55. Rewriting equals() via type patterns for instanceof
It is not mandatory to rely on instanceof for implementing the equals() method, but it is a convenient approach to write something as follows:
public class MyPoint {
private final int x;
private final int y;
private final int z;
public MyPoint(int x, int y, int z) {
this.x = x;
this.y = y;
this.z = z;
}
@Override
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (!(obj instanceof MyPoint)) {
return false;
}
final MyPoint other = (MyPoint) obj;
return (this.x == other.x && this.y == other.y
&& this.z == other.z);
}
}
If you are a fan of the previous approach for implementing equals() then you’ll love to rewrite it via type pattern for instanceof. Check out the following snippet:
@Override
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
return obj instanceof MyPoint other
&& this.x == other.x && this.y == other.y
&& this.z == other.z;
}
If MyPoint is generic (MyPoint<E>) then simply use a wildcard as follows (more details are available in the next problem):
return obj instanceof MyPoint<?> other
&& this.x == other.x && this.y == other.y
&& this.z == other.z;
Cool, right?!
56. Tackling type patterns for instanceof and generics
Consider the following snippet of code that uses instanceof in the old-school fashion:
public static <K, V> void process(Map<K, ? extends V> map) {
if (map instanceof EnumMap<?, ? extends V>) {
EnumMap<?, ? extends V> books
= (EnumMap<?, ? extends V>) map;
if (books.get(Status.DRAFT) instanceof Book) {
Book book = (Book) books.get(Status.DRAFT);
book.review();
}
}
}
// use case
EnumMap<Status, Book> books = new EnumMap<>(Status.class);
books.put(Status.DRAFT, new Book());
books.put(Status.READY, new Book());
process(books);
As we know from Problem 51, we can combine instanceof with generic types via unbounded wildcards, such as our EnumMap<?, ? extends V> (or EnumMap<?, ?>, but not EnumMap<K, ? extends V>, EnumMap<K, ?>, or EnumMap<K, V>).This code can be written more concisely via the type pattern for instanceof as follows:
public static <K, V> void process(Map<K, ? extends V> map) {
if (map instanceof EnumMap<?, ? extends V> books
&& books.get(Status.DRAFT) instanceof Book book) {
book.review();
}
}
In the example based on plain instanceof, we can also replace EnumMap<?, ? extends V> with Map<?, ? extends V>. But, as we know from Problem 53, this is not possible with type patterns because the expression type cannot be a subtype of pattern type (up-casting is allowed).