我正在制作一个应用程序，它将显示名人的图像和4个名称，其中一个是正确的。
我添加了一个按钮＆＃34;更改＆＃34;更改图像中的名人，但按下该按钮我的应用程序崩溃显示null java.lang.null指针异常。
我有用于加载图像的代码和oncreate方法中的四个名称，它们在应用程序首次启动时加载图像和四个名称，但是在调用函数时按下按钮，虽然我有错误但是相同的代码。
public class MainActivity extends AppCompatActivity {
//declaring all the widgets to be used
Bitmap celebimg;
Random rand;
ImageView img;
Button btn0, btn1, btn2, btn3;
int optionnum, imgnum;
ArrayList<String> celebnames = new ArrayList<String>();
ArrayList<String> celebimages = new ArrayList<String>();
ArrayList<String> buttonoptions = new ArrayList<String>();
//Invoked when one out of the four options is clicked
public void row(View view) {
if (view.getTag().toString().equals(Integer.toString(optionnum))){
Toast.makeText(this, "correct Answer", Toast.LENGTH_SHORT).show();
} else {
Toast.makeText(this, "wrong Answer, It was" +
celebnames.get(imgnum), Toast.LENGTH_SHORT).show();
}
}
//Class for downloading of image
public class Imagedownload extends AsyncTask<String, Void, Bitmap> {
@Override
protected Bitmap doInBackground(String... strings) {
try {
URL url = new URL(strings[0]);
HttpURLConnection connection = (HttpURLConnection)url.openConnection();
connection.connect();
InputStream in = connection.getInputStream();
Bitmap myimage = BitmapFactory.decodeStream(in);
return myimage;
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
return null;
}
}
/**
* Class for downloading the website html code
*/
public class Download extends AsyncTask<String, Void, String > {
@Override
protected String doInBackground(String... strings) {
String result = "";
URL url;
HttpURLConnection connection = null;
try {
url = new URL(strings[0]);
connection = (HttpURLConnection)url.openConnection();
InputStream input = connection.getInputStream();
InputStreamReader reader = new InputStreamReader(input);
int data = reader.read();
while(data != -1) {
char current = (char)data;
result += current;
data = reader.read();
}
return result;
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
return null;
}
}
单击更改按钮时执行的代码
public void change(View view) {
rand = new Random();
imgnum = rand.nextInt(celebimages.size());
Imagedownload image = new Imagedownload();
try {
celebimg = image.execute(celebimages.get(imgnum)).get();
if (celebimg == null)
Log.i("its", "null douchebag");
img.setImageBitmap(celebimg);
} catch (InterruptedException e) {
if (celebimg == null)
Log.i("its", "null douchebag");
e.printStackTrace();
} catch (ExecutionException e) {
if (celebimg == null)
Log.i("its", "null douchebag");
e.printStackTrace();
}
optionnum = rand.nextInt(4);
for(int i = 0;i < 4;i++){
if(i == optionnum)
buttonoptions.add(celebnames.get(imgnum));
else {
int random = rand.nextInt(celebnames.size());
while (celebnames.get(imgnum) == celebnames.get(random)){
random = rand.nextInt(celebnames.size());
}
buttonoptions.add(celebnames.get(random));
}
}
btn0.setText(buttonoptions.get(0));
btn1.setText(buttonoptions.get(1));
btn2.setText(buttonoptions.get(2));
btn3.setText(buttonoptions.get(3));
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
ImageView img = (ImageView)findViewById(R.id.celeb);
btn0 = (Button)findViewById(R.id.btn0);
btn1 = (Button)findViewById(R.id.btn1);
btn2 = (Button)findViewById(R.id.btn2);
btn3 = (Button)findViewById(R.id.btn3);
String data = "";
Download load = new Download();
try {
data = load.execute("http://www.posh24.se/kandisar").get();
String[] splitdata = data.split("<div class=\"title\">Lista:</div>");
// seperating out the required img src from the html code
Pattern p = Pattern.compile("src=\"(.*?)\"");
Matcher M = p.matcher(splitdata[1]);
while (M.find()){
//adding all the img src values to celebimages arraylist
celebimages.add(M.group(1));
}
Pattern pi = Pattern.compile("alt=\"(.*?)\"");
Matcher Mi = pi.matcher(splitdata[1]);
while (Mi.find()) {
// adding all the alt values to celebnames arraylist
celebnames.add(Mi.group(1));
}
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
Random rand = new Random();
imgnum = rand.nextInt(celebimages.size());
Imagedownload image = new Imagedownload();
Bitmap celebimg;
try {
//downloading the image from stored img src values from celebimages
//arraylist
celebimg = image.execute(celebimages.get(imgnum)).get();
img.setImageBitmap(celebimg);
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
//Setting the correct value in one button and random values in other
//three
optionnum = rand.nextInt(4);
for(int i = 0;i < 4;i++){
if(i == optionnum)
buttonoptions.add(celebnames.get(imgnum));
else {
int random = rand.nextInt(85);
while (celebnames.get(imgnum) == celebnames.get(random)){
random = rand.nextInt(celebnames.size());
}
buttonoptions.add(celebnames.get(random));
}
}
btn0.setText(buttonoptions.get(0));
btn1.setText(buttonoptions.get(1));
btn2.setText(buttonoptions.get(2));
btn3.setText(buttonoptions.get(3));
}
错误：
引起：java.lang.NullPointerException：尝试调用虚方法＆＃39; void android.widget.ImageView.setImageBitmap（android.graphics.Bitmap）＆＃39;在空对象引用上
答案 0 :(得分：2)
问题是因为您正在为图像创建另一个变量而不是重复使用上一个变量：
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
ImageView img = (ImageView)findViewById(R.id.celeb);
...
}
应该是
img = (ImageView)findViewById(R.id.celeb);
问题通常发生是因为没有使用可读的命名约定，如下所示：
Bitmap celebimg;
Random rand;
ImageView img;
Button btn0, btn1, btn2, btn3;
对类范围变量使用类似的东西：
Bitmap mBmpCeleb;
Random mRndSomeId;
ImageView mImvCeleb;
Button mBtnImageOne, mBtnImageTwo, mBtnImageThree, mBtnImageThree;
其中m字符是成员的缩写，表示该变量是该类的成员。
对方法范围变量使用类似以下内容：
ImageView imvCeleb = (ImageView) findViewById(R.id.celeb);