lookup filter

Hi I need to filter lookup with out null values.

it should not show null values.

can anybody  help me. whats the logic.

I wrote following code for filter ..

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public void lookup()
{
PurchTable purchTable;
SysTableLookup sysTableLookup = SysTableLookup::newParameters(tablenum(PurchTable), this);
Query query = new Query();
QueryBuildDataSource queryBuildDataSource = query.addDataSource(tablenum(PurchTable));
;
sysTableLookup.addLookupfield(fieldnum(PurchTable, PurchId));

queryBuildDataSource.addRange(fieldnum(PurchTable, PurchId)).value("000010..000020");


sysTableLookup.parmQuery(query);
sysTableLookup.performFormLookup();
}

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But I am not getting how to avoid null values.