我是Android Studio中的新手，我有一点问题。当我按下按钮选择活动时，应用程序崩溃，并在“Android监视器”中显示：
05-17 21:46:25.309 10393-10393/bodo.odysseuscontest E/AndroidRuntime: FATAL EXCEPTION: main
Process: bodo.odysseuscontest, PID: 10393
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{bodo.odysseuscontest/bodo.odysseuscontest.Activity3}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2567)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
at android.support.v7.app.AppCompatDelegateImplBase.<init>(AppCompatDelegateImplBase.java:120)
at android.support.v7.app.AppCompatDelegateImplV9.<init>(AppCompatDelegateImplV9.java:155)
at android.support.v7.app.AppCompatDelegateImplV11.<init>(AppCompatDelegateImplV11.java:31)
at android.support.v7.app.AppCompatDelegateImplV14.<init>(AppCompatDelegateImplV14.java:55)
at android.support.v7.app.AppCompatDelegateImplV23.<init>(AppCompatDelegateImplV23.java:33)
at android.support.v7.app.AppCompatDelegateImplN.<init>(AppCompatDelegateImplN.java:33)
at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:201)
at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:185)
at android.support.v7.app.AppCompatActivity.getDelegate(AppCompatActivity.java:519)
at android.support.v7.app.AppCompatActivity.findViewById(AppCompatActivity.java:190)
at bodo.odysseuscontest.Activity3.<init>(Activity3.java:18)
at java.lang.Class.newInstance(Native Method)
at android.app.Instrumentation.newActivity(Instrumentation.java:1078)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2557)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
这是.java代码：
package bodo.odysseuscontest;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.OverScroller;
import android.widget.Spinner;
import android.widget.TextView;
public class Activity3 extends AppCompatActivity {
private TextView totalTextView;
private EditText valuetxt;
Spinner chooses= (Spinner) findViewById(R.id.spinner);
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_3);
ArrayAdapter<String> myAdapter = new ArrayAdapter<String>(Activity3.this,
android.R.layout.simple_list_item_1, getResources().getStringArray(R.array.names));
myAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
chooses.setAdapter(myAdapter);
Button calcbtn = (Button)findViewById(R.id.calcbtn);
totalTextView = (TextView)findViewById(R.id.totalTextView);
valuetxt = (EditText)findViewById(R.id.valuetxt);
calcbtn.setOnClickListener(new View.OnClickListener() {
public void onClick(View view) {
float value = Float.parseFloat(valuetxt.getText().toString());
if (chooses.equals("Km")) {
float total = value * 100;
totalTextView.setText(Float.toString(total));
}
}
});
}
}
这是android清单文件：
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="bodo.odysseuscontest">
<application
android:allowBackup="true"
android:background="@color/Background"
android:icon="@mipmap/ic_launcher"
android:label="@string/app_name"
android:roundIcon="@mipmap/ic_launcher_round"
android:screenOrientation="portrait"
android:supportsRtl="true"
android:theme="@style/AppTheme">
<activity android:name=".MainActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity
android:name=".Activity2"
android:screenOrientation="portrait" />
<activity
android:name="Activity3"
android:label="Conversion"
android:parentActivityName=".Activity2"
android:screenOrientation="portrait" />
<activity android:name="Activity4"
android:label="Rockets"
android:parentActivityName=".Activity2"/>
<activity android:name="Activity5"
android:label="Missions"
android:parentActivityName=".Activity2"/>
<activity android:name="Activity6"
android:label="Credits"
android:parentActivityName=".Activity2"/>
</application>
答案 0 :(得分：0)
在创建活动的上下文之前，您正在调用findViewById。
尝试改变这一点：
...
...
private TextView totalTextView;
private EditText valuetxt;
Spinner chooses= (Spinner) findViewById(R.id.spinner);
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_3);
....
....
使用此代码：
...
...
private TextView totalTextView;
private EditText valuetxt;
Spinner chooses;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_3);
chooses= (Spinner) findViewById(R.id.spinner);
....
....