"The Devoted Friend" by Oscar Wilde is a short story whose main idea is the exposure of the double standards of what is thought to be as the proper, righteous, and polite society of Victorian London.
Therefore, irony would be the first technique as well as the personification of the marsh animals who are taking part of the story and tell each other the tale of the miller and Hans.
The nested story is also figurative. The animals tell a story within their own story.
Epigrams and aphorisms are rampant in every work of Oscariana and is evident in the juxtaposition of ideas such as the idea that having a moral in a story is a negative thing, and how (even though the moral of the story was clear) nobody seemed to care to recognize it.
That is what reinforces Wilde's use of irony in his tales, and how he wants to expose society for what it is.
Oscar Wilde’s “A Devoted Friend” is an example of a type of story called a “fable.” Fables usually sound like children’s stories because they are meant to instruct or demonstrate a moral, and they often feature animals as characters. This type of story is, by its very nature, figurative in meaning, because it is not meant to be taken literally. If we took this story literally we would have ducks and rats and linnets talking to each other.
The figurative nature of the story is also a bit ironic because the linnet is telling a story about humans. Within the story’s reality, he is using human behavior to teach a moral to other animals. Wilde has succeeded in turning the normal pattern upside down. Again, none of this is meant to be taken literally, so its meaning is figurative.
You might also think of the story as symbolic. Symbolism is a type of figurative language (along with simile, metaphor, personification). The animals represent, or symbolize, different kinds of people. The duck is a concerned parent, the linnet is something of a philosopher, and the water rat is a selfish, ignorant person who can only look at things in terms of his own well-being.
This story is also an example of a “frame story” because it consists of a story within a story. This happens when the linnet tells the duck and water rat a story about two humans.