Java API

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Java API documentation

Handling Kuzu output using getNext()

For the examples in this section we will be using the following schema:

CREATE NODE TABLE person(id INT64 PRIMARY KEY);

The getNext() function in a QueryResult returns a reference to the resulting FlatTuple. Additionally, to reduce resource allocation all calls to getNext() reuse the same FlatTuple object. This means that for a QueryResult, each call to getNext() actually overwrites the FlatTuple previously returned by the previous call to getNext().

Thus, we don’t recommend using QueryResult like this:

QueryResult result = conn.query("MATCH (p:person) RETURN p.*");
List<FlatTuple> tuples = new ArrayList<FlatTuple>();
while (result.hasNext()) {
  // Each call to getNext() actually returns a reference to the same tuple object
  tuples.add(result.getNext());
}

// This is wrong!
// The list stores a bunch of references to the same underlying tuple object
for (FlatTuple resultTuple: tuples) {
  doSomething(resultTuple);
}

Instead, we recommend processing each tuple immediately before making the next call to getNext:

QueryResult result = conn.query("MATCH (p:person) RETURN p.*");
while (result.hasNext()) {
  FlatTuple tuple = result.getNext();
  doSomething(tuple);
}

If wish to process the tuples later, you must explicitly make a copy of each tuple:

List<Value> copyFlatTuple(FlatTuple tuple, long tupleLen) throws ObjectRefDestroyedException {
  List<Value> ret = new ArrayList<Value>();
  for (int i = 0; i < tupleLen; i++) {
      ret.add(tuple.getValue(i).clone());
  }
  return ret;
}

void mainFunction() throws ObjectRefDestroyedException {
  QueryResult result = conn.query("MATCH (p:person) RETURN p.*");
  List<List<Value>> tuples = new ArrayList<List<Value>>();
  while (result.hasNext()) {
    FlatTuple tuple = result.getNext();
    tuples.add(copyFlatTuple(tuple, result.getNumColumns()));
  }

  for (List<Value> tuple: tuples) {
    doSomething(tuple);
  }
}