我制作了一个计算二次方程的android应用程序。我建立了一个apk并在我的手机上测试它并没有用。每次我运行应用程序时，它甚至会崩溃按钮，文本字段也不会显示。只有包含应用程序名称的窗口才会崩溃。
这是代码：
package com.islamatabl.quadraticequationsfoa;
import android.support.v7.app.ActionBarActivity;
import android.support.v7.app.ActionBar;
import android.support.v4.app.Fragment;
import android.content.Context;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.View.*;
import android.view.ViewGroup;
import android.os.Build;
import android.widget.*;
import java.lang.Math;
public class MainActivity extends ActionBarActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Button calc = (Button) findViewById(R.id.button1);
calc.setOnClickListener(new View.OnClickListener(){
public void onClick(View v)
{
int duration = 5;
Context context = getApplicationContext();
EditText f1 = (EditText) findViewById(R.id.editText1);
EditText f2 = (EditText) findViewById(R.id.editText2);
EditText f3 = (EditText) findViewById(R.id.editText3);
TextView disp1 = (TextView) findViewById(R.id.textView4);
TextView disp2 = (TextView) findViewById(R.id.textView5);
double a,b,c,root,inroot,x1,x2;
a = Double.parseDouble(f1.getText().toString());
b = Double.parseDouble(f2.getText().toString());
c = Double.parseDouble(f3.getText().toString());
inroot = Math.pow(b,2.0) - 2*a*c;
if (inroot<0)
{
Toast.makeText(context, "Math Error", duration).show();
}
else if (a==0)
{
Toast.makeText(context, "a cannot be 0", duration).show();
}
else
{
root = Math.sqrt(inroot);
x1 = (-b+root)/2*a;
x2 = (-b-root)/2*a;
if (x1==x2)
{
disp1.setText("X = " + String.valueOf(x1));
disp2.setText("");
}
else
{
disp1.setText("X1 = " + String.valueOf(x1));
disp2.setText("X2 = " + String.valueOf(x2));
}
}
}
});
if (savedInstanceState == null) {
getSupportFragmentManager().beginTransaction()
.add(R.id.container, new PlaceholderFragment()).commit();
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.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();
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
/**
* A placeholder fragment containing a simple view.
*/
public static class PlaceholderFragment extends Fragment {
public PlaceholderFragment() {
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_main, container,
false);
return rootView;
}
}
}
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: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.islamatabl.quadraticequationsfoa.MainActivity$PlaceholderFragment" >
<EditText
android:id="@+id/editText1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentRight="true"
android:layout_alignParentTop="true"
android:layout_marginRight="23dp"
android:layout_marginTop="100dp"
android:ems="10"
android:inputType="number|numberSigned|numberDecimal" >
<requestFocus />
</EditText>
<EditText
android:id="@+id/editText2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/editText1"
android:layout_below="@+id/editText1"
android:ems="10"
android:inputType="number|numberSigned|numberDecimal" />
<EditText
android:id="@+id/editText3"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/editText2"
android:layout_below="@+id/editText2"
android:ems="10"
android:inputType="number|numberSigned|numberDecimal" />
<TextView
android:id="@+id/textView1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_above="@+id/editText2"
android:layout_marginRight="17dp"
android:layout_toLeftOf="@+id/editText1"
android:text="a:"
android:textAppearance="?android:attr/textAppearanceMedium" />
<TextView
android:id="@+id/textView2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignBottom="@+id/editText2"
android:layout_alignLeft="@+id/textView1"
android:text="b:"
android:textAppearance="?android:attr/textAppearanceMedium" />
<TextView
android:id="@+id/textView3"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignBottom="@+id/editText3"
android:layout_alignLeft="@+id/textView2"
android:text="c:"
android:textAppearance="?android:attr/textAppearanceMedium" />
<TextView
android:id="@+id/textView4"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@+id/editText3"
android:layout_centerHorizontal="true"
android:layout_marginTop="34dp"
android:text="X1 ="
android:textAppearance="?android:attr/textAppearanceMedium" />
<TextView
android:id="@+id/textView5"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@+id/textView4"
android:layout_centerHorizontal="true"
android:layout_marginTop="28dp"
android:text="X2 ="
android:textAppearance="?android:attr/textAppearanceMedium" />
<Button
android:id="@+id/button1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@+id/textView5"
android:layout_centerHorizontal="true"
android:layout_marginTop="24dp"
android:text="Calculate" />
</RelativeLayout>
这是logcat：
04-19 17:18:54.312: D/AndroidRuntime(27681): Shutting down VM
04-19 17:18:54.312: W/dalvikvm(27681): threadid=1: thread exiting with uncaught exception (group=0x41f862a0)
04-19 17:18:54.322: E/AndroidRuntime(27681): FATAL EXCEPTION: main
04-19 17:18:54.322: E/AndroidRuntime(27681): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.islamatabl.quadraticequationsfoa/com.islamatabl.quadraticequationsfoa.MainActivity}: java.lang.NullPointerException
04-19 17:18:54.322: E/AndroidRuntime(27681): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2081)
04-19 17:18:54.322: E/AndroidRuntime(27681): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2106)
04-19 17:18:54.322: E/AndroidRuntime(27681): at android.app.ActivityThread.access$700(ActivityThread.java:134)
04-19 17:18:54.322: E/AndroidRuntime(27681): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1217)
04-19 17:18:54.322: E/AndroidRuntime(27681): at android.os.Handler.dispatchMessage(Handler.java:99)
04-19 17:18:54.322: E/AndroidRuntime(27681): at android.os.Looper.loop(Looper.java:137)
04-19 17:18:54.322: E/AndroidRuntime(27681): at android.app.ActivityThread.main(ActivityThread.java:4856)
04-19 17:18:54.322: E/AndroidRuntime(27681): at java.lang.reflect.Method.invokeNative(Native Method)
04-19 17:18:54.322: E/AndroidRuntime(27681): at java.lang.reflect.Method.invoke(Method.java:511)
04-19 17:18:54.322: E/AndroidRuntime(27681): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1007)
04-19 17:18:54.322: E/AndroidRuntime(27681): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:774)
04-19 17:18:54.322: E/AndroidRuntime(27681): at dalvik.system.NativeStart.main(Native Method)
04-19 17:18:54.322: E/AndroidRuntime(27681): Caused by: java.lang.NullPointerException
04-19 17:18:54.322: E/AndroidRuntime(27681): at com.islamatabl.quadraticequationsfoa.MainActivity.onCreate(MainActivity.java:26)
04-19 17:18:54.322: E/AndroidRuntime(27681): at android.app.Activity.performCreate(Activity.java:5047)
04-19 17:18:54.322: E/AndroidRuntime(27681): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1094)
04-19 17:18:54.322: E/AndroidRuntime(27681): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2045)
04-19 17:18:54.322: E/AndroidRuntime(27681): ... 11 more
答案 0 :(得分：0)
在设置onclick侦听器之前初始化所有这些并尝试
public class MainActivity extends ActionBarActivity {
EditText f1,f2,f3;
Button calc;
TextView disp1,disp2;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Context context = getApplicationContext();
calc = (Button) findViewById(R.id.button1);
f1 = (EditText) findViewById(R.id.editText1);
f2 = (EditText) findViewById(R.id.editText2);
f3 = (EditText) findViewById(R.id.editText3);
disp1 = (TextView) findViewById(R.id.textView4);
TextView disp2 = (TextView) findViewById(R.id.textView5);
现在设置onclick侦听器并尝试