code: public TypeInformation<T> getOutputType() {
  		if (outputType instanceof MissingTypeInfo) {
  			MissingTypeInfo typeInfo = (MissingTypeInfo) this.outputType;
  			throw new InvalidTypesException(
  					"The return type of function '"
  							+ typeInfo.getFunctionName()
  							+ "' could not be determined automatically, due to type erasure. "
  							+ "You can give type information hints by using the returns(...) "
  							+ "method on the result of the transformation call, or by letting "
  							+ "your function implement the 'ResultTypeQueryable' "
  							+ "interface.", typeInfo.getTypeException());
  		}
  		typeUsed = true;
  		return this.outputType;
  	}
docs: Returns the output type of this {@code StreamTransformation} as a {@link TypeInformation}. Once
  this is used once the output type cannot be changed anymore using {@link #setOutputType}.
  
  @return The output type of this {@code StreamTransformation}