Mis vahe on märksõnadel extends ja super Java generics kasutamisel?

In Java, when working with generics, the keywords extends and super are used to restrict types and influence covariance and contravariance.

• ? extends T — upper bound (covariance). Allows using type T or any of its subclasses. This means you can read objects of type T from the collection, but cannot safely add objects to such a collection (except null).

• ? super T — lower bound (contravariance). Allows using type T or any of its superclasses. This means you can add objects of type T to the collection, but when reading, you only get objects of type Object or more general types.

Example:

List<? extends Number> listExtends = new ArrayList<Integer>();
Number num = listExtends.get(0); // Can read as Number
// listExtends.add(10); // Compilation error

List<? super Integer> listSuper = new ArrayList<Number>();
listSuper.add(10); // Can add Integer
Object obj = listSuper.get(0); // When reading, type is Object

Thus, extends is used when you need to read data from the structure, and super when you need to write data.