Key Java 17 Features

1. Records

• Introduced in Java 14 (preview), finalized in Java 16.

• Purpose: A concise way to create immutable data classes.
  
  
  

Before : 

class User {

    private final String name;

    private final int age;

    User(String name, int age) {

        this.name = name;

        this.age = age;

    }

    public String name() { return name; }

    public int age() { return age; }

    @Override public String toString() { return name + " (" + age + ")"; }

}

with Records : 

record User(String name, int age) {}

2) Sealed Classes

• Introduced in Java 15 (preview), finalized in Java 17.

• Purpose: Restrict which classes can extend or implement a type.

• Provides more control over inheritance.

sealed interface Shape permits Circle, Rectangle {}

final class Circle implements Shape {

    double radius;

}

final class Rectangle implements Shape {

    double length, width;

}

3. Enhanced Switch (Switch Expressions + Pattern Matching)

• Modernized since Java 12 → finalized in Java 17.

• Switch can return a value and use arrow syntax.

Old :

String dayType;

switch (day) {

    case "MONDAY": case "TUESDAY": case "WEDNESDAY":

        dayType = "Weekday";

        break;

    default:

        dayType = "Weekend";

}

NEW : 

String dayType = switch (day) {

    case "MONDAY", "TUESDAY", "WEDNESDAY" -> "Weekday";

    default -> "Weekend";

};

Also supports pattern matching (useful with sealed classes):

static String describeShape(Shape shape) {

    return switch (shape) {

        case Circle c -> "Circle with radius " + c.radius;

        case Rectangle r -> "Rectangle " + r.length + " x " + r.width;

    };

}

Quick Practice Checklist

• Convert an old POJO (like User) → record.

• Create a sealed interface Vehicle with Car, Bike, Truck.

• Write a switch expression that works with the Vehicle hierarchy.

• Notice how exhaustiveness is enforced (compiler warns if you miss a case).

4)  Pattern Matching for instanceof

• Simplifies type checks + casting.
  
  

  if (obj instanceof String) {

      String s = (String) obj;

      System.out.println(s.length());

  }
  
  NOW : 
  
  

  if (obj instanceof String s) {

      System.out.println(s.length());

  }

5) 

 Text Blocks (Multi-line Strings)

• Introduced earlier (Java 13+), standard in Java 15+.

• Makes multi-line strings easier (useful for JSON, SQL, XML in code).

• String json = """

  {

    "name": "Alice",

    "age": 30

  }

  """;

Stream.toList() and Collection Factory Methods

• Instead of .collect(Collectors.toList()), use:

List<String> list = Stream.of("a","b","c").toList();

• Immutable by default.

• Collection factory methods:

Set<String> s = Set.of("a", "b", "c");
Map<String, Integer> m = Map.of("a",1,"b",2);