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Android 中的 java.lang.nullpointer 错误

转载 作者:行者123 更新时间:2023-12-02 02:05:42 27 4
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我正在制作一个应用程序,它将显示名人的图像和 4 个名字,其中一个是正确的。

我添加了一个“更改”按钮来更改图像中的名人,但按下该按钮时我的应用程序崩溃,显示空 java.lang.null 指针异常。

我在 oncreate 方法中有加载图像和四个名称的代码,该方法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时,会生成错误,尽管我有其中包含相同的代码。

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:

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ImageView.setImageBitmap(android.graphics.Bitmap)' on a null object reference

最佳答案

问题是因为您正在为图像创建另一个变量,而不是重用以前的变量:

@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);
<小时/>

该问题通常是由于未使用如下所示的可读命名约定而发生的:

Bitmap celebimg;
Random rand;
ImageView img;
Button btn0, btn1, btn2, btn3;

对类范围变量使用类似的内容:

Bitmap mBmpCeleb;
Random mRndSomeId;
ImageView mImvCeleb;
Button mBtnImageOne, mBtnImageTwo, mBtnImageThree, mBtnImageThree;

其中 m 字符是 member 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);

关于Android 中的 java.lang.nullpointer 错误,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/50832068/

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