【发布时间】：2016-08-27 15:43:27
【问题描述】：
我是 Android 开发新手，遇到了一个渲染问题，我在 XML 选项卡（显示布局的位置）上遇到了问题。错误消息说
The following classes could not be instantiated:
- com.project.simon.project.Segment
java.lang.NullPointerException
at com.project.simon.project.Segment.<init>(Segment.java:22)
它与我创建的自定义类 Segment 有关，以扩展 Button 并添加我自己的方法和变量。 当我运行它并吐出相同的错误消息时，该应用程序也会崩溃。我尝试清理项目、更新 android studio、重建、使缓存无效/重新启动，但问题仍然存在。
我正在使用 Android Studio 2.1，在我的应用程序的第二个活动中，我从一个实现 Button 的单独文件（与活动在同一个项目中）中定义的类 Segment 实例化一个对象。
Activity中的Segment对象初始化：
package com.project.simon.project;
import android.content.Context;
import android.media.AudioManager;
import android.media.MediaPlayer;
import android.os.Bundle;
import android.app.Activity;
import android.os.CountDownTimer;
import android.util.AttributeSet;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Timer;
import java.util.TimerTask;
import java.util.concurrent.Semaphore;
public class GameActivity extends Activity {
static Integer numSeg = 0;
Integer selectedPart;
Segment songSegment;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_game);
songSegment = (Segment) findViewById(R.id.segment1);
...
}
Segment 对象 ID 的 XML：
<com.project.simon.project.Segment
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="0dp"
android:layout_weight="30"
android:layout_height="120dp"
android:id="@+id/segment1"
android:layout_column="0"
android:background="@drawable/petolinie_part"
android:clickable="true"
android:layout_gravity="center_vertical"
android:baselineAlignBottom="false"
android:cropToPadding="false"
android:adjustViewBounds="false"
android:text="1"
android:textSize="30dp"
android:textColor="@android:color/black" />
该类具有来自 Button 类的三个构造函数。 numSeg 是我实例化 Segment 对象的活动中的静态成员变量（在 onCreate 之外）。
package com.project.simon.project;
import android.content.Context;
import android.util.AttributeSet;
import android.view.View;
import android.widget.Button;
/**
* Created by simon on 28.4.2016.
*/
public class Segment extends Button{
boolean emptiness = false;
Integer part;
public Segment(Context context) {
super(context);
part = GameActivity.numSeg;
(GameActivity.numSeg)++;
}
public Segment(Context context, AttributeSet attrs){
super(context, attrs);
part = GameActivity.numSeg;
(GameActivity.numSeg)++; //Segment.java:22
}
public Segment(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
part = GameActivity.numSeg;
(GameActivity.numSeg)++;
}
Integer whichPart(){
return part;
}
void setPart(int part){
this.part = part;
}
boolean isEmpty(){
return emptiness;
}
void setEmpty(Boolean emptiness){
this.emptiness = emptiness;
}
}
我得到的渲染问题堆栈是一个错误：
Rendering Problems The following classes could not be instantiated:
- com.project.simon.project.Segment
java.lang.NullPointerException
at com.project.simon.project.Segment.<init>(Segment.java:22)
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:422)
at org.jetbrains.android.uipreview.ViewLoader.createNewInstance(ViewLoader.java:465)
at org.jetbrains.android.uipreview.ViewLoader.loadClass(ViewLoader.java:172)
at org.jetbrains.android.uipreview.ViewLoader.loadView(ViewLoader.java:105)
at com.android.tools.idea.rendering.LayoutlibCallbackImpl.loadView(LayoutlibCallbackImpl.java:176)
at android.view.BridgeInflater.loadCustomView(BridgeInflater.java:247)
at android.view.BridgeInflater.createViewFromTag(BridgeInflater.java:171)
at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:704)
at android.view.LayoutInflater.rInflate_Original(LayoutInflater.java:835)
at android.view.LayoutInflater_Delegate.rInflate(LayoutInflater_Delegate.java:70)
at android.view.LayoutInflater.rInflate(LayoutInflater.java:811)
at android.view.LayoutInflater.rInflateChildren(LayoutInflater.java:798)
at android.view.LayoutInflater.rInflate_Original(LayoutInflater.java:838)
at android.view.LayoutInflater_Delegate.rInflate(LayoutInflater_Delegate.java:70)
at android.view.LayoutInflater.rInflate(LayoutInflater.java:811)
at android.view.LayoutInflater.rInflateChildren(LayoutInflater.java:798)
at android.view.LayoutInflater.rInflate_Original(LayoutInflater.java:838)
at android.view.LayoutInflater_Delegate.rInflate(LayoutInflater_Delegate.java:70)
at android.view.LayoutInflater.rInflate(LayoutInflater.java:811)
at android.view.LayoutInflater.rInflateChildren(LayoutInflater.java:798)
at android.view.LayoutInflater.inflate(LayoutInflater.java:515)
at android.view.LayoutInflater.inflate(LayoutInflater.java:394)
at com.android.layoutlib.bridge.impl.RenderSessionImpl.inflate(RenderSessionImpl.java:229)
at com.android.layoutlib.bridge.Bridge.createSession(Bridge.java:426)
at com.android.ide.common.rendering.LayoutLibrary.createSession(LayoutLibrary.java:350)
at com.android.tools.idea.rendering.RenderTask$2.compute(RenderTask.java:520)
at com.android.tools.idea.rendering.RenderTask$2.compute(RenderTask.java:508)
at com.intellij.openapi.application.impl.ApplicationImpl.runReadAction(ApplicationImpl.java:967)
at com.android.tools.idea.rendering.RenderTask.createRenderSession(RenderTask.java:508)
at com.android.tools.idea.rendering.RenderTask.access$600(RenderTask.java:75)
at com.android.tools.idea.rendering.RenderTask$3.call(RenderTask.java:620)
at com.android.tools.idea.rendering.RenderTask$3.call(RenderTask.java:617)
at com.android.tools.idea.rendering.RenderService.runRenderAction(RenderService.java:371)
at com.android.tools.idea.rendering.RenderTask.render(RenderTask.java:617)
at com.android.tools.idea.rendering.RenderTask.render(RenderTask.java:639)
at org.jetbrains.android.uipreview.AndroidLayoutPreviewToolWindowManager.doRender(AndroidLayoutPreviewToolWindowManager.java:654)
at org.jetbrains.android.uipreview.AndroidLayoutPreviewToolWindowManager.access$1700(AndroidLayoutPreviewToolWindowManager.java:82)
at org.jetbrains.android.uipreview.AndroidLayoutPreviewToolWindowManager$7$1.run(AndroidLayoutPreviewToolWindowManager.java:596)
at com.intellij.openapi.progress.impl.CoreProgressManager$2.run(CoreProgressManager.java:142)
at com.intellij.openapi.progress.impl.CoreProgressManager.registerIndicatorAndRun(CoreProgressManager.java:446)
at com.intellij.openapi.progress.impl.CoreProgressManager.executeProcessUnderProgress(CoreProgressManager.java:392)
at com.intellij.openapi.progress.impl.ProgressManagerImpl.executeProcessUnderProgress(ProgressManagerImpl.java:54)
at com.intellij.openapi.progress.impl.CoreProgressManager.runProcess(CoreProgressManager.java:127)
at org.jetbrains.android.uipreview.AndroidLayoutPreviewToolWindowManager$7.run(AndroidLayoutPreviewToolWindowManager.java:591)
at com.intellij.util.ui.update.MergingUpdateQueue.execute(MergingUpdateQueue.java:337)
at com.intellij.util.ui.update.MergingUpdateQueue.execute(MergingUpdateQueue.java:327)
at com.intellij.util.ui.update.MergingUpdateQueue$3.run(MergingUpdateQueue.java:271)
at com.intellij.util.ui.update.MergingUpdateQueue.flush(MergingUpdateQueue.java:286)
at com.intellij.util.ui.update.MergingUpdateQueue.flush(MergingUpdateQueue.java:244)
at com.intellij.util.ui.update.MergingUpdateQueue.run(MergingUpdateQueue.java:234)
at com.intellij.util.concurrency.QueueProcessor.runSafely(QueueProcessor.java:238)
at com.intellij.util.Alarm$Request$1.run(Alarm.java:352)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
【问题讨论】：
标签： android button nullpointerexception rendering extends