public <T> T pop() {
    @SuppressWarnings("unchecked")
    final T obj = (T) stack.pop();
    if (logger.isTraceEnabled()) {
      logger.trace("QCompiler.pop: {}", obj);
    }
    return obj;
  }
@return null if null is on the stack
geode-core/src/main/java/org/apache/geode/cache/query/internal/QCompiler.java