Java 14: Pattern Matching for instanceof

Java 14 introduces Pattern Matching for instanceof, another preview language feature, that eliminates the need for casts when using instanceof. For example, consider the following code:

if (obj instanceof String) {
    String s = (String) obj;
    System.out.println(s.length());
}

This code can now be rewritten as:

if (obj instanceof String s) {
    System.out.println(s.length());
}

As shown above, the instanceof operator now takes a "binding variable" and the cast to String is no longer required. If obj is an instance of String, then it is cast to String and assigned to the binding variable s. The binding variable is only in scope in the true block of the if-statement.

In particular, this feature makes equals methods a lot more concise as shown in the example below:

@Override
public boolean equals(Object obj) { 
  return this == obj || 
    (obj instanceof Person other) && other.name.equals(name);
}

This feature is an example of pattern matching, which is already available in many other programming languages, and allows us to conditionally extract components from objects. It opens the door for more general pattern matching in the future which I am very excited about!

Java 14: Records

Java 14 arrived a few weeks ago and introduces the Record type, which is an immutable data carrier class designed to hold a fixed set of fields. Note that this is a preview language feature, which means that it must be explicitly enabled in the Java compiler and runtime using the --enable-preview flag.

I'm going to jump straight in with an example of a Book record designed to hold the title, author, publish date and price of a book. This is how the record class is declared:

public record Book(String title, String author, LocalDate publishDate, double price) {
}

You can use javap to see the code that the compiler has autogenerated:

public final class Book extends java.lang.Record {
  public Book(java.lang.String, java.lang.String, java.time.LocalDate, double);
  public java.lang.String title();
  public java.lang.String author();
  public java.time.LocalDate publishDate();
  public double price();
  public java.lang.String toString();
  public final int hashCode();
  public final boolean equals(java.lang.Object);
}

As shown above, the compiler has automatically generated the constructor, getter methods, hashCode, equals and toString, thus saving us from having to type a lot of boilerplate code.

However, records do not just save on typing. They also make your intent clear that you want to model an immutable data item as a group of related fields.

Compact Constructors for Field Validation

Now let's say that you want to add validation and default values to your record. For example, you might want to validate that Book records are not created with negative prices or future publish dates. This can be done with a compact constructor as shown below:

public record Book(String title, String author, LocalDate publishDate, double price) {

  //compact constructor (no parameter list), used for validation and setting defaults
  public Book {
    if (price < 0.0) {
      throw new IllegalArgumentException("price must be positive");
    }
    if (publishDate != null && publishDate.isAfter(LocalDate.now())) {
      throw new IllegalArgumentException("publishDate cannot be in the future");
    }
    this.author = author == null ? "Unknown" : author;
  }
}

The compact constructor does not have a parameter list. It validates the price and publish date, and also sets a default value for the author. The fields that have not been assigned in this constructor (i.e. title, publishDate and price) are implicitly initialised at the end of this constructor.

Alternative Constructors and Additional Methods

Records allow you to define additional methods, constructors, and static fields, as shown in the code below. However, remember that semantically a record is designed to be a data carrier, so if you feel that are adding extra methods, it might be that you need a class instead of a record.

public record Book(String title, String author, LocalDate publishDate, double price) {

  // static field
  private static final String UNKNOWN_AUTHOR = "UNKNOWN";

  // compact constructor, used for validation and setting defaults
  public Book {
    if (price < 0) {
      throw new IllegalArgumentException("price must be positive");
    }
    if (publishDate != null && publishDate.isAfter(LocalDate.now())) {
      throw new IllegalArgumentException("publishDate cannot be in the future");
    }
    this.author = author == null ? UNKNOWN_AUTHOR : author;
  }

  // static factory constructor
  public static Book freeBook(String title, String author, LocalDate publishDate) {
    return new Book(title, author, publishDate, 0.0);
  }

  // alternative constructor, without an author
  public Book(String title, LocalDate publishDate, double price) {
    this(title, null, publishDate, price);
  }

  // additional method to get the year of publish
  public int publishYear() {
    return publishDate.getYear();
  }

  // override toString to make it more user friendly
  @Override
  public String toString() {
    return String.format("%s (%tY) by %s for £%.2f", title, publishDate, author, price);
  }
}