Attempt to invoke interface method ' Boolean java.util.List.add (Java.lang.Object) ' on a null Object reference

Source: Internet
Author: User



Try to refer to the Boolean Java.util.List.add () method on an empty object reference;
Error Examples:


Private Arraylist<string> Classlist;classlist.add ("2014211501");
Equivalent to classlist is null.


Correct example:


Private arraylist<string> classlist=New arraylist<string>(); Classlist.add ("2014211501");





P.s. This is also a relatively typical novice's mistake, nullpointexception, there are a few possible, and basically are not initialized with the relationship. Although not summarized here, it is possible to record:



1. A more typical



int xxx;



And then report the mistake;



directly modified into int xxx=0;






2. There is another, such as:


Tv2= (TextView) Findviewbyid (R.id.textview);


The R.id.textview in the back is wrong.






3. And then the above one. Take a good look, tart!



Attempt to invoke interface method ' Boolean java.util.List.add (Java.lang.Object) ' on a null Object reference


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