The pattern matching enhancement that conditionally extract data from an object if the instanceof operator was satisfied and assigns it to a variable.
The variable in this pattern matching is following a flow scope. This means that the variable is in scope if it is guaranted that it can satisfies the instanceof operator.
Code Without Pattern Matching
record Person(String firstName, String lastName) {}
Record rec = new Person("Juan", "Dela Cruz");
//Check if rec is a Person.
if (rec instanceof Person) {
//rec can safely be casted to Person.
var person = (Person) rec;
System.out.println(person);
System.out.println(person.firstName());
System.out.println(person.lastName());
}
Notice that casting the rec to Person once the check succeded is required.
Code With Pattern Matching
record Person(String firstName, String lastName) {}
Record rec = new Person("Juan", "Dela Cruz");
//If rec is a Person extract it to a variable person.
if (rec instanceof Person person) {
System.out.println(person);
System.out.println(person.firstName());
System.out.println(person.lastName());
}
Notice that casting is not required here and Person instance is extracted to person variable. Moreover, the scope of the variable is in the truth block of the if statement.
Flow Scope
Example 1
Record rec = new Person("Juan", "Dela Cruz");
if (!(rec instanceof Person person)) {
System.out.println("In human.");
}
else {
System.out.println(person);
System.out.println(person.firstName());
System.out.println(person.lastName());
}
Notice that person variable usage is on the else block of the if statement.
Example 2
Record rec = new Person("Juan", "Dela Cruz");
if (rec instanceof Person person && "JUAN DELA CRUZ".equals(String.join(person.firstName(), " ", person.lastName().toUpperCase()))) {
System.out.println("You won a price");
}
Notice that the person variable is usable after the AND (&&) operator. This is because the and operator guarantees that the instanceof operator was satisfied before evaluating the following condition. Changing it to OR (||) operator will be an error, since the person variable is out of scope.
Practical Usage
equals implementation
import java.util.Objects;
record Person(String firstName, String lastName) {
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (null == o) return false;
if (o instanceof Person person) {
return Objects.equals(firstName, person.firstName()) && Objects.equals(lastName, person.lastName());
}
return false;
}
}
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