java - android studio中带有setImageDrawable方法的NullPointerException
问题描述
我刚刚为一个项目启动了一个应用程序，我想要一个进入滑块。除了页面点之外，我得到了一切工作，当 IntroSlider 活动（显示所有幻灯片的活动）启动时不显示页面点，并在尝试转到下一张幻灯片时使应用程序崩溃。我不明白是什么导致了错误，我将不胜感激。在 IntroSlider.java 中查找注释，它指定了 Android Studio 所说的导致错误的行号。谢谢
进入Slider.java：
package com.businessbrains.businessbrains.Activities;
import android.content.Intent;
import android.graphics.drawable.Drawable;
import android.os.Build;
import android.support.v4.content.ContextCompat;
import android.support.v4.view.ViewPager;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.view.ViewGroup;
import android.view.WindowManager;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.LinearLayout;
import com.businessbrains.businessbrains.Adapters.SlidesAdapter;
import com.businessbrains.businessbrains.Internals.PreferenceManager;
import com.businessbrains.businessbrains.R;
public class IntroSlider extends AppCompatActivity implements View.OnClickListener{
private ViewPager mPager;
private int[] layouts = {R.layout.slide_welcome, R.layout.slide_account, R.layout.slide_selftest, R.layout.slide_mutliplayer};
private SlidesAdapter slidesAdapter;
private LinearLayout Dots_Layout;
private ImageView[] dots;
private Button skipBtn, nextBtn;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
if(new PreferenceManager(this).isSlidesEnabled()){
loadLogin();
}
if(Build.VERSION.SDK_INT >= 19){
getWindow().addFlags(WindowManager.LayoutParams.FLAG_TRANSLUCENT_STATUS);
} else{
getWindow().clearFlags(WindowManager.LayoutParams.FLAG_TRANSLUCENT_STATUS);
}
setContentView(R.layout.activity_intro_slider);
mPager= (ViewPager) findViewById(R.id.viewPager);
slidesAdapter = new SlidesAdapter(layouts, this);
mPager.setAdapter(slidesAdapter);
Dots_Layout = findViewById(R.id.dotsLayout);
nextBtn = (Button) findViewById(R.id.nextBtn);
nextBtn.setOnClickListener(this);
skipBtn = (Button) findViewById(R.id.skipBtn);
skipBtn.setOnClickListener(this);
mPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
@Override
public void onPageScrolled(int i, float v, int i1) {
}
@Override
public void onPageSelected(int i) {
createDots(i);
if(i==layouts.length-1){
nextBtn.setText("Start");
skipBtn.setVisibility(View.INVISIBLE);
}
else{
nextBtn.setText("Next");
skipBtn.setVisibility(View.VISIBLE);
}
}
@Override
public void onPageScrollStateChanged(int i) {
}
});
}
private void createDots(int current_position){
if(Dots_Layout!=null){
Dots_Layout.removeAllViews();
}
dots = new ImageView[layouts.length];
for(int i = 0; i < layouts.length; i++){
if(i==current_position){
dots[i].setImageDrawable((Drawable)ContextCompat.getDrawable(getApplicationContext(), R.drawable.accessory_active_dots));
}
else{
dots[i].setImageDrawable((Drawable)(ContextCompat.getDrawable(getApplicationContext(), R.drawable.accessory_inactive_dots)));
//line which is causing the error
}
LinearLayout.LayoutParams params = new LinearLayout.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT);
params.setMargins(4,0,4,0);
Dots_Layout.addView(dots[i], params);
}
}
@Override
public void onClick(View view) {
switch(view.getId()){
case R.id.nextBtn:
loadNextSlide();
break;
case R.id.skipBtn:
loadLogin();
new PreferenceManager(this).writeDisableSlides();
break;
}
}
private void loadLogin(){
startActivity(new Intent(this, LoginOptionsActivity.class));
finish();
}
private void loadNextSlide(){
int nextslide = mPager.getCurrentItem() + 1;
if(nextslide < layouts.length){
mPager.setCurrentItem(nextslide);
}
else{
loadLogin();
new PreferenceManager(this).writeDisableSlides();
}
}
}
对应的.xml，activity_into_slider.xml：
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".Activities.IntroSlider">
<android.support.v4.view.ViewPager
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/viewPager"
/>
<View
android:layout_width="match_parent"
android:layout_height="1dp"
android:alpha=".5"
android:layout_above="@+id/dotsLayout"
android:background="@android:color/white"
android:layout_marginBottom="15dp"
></View>
<LinearLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:id="@+id/dotsLayout"
android:layout_centerHorizontal="true"
android:layout_alignParentBottom="true"
android:orientation="horizontal"
android:gravity="center"
android:layout_marginBottom="55sp"
/>
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Skip"
android:textSize="16sp"
android:layout_alignParentBottom="true"
android:layout_alignParentLeft="true"
android:background="@android:color/transparent"
android:id="@+id/skipBtn"
android:layout_marginBottom="8dp"
android:textColor="@android:color/white"
/>
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Next"
android:textSize="16sp"
android:layout_alignParentBottom="true"
android:layout_alignParentRight="true"
android:background="@android:color/transparent"
android:id="@+id/nextBtn"
android:layout_marginBottom="8dp"
android:textColor="@android:color/white"
/>
</RelativeLayout>
其中一张幻灯片：
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="@color/colorSlide_welcome">
<ImageView
android:layout_width="100dp"
android:layout_height="100dp"
android:src="@drawable/ic_welcome"
android:layout_centerHorizontal="true"
android:scaleType="centerCrop"
android:layout_marginTop="150dp"
android:id="@+id/img"
android:tint="@android:color/black"
/>
<TextView
android:layout_width="132dp"
android:layout_height="100dp"
android:text="@string/slide1title"
android:textColor="@android:color/white"
android:textSize="30sp"
android:textStyle="bold"
android:layout_centerHorizontal="true"
android:layout_below="@+id/img"
android:layout_marginTop="12dp"
android:id="@+id/slidetitle"
/>
<TextView
android:layout_width="match_parent"
android:layout_height="match_parent"
android:text="@string/slide1desc"
android:textColor="@android:color/white"
android:textSize="18sp"
android:gravity="center_horizontal"
android:layout_below="@+id/slidetitle"
android:layout_marginTop="10dp"
android:id="@+id/slide_desc"
android:layout_marginRight="40sp"
android:layout_marginLeft="40sp"
/>
</RelativeLayout>
正在抛出的错误：
07-11 00:30:30.253 6010-6010/com.businessbrains.businessbrains E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.businessbrains.businessbrains, PID: 6010
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ImageView.setImageDrawable(android.graphics.drawable.Drawable)' on a null object reference
at com.businessbrains.businessbrains.Activities.IntroSlider.createDots(IntroSlider.java:89)
at com.businessbrains.businessbrains.Activities.IntroSlider.access$000(IntroSlider.java:21)
at com.businessbrains.businessbrains.Activities.IntroSlider$1.onPageSelected(IntroSlider.java:62)
at android.support.v4.view.ViewPager.dispatchOnPageSelected(ViewPager.java:1947)
at android.support.v4.view.ViewPager.scrollToItem(ViewPager.java:686)
at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:670)
at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:631)
at android.support.v4.view.ViewPager.setCurrentItem(ViewPager.java:612)
at com.businessbrains.businessbrains.Activities.IntroSlider.loadNextSlide(IntroSlider.java:118)
at com.businessbrains.businessbrains.Activities.IntroSlider.onClick(IntroSlider.java:101)
at android.view.View.performClick(View.java:6256)
at android.view.View$PerformClick.run(View.java:24701)
at android.os.Handler.handleCallback(Handler.java:789)
at android.os.Handler.dispatchMessage(Handler.java:98)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6541)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
附件主动点.xml：
<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
android:shape="oval"
android:useLevel="true"
android:dither="true">
<size android:height="8dp" android:width="8dp"/>
<solid android:color="@color/active_dots"/>
</shape>
附件_inactive_dots.xml：
<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
android:shape="oval"
android:useLevel="true"
android:dither="true">
<size android:height="8dp" android:width="8dp"/>
<solid android:color="@color/inactive_dots"/>
</shape>
解决方案
dots = new ImageView[layouts.length];创建一个大小数组，layouts.length但该数组的所有元素均为空。
因此，当您调用setImageDrawable(..)on时dots[i]，您实际上是在调用setImageDrawable(..)null 对象上的方法。
修复：使用前用非空值初始化dots[i]（即）。dots[i] =例如，dots[i] = new ImageView()在调用setImageDrawable(..)on之前执行dots[i]。
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