How do I use Stream.toList() instead of collect(Collectors.toList())?

By Wayan in Java 16, Util Package 0 Comment

In Java 16, a convenient method Stream.toList() was introduced to simplify collecting elements of a Stream into a List. It provides a more concise alternative to collect(Collectors.toList()), which was used in older versions of Java.

Key Differences

  • Stream.toList() produces an immutable list, whereas collect(Collectors.toList()) produces a mutable list.
  • Stream.toList() guarantees immutability, meaning the resulting list cannot be structurally modified (additions, deletions, updates).
  • collect(Collectors.toList()) does not enforce immutability. It typically returns an ArrayList.

How to Replace collect(Collectors.toList()) with Stream.toList()

If you want to update your code to use Stream.toList() (introduced in Java 16), here’s how you can do it.

Using collect(Collectors.toList()) (Old Style):

package org.kodejava.util.stream;

import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class Main {
    public static void main(String[] args) {
        List<String> result = Stream.of("a", "b", "c")
                                    .collect(Collectors.toList());
        System.out.println(result);
    }
}

Using Stream.toList() (New Style):

package org.kodejava.util.stream;

import java.util.List;
import java.util.stream.Stream;

public class Main {
    public static void main(String[] args) {
        List<String> result = Stream.of("a", "b", "c")
                                    .toList(); // Simpler, concise, and immutable
        System.out.println(result);
    }
}

How to Modify Your Code:

  1. Replace .collect(Collectors.toList()) with .toList().
  2. Ensure your code works well with an immutable list because Stream.toList() returns a list that does not allow structural modifications.

Example Comparison:

Immutable List with Stream.toList():

List<String> result = Stream.of("a", "b", "c").toList();
result.add("d"); // Throws UnsupportedOperationException

Mutable List with collect(Collectors.toList()):

List<String> result = Stream.of("a", "b", "c").collect(Collectors.toList());
result.add("d"); // Works fine

Compatibility Note

  • If you are using Java 16 or above, prefer Stream.toList() for conciseness and immutability.
  • If you need a mutable list (e.g., you want to add or remove elements later), stick to collect(Collectors.toList()).

When to Use Each

  • Use Stream.toList() when immutability is preferred or sufficient.
  • Use collect(Collectors.toList()) when you need a list you can modify after creation.

How do I use record with generics?

By Wayan in Basic, Java 16 0 Comment

Using records with generics in Java allows you to create immutable data structures while also providing the benefits of generics, such as type safety and flexibility. Since Java 16, the record feature was introduced, and it can be combined with generics like other classes. Here’s an example of how to use records with generics.

Syntax

To define a record with generics, you specify the type parameter(s) in the record declaration just like in a class declaration.

public record MyRecord<T>(T value) { }

Examples

1. Basic Generic Record

A simple record that accepts a generic type parameter:

// Define record with a generic parameter T
public record Box<T>(T content) { }

Usage:

public class Main {
    public static void main(String[] args) {
        // Create a record with a String type
        Box<String> stringBox = new Box<>("Hello, generics!");
        System.out.println(stringBox.content()); // Output: Hello, generics!

        // Create a record with an Integer type
        Box<Integer> integerBox = new Box<>(123);
        System.out.println(integerBox.content()); // Output: 123
    }
}

2. Records with Multiple Generics

You can define records with multiple type parameters, just like generic classes:

// Define a record with two generic types
public record Pair<K, V>(K key, V value) { }

Usage:

public class Main {
    public static void main(String[] args) {
        // Create a Pair with String and Integer
        Pair<String, Integer> pair = new Pair<>("Age", 30);
        System.out.println(pair.key() + ": " + pair.value()); // Output: Age: 30

        // Create a Pair with two different types
        Pair<Double, String> anotherPair = new Pair<>(3.14, "Pi");
        System.out.println(anotherPair.key() + " -> " + anotherPair.value()); // Output: 3.14 -> Pi
    }
}

3. Generics with Constraints

Generic records can include bounded type parameters to restrict the types allowed:

// Generic type T is constrained to subclasses of Number
public record NumericBox<T extends Number>(T number) { }

Usage:

public class Main {
    public static void main(String[] args) {
        // Only Number or subclasses of Number are allowed
        NumericBox<Integer> intBox = new NumericBox<>(42);
        System.out.println(intBox.number()); // Output: 42

        NumericBox<Double> doubleBox = new NumericBox<>(3.14);
        System.out.println(doubleBox.number()); // Output: 3.14

        // Compiler error: String is not a subclass of Number
        // NumericBox<String> stringBox = new NumericBox<>("Not a number");
    }
}

4. Working with Wildcards

You can use wildcards in generic records when specifying their types:

public class Main {
    public static void main(String[] args) {
        // Using a wildcard
        Box<?> anyBox = new Box<>("Wildcard content");
        System.out.println(anyBox.content()); // Output: Wildcard content

        // Using bounded wildcards
        NumericBox<? extends Number> numBox = new NumericBox<>(42);
        System.out.println(numBox.number()); // Output: 42
    }
}

Benefits of Using Generics with Records

  1. Type Safety: With generics, the compiler ensures the record is used correctly for the intended type.
  2. Reusability: You can use the same record with different data types.
  3. Immutability: Records’ inherent immutability, coupled with generics, allows you to encapsulate type-safe, immutable data structures.

This approach works seamlessly with the other features of records, such as pattern matching and compact constructors. Let me know if you’d like more advanced scenarios!

How do I use records for immutable data carriers?

By Wayan in Java 16, Lang Package 0 Comment

In Java, records are special classes introduced in Java 14 (as a preview feature) and became a standard feature in Java 16. They are specifically designed to model immutable data carriers with minimal boilerplate code. A record in Java automatically generates boilerplate code such as constructors, getters, toString, equals, and hashCode methods, making it a great choice for representing immutable data.

Here’s how you can use records as immutable data carriers:

1. Define a Record

To define a record, use the record keyword. A record automatically generates:

  • A constructor.
  • Accessors (getters) for all fields.
  • toString(), equals(), and hashCode() based on the fields.

Example:

public record User(String name, int age) {}

This creates an immutable User record class with:

  • Fields: name and age
  • Automatically provides:
    • Constructor: User(String name, int age)
    • name() and age() as accessors for the fields
    • A meaningful `toString(), method
    • Implementations of equals() and hashCode()

2. Using a Record

Once defined, you can use the record class as follows:

public class Main {
    public static void main(String[] args) {
        // Creating and using a User record
        User user = new User("Alice", 30);

        // Access fields (no need for `getName()` or `getAge()`)
        System.out.println(user.name());  // Alice
        System.out.println(user.age());  // 30

        // Automatic toString()
        System.out.println(user);        // User[name=Alice, age=30]

        // Automatic equals() and hashCode()
        User anotherUser = new User("Alice", 30);
        System.out.println(user.equals(anotherUser)); // true
    }
}

3. Immutability

Records are immutable by default:

  • The fields of a record are implicitly private final.
  • Once an object is created, its fields cannot be changed.
  • Records make it easier to declare immutable objects compared to manually writing getters and using final.

4. Customizing a Record

While records are concise, you can still customize them if needed:

  • Add extra methods.
  • Implement additional interfaces.
  • Preprocess fields in the constructor or validate input.

Example:

public record User(String name, int age) {
   public User {
       // Compact constructor for validation
       if (age < 0) {
           throw new IllegalArgumentException("Age cannot be negative");
       }
   }

   // Additional method
   public String greeting() {
       return "Hello, " + name + "!";
   }
}

Usage:

User user = new User("Alice", 30);
System.out.println(user.greeting()); // Hello, Alice!

5. Limitations of Records

While records are extremely powerful for data carrier use cases, they are not suitable for every situation:

  1. Records cannot extend other classes (but they can implement interfaces).
  2. Fields in records cannot be modified after object creation.
  3. Records are designed primarily for data aggregation and are not meant for behavior-heavy classes.

6. Common Use Cases

  • Representing DTOs (Data Transfer Objects).
  • Creating immutable models for APIs.
  • Storing simple structured data (e.g., key-value pairs, coordinates).

Summary

To use records for immutable data carriers:

  1. Define them with record. The syntax automatically generates boilerplate code.
  2. Use the generated constructors and field accessors (name() instead of getName()).
  3. Optionally, customize validation or add methods if you need additional behavior.

By leveraging records, you simplify your code, reduce boilerplate, and ensure your data class is immutable by design!