activity_horizontal_margin" 
    android:paddingRight="@dimen/activity_horizontal_margin" 
    android:paddingTop="@dimen/activity_vertical_margin" 
    app:layout_behavior="@string/appbar_scrolling_view_behavior" 
    tools:context="com.masesk.kalkulator.Main2Activity" 
    tools:showIn="@layout/activity_main2"> 

    <ToggleButton 
     android:text="ToggleButton" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_alignParentTop="true" 
     android:layout_alignParentStart="true" 
     android:id="@+id/textToBinary" 
     android:layout_alignParentEnd="true" 
     android:textOff="Binary To Text" 
     android:textOn="Text to Binary" /> 

    <TextView 
     android:text="Text:" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_below="@+id/textToBinary" 
     android:layout_alignParentStart="true" 
     android:id="@+id/textView2" 
     android:layout_alignParentEnd="true" /> 

    <EditText 
     android:background="@drawable/back" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:inputType="textMultiLine" 
     android:ems="10" 
     android:layout_below="@+id/textView2" 
     android:layout_alignParentStart="true" 
     android:id="@+id/text" 
     android:layout_alignParentEnd="true" /> 

    <TextView 
     android:text="Binary Code:" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_below="@+id/text" 
     android:layout_alignParentStart="true" 
     android:id="@+id/textView3" /> 

    <EditText 
     android:background="@drawable/back" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:inputType="textMultiLine" 
     android:ems="10" 
     android:layout_below="@+id/textView3" 
     android:layout_alignParentStart="true" 
     android:id="@+id/binary" 
     android:layout_alignParentEnd="true" /> 

    <Button 
     android:text="convert" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_below="@+id/binary" 
     android:layout_alignParentStart="true" 
     android:layout_marginTop="14dp" 
     android:id="@+id/trace" 
     android:layout_alignParentEnd="true" /> 

</RelativeLayout> 

Answer 1

在OnCreate方法，你應該總是第一張放入

super.onCreate(savedInstanceState); 
setContentView(R.layout.activity_main2); 

這是因爲在您將其設置爲setContentView之前，視圖不會膨脹。這意味着任何findViewById之前的電話將返回null。

如Android文檔所述，您還必須設置super.onResume(...)呼叫開始onResume()。這也適用於其他生命週期方法。這也應該可以解決您的setText問題。