Effective Java 3/E summary chanpter 3 (Shorter version)

28 Dec 2020

Chanpter 4 : Generic

You can find the longer version of this summary here.

When to use Generics?

• If something is expressed as Object, use Generics.
• If there is a class or method in which type conversion occurs, use Generics. (For methods)
• When using Generics, the type is specified at compile time and the client doesn’t need type-casting.
• When using a container of a variable type, consider using a type-safe heterogeneous container with Class<T> as the type token.

Characteristics of Generics

• Invariant : Generic classes declared as different types are of different types.
• Type check at compile time.
• Type information is erased at runtime.

Precautions for Generics

• Do not use the raw type because it exists due to compatibility with existing Java (it is difficult to find a bug at compile time)
• Do not use Generics with arrays
  • Generics don’t reify, whereas arrays reify. (The array type is unknown at runtime)
• Since variable arguments are arrays, make sure to be type-safe to use with Generics and attach @SafeVarargs.

Use of Wildcards

• Using wildcards makes the public API much more flexible. (Client doesn’t need type declaration)
• If type parameter appears only once in the method declaration, replace it with a wildcard.
• Recursive type limitation ```java

<E extends Comparable> // Any type E can be compared. <T extends Builder> // Any type T implement Builder. ```

• PECS: producer-extends, consumer-super