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

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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