顯示java.lang.NullPointerException在主要活動

Question

我前幾天有一個正常運作的程序，我換了幾個按鈕的點左右，現在收到此錯誤：

E/AndroidRuntime: FATAL EXCEPTION: main 
       Process: com.example.jack.synthesiser, PID: 10903 
       java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.jack.synthesiser/com.example.jack.synthesiser.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:2387) 
        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:5254) 
        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.jack.synthesiser.MainActivity.initializeView(MainActivity.java:59) 
        at com.example.jack.synthesiser.MainActivity.onCreate(MainActivity.java:38) 
        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:2387)  
        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:5254)  
        at java.lang.reflect.Method.invoke(Native Method)  
        at java.lang.reflect.Method.invoke(Method.java:372)  

這可能是一個簡單的錯誤，但不能似乎找到一個修復它，任何幫助將不勝感激。下面是我的XML。

<?xml version="1.0" encoding="utf-8"?> 
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" 
xmlns:tools="http://schemas.android.com/tools" 
android:id="@+id/activity_main" 
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" 
tools:context="com.example.jack.synthesiser.MainActivity"> 

<EditText 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:hint ="Enter frequency" 
    android:inputType="textPersonName" 

    android:ems="10" 
    android:layout_alignParentTop="true" 
    android:layout_alignParentEnd="true" 
    android:layout_marginEnd="218dp" 
    android:id="@+id/editText" /> 


<RadioGroup 
android:layout_width="wrap_content" 
android:layout_height="wrap_content" 
    android:orientation="vertical" 
    android:layout_alignParentTop="true" 
    android:layout_alignParentStart="true" 
    android:id="@+id/radioGroup"> 

    <RadioButton 
     android:text="Sine" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_below="@+id/editText" 
     android:layout_alignParentStart="true" 
     android:layout_marginStart="15dp" 
     android:layout_marginTop="15dp" 
     android:id="@+id/radioButton" /> 

    <RadioButton 
    android:text="Square" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:id="@+id/radioButton2" 
    android:layout_above="@+id/toggleButton" 
    android:layout_toEndOf="@+id/radioButton" 
    android:layout_marginStart="15dp" /> 

    <RadioButton 
    android:text="Saw" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:id="@+id/radioButton3" 
    android:layout_marginStart="15dp" 
    android:layout_alignBaseline="@+id/radioButton2" 
    android:layout_alignBottom="@+id/radioButton2" 
    android:layout_toEndOf="@+id/radioButton2" /> 

</RadioGroup> 

<Button 
    android:text="Button" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:id="@+id/button" 
    android:layout_marginStart="55dp" 
    android:layout_below="@+id/radioGroup" 
    android:layout_alignStart="@+id/editText" 
    android:layout_marginTop="44dp" /> 

<SeekBar 
    android:layout_width="wrap_content" 
    android:layout_height="100dp" 
    android:id="@+id/Volume" 
    android:layout_marginStart="52dp" 
    android:layout_alignParentTop="true" 
    android:layout_toEndOf="@+id/button" /> 

<TextView 
    android:text="@string/textview" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:id="@+id/textView" 
    android:layout_alignTop="@+id/button" 
    android:layout_alignStart="@+id/Volume" /> 

</RelativeLayout> 

這是主要的活動文件：

package com.example.jack.synthesiser; 

import android.content.Context; 
import android.media.AudioManager; 
import android.support.v7.app.AppCompatActivity; 
import android.os.Bundle; 
import android.view.MotionEvent; 
import android.view.View; 
import android.widget.Button; 
import android.widget.EditText; 
import android.widget.SeekBar; 
import android.widget.TextView; 
import android.widget.ToggleButton; 
import android.widget.RadioButton; 


public class MainActivity extends AppCompatActivity implements  View.OnClickListener, View.OnTouchListener, SeekBar.OnSeekBarChangeListener { 

EditText frequencyInput; 
TextView displayFrequency; 
Button startStop; 
RadioButton sine; 
RadioButton square; 
RadioButton saw; 
PlaySine sineWave = new PlaySine(); 
PlaySquare squareWave = new PlaySquare(); 
PlaySaw sawWave = new PlaySaw(); 
//Filter attack = new Filter(); 
SeekBar amplitude; 
AudioManager audioManager; 
float curVolume, maxVolume, volume; 



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

//protected void volumeSounds() { 
// audioManager = (AudioManager) getSystemService(AUDIO_SERVICE); 
// curVolume = (float)audioManager.getStreamVolume(AudioManager.STREAM_MUSIC); 
// maxVolume = (float)audioManager.getStreamMaxVolume(AudioManager.STREAM_MUSIC); 

    // volume = curVolume/maxVolume; 
//} 
private void initializeView() { 

    frequencyInput = (EditText) findViewById(R.id.editText); 
    startStop = (Button) findViewById(R.id.toggleButton); 
    sine = (RadioButton) findViewById(R.id.radioButton); 
    square = (RadioButton) findViewById(R.id.radioButton2); 
    saw = (RadioButton) findViewById(R.id.radioButton3); 
    //startStopSquare = (ToggleButton) findViewById(R.id.toggleButton2); 
    //startStopSaw = (ToggleButton) findViewById(R.id.toggleButton3); 
    displayFrequency = (TextView) findViewById(R.id.textView); 
    startStop.setOnTouchListener(this); 
    amplitude = (SeekBar) findViewById(R.id.Volume); 
    audioManager = (AudioManager)getSystemService(Context.AUDIO_SERVICE); 
    maxVolume = audioManager.getStreamMaxVolume(AudioManager.STREAM_MUSIC); 
    volume = audioManager.getStreamVolume(AudioManager.STREAM_MUSIC); 
    amplitude.setOnSeekBarChangeListener(this); 
    //startStopSquare.setOnClickListener(this); 
    //startStopSaw.setOnClickListener(this); 
} 

@Override 
public void onClick(View v) { 
    // TODO Auto-generated method stub 
    int frequency = Integer.parseInt(frequencyInput.getText().toString()); 
    displayFrequency.setText(String.valueOf(frequency)); 
    sineWave.setSine(frequency); 
    squareWave.setSquareWave(frequency); 
    sawWave.setSawWave(frequency); 

} 

@Override 
public void onProgressChanged(SeekBar seekBar, int progress, boolean fromUser) { 

} 

@Override 
public void onStartTrackingTouch(SeekBar seekBar) { 

} 

@Override 
public void onStopTrackingTouch(SeekBar seekBar) { 

} 

@Override 
public boolean onTouch(View v, MotionEvent event) { 
    boolean on = ((startStop.isPressed()) && sine.isChecked()); 
    boolean sqOn = ((startStop.isPressed()) && square.isChecked()); 
    boolean sawOn =((startStop.isPressed()) && saw.isChecked()); 
    if (on) { 
     sineWave.start(); 
    } else if (!on){ 
     sineWave.stop(); 
    } 
    if (sqOn) { 
     squareWave.start(); 
    } else if (!sqOn) { 
     squareWave.stop(); 
    } 
    if (sawOn) { 
     sawWave.start(); 
    } else if (!sawOn) { 
     sawWave.stop(); 
    } 
    return false; 
} 
} 

Answer 1

，因爲當你嘗試調用.setOnClickListener()上的按鈕變量是null的一個你所得到的錯誤。例如，確保您首先使用findViewById()初始化變量。