当我点击按钮时，为什么我的应用程序崩溃？

Question

我有一个按钮，id = button1，点击它时，它会运行函数onEnterClick();，但每当我点击它时，我的应用程序崩溃并且说它是unexpectedly quit。我将在下面提供java文件和xml，非常感谢任何帮助。

package tip.calculator; 

import android.os.Bundle; 
import android.app.Activity; 
import android.view.Menu; 
import android.widget.EditText; 
import android.widget.TextView; 
import android.view.View; 

public class MainActivity extends Activity { 

    final EditText myEditField = (EditText) findViewById(R.id.percentText); 
    final EditText myEditField2 = (EditText) findViewById(R.id.billText); 


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

    @Override 
    public boolean onCreateOptionsMenu(Menu menu) { 
     getMenuInflater().inflate(R.menu.activity_main, menu); 
     return true; 
    } 

    public void onEnterClick(View view){ //This Function Runs When The Button is clicked 

     TextView textView; 

     textView = 
       (TextView) findViewById(R.id.textView1); 

     final EditText myEditField = (EditText) findViewById(R.id.percentText); 
     final EditText myEditField2 = (EditText) findViewById(R.id.billText); 

     int percentage; 
     int percentageInp; 
     int billAmount; 
     int output; 
     String output1; 

     percentageInp = Integer.parseInt(myEditField.getText().toString()); 
     billAmount = Integer.parseInt(myEditField2.getText().toString()); 

     percentage = percentageInp /100; 
     output = (billAmount * percentage); 
     output1 = Integer.toString(output); 

     textView.setText(output1); 
    } 


} 

这是Java的^^ 这是XML：

<RelativeLayout 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:onClick="onEnterClick" > 

    <EditText 
     android:id="@+id/billText" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_centerHorizontal="true" 
     android:ems="10" 
     android:hint="Bill Amount" 
     android:inputType="numberDecimal|textAutoComplete" /> 

    <EditText 
     android:id="@+id/percentText" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_alignLeft="@+id/billText" 
     android:layout_below="@+id/billText" 
     android:ems="10" 
     android:hint="Percent" 
     android:inputType="number" > 

     <requestFocus /> 
    </EditText> 

    <Button 
     android:id="@+id/button1" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_below="@+id/percentText" 
     android:layout_centerHorizontal="true" 
     android:onClick="onEnterClick" 
     android:text="Calculate" /> 

    <TextView 
     android:id="@+id/textView1" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_alignParentTop="true" 
     android:layout_centerHorizontal="true" 
     android:layout_marginTop="287dp" 
     android:text="Solution Will Appear Here:" 
     android:textAppearance="?android:attr/textAppearanceLarge" /> 

</RelativeLayout> 

登录猫：

07-24 03:12:26.563: D/AndroidRuntime(412): Shutting down VM 
07-24 03:12:26.563: W/dalvikvm(412): threadid=1: thread exiting with uncaught exception (group=0x40014760) 
07-24 03:12:26.763: E/AndroidRuntime(412): FATAL EXCEPTION: main 
07-24 03:12:26.763: E/AndroidRuntime(412): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{tip.calculator/tip.calculator.MainActivity}: java.lang.NullPointerException 
07-24 03:12:26.763: E/AndroidRuntime(412): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1739) 
07-24 03:12:26.763: E/AndroidRuntime(412): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1831) 
07-24 03:12:26.763: E/AndroidRuntime(412): at android.app.ActivityThread.access$500(ActivityThread.java:122) 
07-24 03:12:26.763: E/AndroidRuntime(412): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1024) 
07-24 03:12:26.763: E/AndroidRuntime(412): at android.os.Handler.dispatchMessage(Handler.java:99) 
07-24 03:12:26.763: E/AndroidRuntime(412): at android.os.Looper.loop(Looper.java:132) 
07-24 03:12:26.763: E/AndroidRuntime(412): at android.app.ActivityThread.main(ActivityThread.java:4123) 
07-24 03:12:26.763: E/AndroidRuntime(412): at java.lang.reflect.Method.invokeNative(Native Method) 
07-24 03:12:26.763: E/AndroidRuntime(412): at java.lang.reflect.Method.invoke(Method.java:491) 
07-24 03:12:26.763: E/AndroidRuntime(412): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:841) 
07-24 03:12:26.763: E/AndroidRuntime(412): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:599) 
07-24 03:12:26.763: E/AndroidRuntime(412): at dalvik.system.NativeStart.main(Native Method) 
07-24 03:12:26.763: E/AndroidRuntime(412): Caused by: java.lang.NullPointerException 
07-24 03:12:26.763: E/AndroidRuntime(412): at android.app.Activity.findViewById(Activity.java:1745) 
07-24 03:12:26.763: E/AndroidRuntime(412): at tip.calculator.MainActivity.<init>(MainActivity.java:12) 
07-24 03:12:26.763: E/AndroidRuntime(412): at java.lang.Class.newInstanceImpl(Native Method) 
07-24 03:12:26.763: E/AndroidRuntime(412): at java.lang.Class.newInstance(Class.java:1301) 
07-24 03:12:26.763: E/AndroidRuntime(412): at android.app.Instrumentation.newActivity(Instrumentation.java:1022) 
07-24 03:12:26.763: E/AndroidRuntime(412): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1730) 
07-24 03:12:26.763: E/AndroidRuntime(412): ... 11 more 
07-24 03:12:34.871: I/Process(412): Sending signal. PID: 412 SIG: 9 
07-24 03:15:54.102: D/AndroidRuntime(453): Shutting down VM 
07-24 03:15:54.216: W/dalvikvm(453): threadid=1: thread exiting with uncaught exception (group=0x40014760) 
07-24 03:15:54.381: E/AndroidRuntime(453): FATAL EXCEPTION: main 
07-24 03:15:54.381: E/AndroidRuntime(453): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{tip.calculator/tip.calculator.MainActivity}: java.lang.NullPointerException 
07-24 03:15:54.381: E/AndroidRuntime(453): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1739) 
07-24 03:15:54.381: E/AndroidRuntime(453): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1831) 
07-24 03:15:54.381: E/AndroidRuntime(453): at android.app.ActivityThread.access$500(ActivityThread.java:122) 
07-24 03:15:54.381: E/AndroidRuntime(453): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1024) 
07-24 03:15:54.381: E/AndroidRuntime(453): at android.os.Handler.dispatchMessage(Handler.java:99) 
07-24 03:15:54.381: E/AndroidRuntime(453): at android.os.Looper.loop(Looper.java:132) 
07-24 03:15:54.381: E/AndroidRuntime(453): at android.app.ActivityThread.main(ActivityThread.java:4123) 
07-24 03:15:54.381: E/AndroidRuntime(453): at java.lang.reflect.Method.invokeNative(Native Method) 
07-24 03:15:54.381: E/AndroidRuntime(453): at java.lang.reflect.Method.invoke(Method.java:491) 
07-24 03:15:54.381: E/AndroidRuntime(453): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:841) 
07-24 03:15:54.381: E/AndroidRuntime(453): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:599) 
07-24 03:15:54.381: E/AndroidRuntime(453): at dalvik.system.NativeStart.main(Native Method) 
07-24 03:15:54.381: E/AndroidRuntime(453): Caused by: java.lang.NullPointerException 
07-24 03:15:54.381: E/AndroidRuntime(453): at android.app.Activity.findViewById(Activity.java:1745) 
07-24 03:15:54.381: E/AndroidRuntime(453): at tip.calculator.MainActivity.<init>(MainActivity.java:12) 
07-24 03:15:54.381: E/AndroidRuntime(453): at java.lang.Class.newInstanceImpl(Native Method) 
07-24 03:15:54.381: E/AndroidRuntime(453): at java.lang.Class.newInstance(Class.java:1301) 
07-24 03:15:54.381: E/AndroidRuntime(453): at android.app.Instrumentation.newActivity(Instrumentation.java:1022) 
07-24 03:15:54.381: E/AndroidRuntime(453): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1730) 
07-24 03:15:54.381: E/AndroidRuntime(453): ... 11 more 

Answer 1

调用setContentView(R.layout.activity_main);节之前，不要初始化UI元素。

改变你的代码，以

public class MainActivity extends Activity 
{ 
private EditText myEditField; 
private EditText myEditField2; 
private Button enter; 

public void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 

    myEditField = (EditText) findViewById(R.id.percentText); 
    myEditField2 = (EditText) findViewById(R.id.billText); 
    enter = (Button)findViewById(R.id.button1); 

    enter.setOnClickListener(new OnClickListener() { 

     public void onClick(View v) { 
      TextView textView; 

      textView = (TextView) findViewById(R.id.textView1); 

      final EditText myEditField = (EditText) findViewById(R.id.percentText); 
      final EditText myEditField2 = (EditText) findViewById(R.id.billText); 

      float percentage;//=======> declate as float 
      int percentageInp; 
      int billAmount; 
      int output; 
      String output1; 

      percentageInp = Integer.parseInt(myEditField.getText().toString()); 
      billAmount = Integer.parseInt(myEditField2.getText().toString()); 

      System.out.println(percentageInp+" "+billAmount); 

      percentage = ((float)percentageInp /100);//=====>check here 

      output = (int)(billAmount * percentage); 
      output1 = Integer.toString(output); 

      textView.setText(output1); 

     } 
    }); 
} 
}