Other Implications – Generics

Other Implications

Enum Types

Because of the way enum types are implemented using the java.lang.Enum class, we cannot declare a generic enum type:

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enum CoinToss<C> { HEAD, TAIL; }                  // Compile-time error!

An enum type can implement a parameterized interface, just like a non-generic class can. Example 11.21 shows the enum type TripleJump that implements the Comparator<TripleJump> interface.

Example 11.21 Enum Implements Parameterized Generic Interface

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enum TripleJump implements java.util.Comparator<TripleJump> {
  HOP, STEP, JUMP;
  @Override public int compare(TripleJump a1, TripleJump a2) {
    return a1.compareTo(a2);
  }
}

Class Literals

Objects of the class Class<T> represent classes and interfaces at runtime. For example, an instance of the Class<String> represents the type String at runtime.

A class literal expression (using .class notation) can only use reifiable types as type parameters, as there is only one class object created for each reifiable type.

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Class<Node> class0 = Node<Integer>.class;// Non-reifiable type. Compile-time error!
Class<Node> class1 = Node.class;        // Reifiable type. Ok.

The getClass() method of the Object class also returns a Class object. The actual result type of this object is Class<? extends |T|> where |T| is the erasure of the static type of the expression on which the getClass() method is called. The following code shows that all invocations of the generic type Node<T> are represented by a single class literal:

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Node<Integer> intNode = new Node<>(2019, null);
Node<String> strNode = new Node<>(“Hi”, null);
Class<?> class2 = strNode.getClass();
Class<?> class3 = intNode.getClass();
assert class1 == class2;
assert class2 == class3;