Here is an example:

public interface List<E>

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The Java "List" type has a generic element type.
When we use "List" we need to substitute in a real type for the generic type "E".
Like this:

List<String> names;

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Here we substitute in the real type "String" for the generic type "E".
Therefore we have a list of String objects.
Another example:

List<Bike> names;

Here we substitute in the real type "Bike" for the generic type "E".
Therefore we have a list of Bike objects.
More details here:

• Generics by example
• Generic Types (Oracle)

More:

• encapsulation