What is the purpose of the PECS method in software development?

PECS is a principle used when working with generics in Java, which helps correctly utilize covariance and contravariance of collections. The abbreviation stands for "Producer Extends, Consumer Super".

• If an object acts as a data producer, the ? extends T restriction is used — this allows reading data of type T or its subtypes.
• If an object acts as a data consumer, the ? super T restriction is used — this allows writing data of type T or its subtypes.

For example:

List<? extends Number> producer = new ArrayList<Integer>();
Number num = producer.get(0); // reading is allowed
// producer.add(10); // error, cannot add

List<? super Integer> consumer = new ArrayList<Number>();
consumer.add(10); // adding is allowed
// Integer i = consumer.get(0); // error, type cannot be guaranteed when reading

Thus, PECS helps avoid type errors when working with generic collections and improves type safety.