public void reduce(Stack<ByteBuffer> operatorStack, Stack<Filter> filterStack,
    ByteBuffer operator) {
    while (
      !operatorStack.empty() && !ParseConstants.LPAREN_BUFFER.equals(operatorStack.peek())
        && hasHigherPriority(operatorStack.peek(), operator)
    ) {
      filterStack.push(popArguments(operatorStack, filterStack));
    }
  }
This function is called while parsing the filterString and an operator is parsed
<p>
@param operatorStack the stack containing the operators and parenthesis
@param filterStack   the stack containing the filters
@param operator      the operator found while parsing the filterString
hbase-client/src/main/java/org/apache/hadoop/hbase/filter/ParseFilter.java