总结一些小细节 ---- Android

1.Null pointer dereference of parent.getItemAtPosition(...) where null comes from constant

This error always happened in the build of spinner ,like this code:

Spinner spinner = (Spinner)findViewById(R.id.selectserver);

 ArrayAdapter adapterserver = new ArrayAdapter<String>(
                    this, android.R.layout.simple_spinner_item, keyServers);
adapterserver.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
spinner.setAdapter(adapterserver);

spinner.setOnItemSelectedListener(new OnItemSelectedListener(){
                @Override
                public void onItemSelected(AdapterView<?> parent, View arg1,
                    int postion, long id) {
                    String lastServer = null;
                    selectedServerName = parent.getItemAtPosition(postion).toString();