java android 中 findViewById() 上的 NullPointerException
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NullPointerException on findViewById() in android
提问by pointour
In the following code i get a NullPointerException on lines 9/10 with findViewById().
In my main class I just instantiated an object from this class, to use .getFrom()
在下面的代码中，我在第 9/10 行使用 findViewById() 得到 NullPointerException。
在我的主类中，我刚刚从这个类中实例化了一个对象，以使用 .getFrom()
public class UserInteraction extends Activity {
EditText etFrom;
int from;
EditText etTill;
int till;
public UserInteraction(){
etFrom = (EditText)findViewById(R.id.et_from);
etTill = (EditText)findViewById(R.id.et_till);
}
public int getFrom() {
String s = etFrom.getText().toString();
int i = Integer.parseInt(s);
return i;
}
public int getTill() {
String s = etTill.getText().toString();
int i = Integer.parseInt(s);
return i;
}
Is it that the contentView is set in my main class ..? What could be the cause ?
是不是我的主类中设置了 contentView ..？可能是什么原因 ？
回答by MByD
The setContentViewmethod should be called with appropriate layout beforecalling findViewById. It is usually called in onCreate(Bundle savedInstance)method.
在调用之前，setContentView应使用适当的布局调用该方法。它通常在方法中调用。findViewByIdonCreate(Bundle savedInstance)
回答by CL22
You have to call it from your Activity's onCreate method, as the resources will not have been made available before that point.
您必须从 Activity 的 onCreate 方法中调用它，因为在此之前资源将不可用。
So expanding MByD's answer, in your onCreate method, first call setContentView(), then findViewById().
因此，扩展 MByD 的答案，在您的 onCreate 方法中，首先调用 setContentView()，然后调用 findViewById()。
回答by Houcine
First , you should call the setContentView(int layout),in order to set the Content of your Activity , and then you can get your Views ( findViewById(int id) ) ;
首先，您应该调用setContentView(int layout)，以设置您的 Activity 的 Content，然后您就可以获取您的 Views ( findViewById(int id) ) ；
So your Activity will be like this :
所以你的 Activity 会是这样的：
public class UserInteraction extends Activity {
EditText etFrom;
int from;
EditText etTill;
int till;
public void onCreate(Bundle savedInstance{
super.onCreate(saveInstance);
this.setContentView(R.layout.main);
etFrom = (EditText)findViewById(R.id.et_from);
etTill = (EditText)findViewById(R.id.et_till);
}
}
}
