java.lang.NullPointerException 无法找出导致错误的原因

Question

我正在为类作业创建一个非常基本的 POS 系统。我不断收到那个可怕的空指针异常。我知道错误来自主要 Activity 中的第 72 行，但它在主要 Activity 和 xml 中都有声明。

这是日志:

10-07 18:05:36.946: D/AndroidRuntime(1279): Shutting down VM
10-07 18:05:36.946: W/dalvikvm(1279): threadid=1: thread exiting with uncaught        exception (group=0x414c4700)
10-07 18:05:36.958: E/AndroidRuntime(1279): FATAL EXCEPTION: main
10-07 18:05:36.958: E/AndroidRuntime(1279): java.lang.RuntimeException: Unable to start activity ComponentInfo{hotchkissmobilesolutions.kudlerpos/hotchkissmobilesolutions.kudlerpos.MainActivity}: java.lang.NullPointerException
10-07 18:05:36.958: E/AndroidRuntime(1279):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)
10-07 18:05:36.958: E/AndroidRuntime(1279):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
10-07 18:05:36.958: E/AndroidRuntime(1279):     at android.app.ActivityThread.access$600(ActivityThread.java:141)
10-07 18:05:36.958: E/AndroidRuntime(1279):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
10-07 18:05:36.958: E/AndroidRuntime(1279):     at android.os.Handler.dispatchMessage(Handler.java:99)
10-07 18:05:36.958: E/AndroidRuntime(1279):     at android.os.Looper.loop(Looper.java:137)
10-07 18:05:36.958: E/AndroidRuntime(1279):     at android.app.ActivityThread.main(ActivityThread.java:5103)
10-07 18:05:36.958: E/AndroidRuntime(1279):     at java.lang.reflect.Method.invokeNative(Native Method)
10-07 18:05:36.958: E/AndroidRuntime(1279):     at java.lang.reflect.Method.invoke(Method.java:525)
10-07 18:05:36.958: E/AndroidRuntime(1279):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
10-07 18:05:36.958: E/AndroidRuntime(1279):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
10-07 18:05:36.958: E/AndroidRuntime(1279):     at dalvik.system.NativeStart.main(Native Method)
10-07 18:05:36.958: E/AndroidRuntime(1279): Caused by: java.lang.NullPointerException
10-07 18:05:36.958: E/AndroidRuntime(1279):     at hotchkissmobilesolutions.kudlerpos.MainActivity.onCreate(MainActivity.java:72)
10-07 18:05:36.958: E/AndroidRuntime(1279):     at android.app.Activity.performCreate(Activity.java:5133)
10-07 18:05:36.958: E/AndroidRuntime(1279):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
10-07 18:05:36.958: E/AndroidRuntime(1279):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)
10-07 18:05:36.958: E/AndroidRuntime(1279):     ... 11 more

这是安卓 list :

    <?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="hotchkissmobilesolutions.kudlerpos"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="8"
        android:targetSdkVersion="18" />

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name="hotchkissmobilesolutions.kudlerpos.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>
    </application>

</manifest>

这是主要 Activity :

package hotchkissmobilesolutions.kudlerpos;
import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.GridLayout;
import android.widget.TextView;

public class MainActivity extends Activity {

double dairy = 4.99, seafood = 15.99, wine = 20, meat = 5.99,
        cheese = 2.99, fruit = 1.99, vegetable = 0.69;
double total, tax = 0.0725;
String total2;
Button btnDairy, btnSeafood, btnWine, btnMeat, btnFruit, btnVegetable,
        btnTotal, btnClear;

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

    btnDairy = (Button) findViewById(R.id.btnDairy);
    btnSeafood = (Button) findViewById(R.id.btnSeafood);
    btnWine = (Button) findViewById(R.id.btnWine);
    btnMeat = (Button) findViewById(R.id.btnMeat);
    btnFruit = (Button) findViewById(R.id.btnFruit);
    btnVegetable = (Button) findViewById(R.id.btnVegetable);
    btnClear = (Button) findViewById(R.id.btnClear);


    btnDairy.setOnClickListener(new OnClickListener() {
        public void onClick(View V) {
            total = (total + dairy) + (total * tax);
            System.out.println(total);
        }

    });
    btnSeafood.setOnClickListener(new OnClickListener() {
        public void onClick(View V) {
            total = (total + seafood) + (total * tax);
            System.out.println(total);
        }
    });
    btnWine.setOnClickListener(new OnClickListener() {
        public void onClick(View V) {
            total = (total + wine) + (total * tax);
            System.out.println(total);
        }
    });
    btnMeat.setOnClickListener(new OnClickListener() {
        public void onClick(View V) {
            total = (total + meat) + (total * tax);
            System.out.println(total);
        }
    });
    btnFruit.setOnClickListener(new OnClickListener() {
        public void onClick(View V) {
            total = (total + fruit) + (total * tax);
            System.out.println(total);
        }
    });
    btnVegetable.setOnClickListener(new OnClickListener() {
        public void onClick(View V) {
            total = (total + vegetable) + (total * tax);
            System.out.println(total);
        }
    });
    btnTotal.setOnClickListener(new OnClickListener() {
        public void onClick(View V) {
            total2 = new Double(total).toString();
            final TextView textView = (TextView) findViewById(R.id.textViewTotal);
            textView.setText(total2);
            System.out.println(total);
            System.out.println(total2);
        }
    });
    btnClear.setOnClickListener(new OnClickListener() {
        public void onClick(View V) {
            total = 0;
            total2 = new Double(total).toString();
            final TextView textView = (TextView) findViewById(R.id.textViewTotal);
            textView.setText(total2);
            System.out.println(total);
        }
    });

}

这是 activity_main.xml:

<?xml version="1.0" encoding="UTF-8"?>
<GridLayout 
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:columnCount="6"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:rowCount="8"
    tools:context=".MainActivity"
    tools:targetApi="14" >

    <TextView android:id="@+id/txtViewInstructions"
        android:layout_column="0"
        android:layout_gravity="left|top"
        android:layout_row="0" 
        android:text="@string/InstructionText"
        android:textAppearance="?android:attr/textAppearanceLarge" />

    <Button android:id="@+id/btnDairy" 
        android:layout_column="0"
        android:layout_gravity="left|top" 
        android:layout_row="3"
        android:onClick="onCLickDairy" 
        android:text="@string/Dairy" />

    <Button android:id="@+id/btnWine" 
        android:layout_column="0"
        android:layout_gravity="center_horizontal|top" 
        android:layout_row="3"
        android:onClick="onCLickWine" 
        android:text="@string/Wine" />

    <Button android:id="@+id/btnMeat" 
        android:layout_column="0"
        android:layout_gravity="left|top" 
        android:layout_row="4"
        android:onClick="onCLickMeat" 
        android:text="@string/Meat" />

    <Button android:id="@+id/btnSeafood" 
        android:layout_column="0"
        android:layout_gravity="center_horizontal|top" 
        android:layout_row="4"
        android:onClick="onCLickSeafood" 
        android:text="@string/Seafood" />

    <Button android:id="@+id/btnVegetable" 
        android:layout_column="0"
        android:layout_gravity="left|top" 
        android:layout_row="5"
        android:onClick="onCLickVegetable" 
        android:text="@string/Vegetable" />

    <TextView android:id="@+id/txtViewTotalDue"
        android:layout_column="0" 
        android:layout_gravity="left|top"
        android:layout_row="7" 
        android:text="@string/TotalDue"
        android:textAppearance="?android:attr/textAppearanceLarge" />

    <TextView android:id="@+id/textViewTotal"
        android:layout_width="260dp" 
        android:layout_height="93dp"
        android:layout_column="0" 
        android:layout_gravity="left|bottom"
        android:layout_row="6" 
        android:text="@string/total"
        android:textAppearance="?android:attr/textAppearanceLarge" />

    <Button android:id="@+id/btnTotal" 
        android:layout_column="0"
        android:layout_gravity="left|center_vertical" 
        android:layout_row="7"
        android:onClick="onCLickTotal" 
        android:text="@string/TotalButton" />

    <Button android:id="@+id/btnFruit" 
        android:layout_column="0"
        android:layout_gravity="center_horizontal|top" 
        android:layout_row="5"
        android:onClick="onCLickFruit" 
        android:text="@string/Fruit" />

    <Button android:id="@+id/btnClear" 
        android:layout_column="0"
        android:layout_gravity="left|bottom" 
        android:layout_row="7"
        android:text="@string/ClearButton" />

</GridLayout>

任何帮助查找主要 activity.java 中 btnTotal 行的问题都会很有帮助。提前致谢!

Answer 1

您没有 btnTotal，因此当您单击它时，值为 null。

参见:

btnDairy = (Button) findViewById(R.id.btnDairy);
    btnSeafood = (Button) findViewById(R.id.btnSeafood);
    btnWine = (Button) findViewById(R.id.btnWine);
    btnMeat = (Button) findViewById(R.id.btnMeat);
    btnFruit = (Button) findViewById(R.id.btnFruit);
    btnVegetable = (Button) findViewById(R.id.btnVegetable);
    btnClear = (Button) findViewById(R.id.btnClear);
    //add this
    btnTotal = (Button) findViewById(R.id.btnTotal);

btnTotal 没有按钮。添加那个按钮，你会没事的。简单的错误。一直在发生。祝你好运。