Viacheslav Eremin | Java Generic, Constraints, Narrowing and Widening conversions

(FRONT) FRONT (2016)

Java Generic, Constraints, Narrowing and Widening conversions

1. Java generic

Generics allow you to write type-safe code that works with different types without needing to know the exact type beforehand. They introduce type parameters (like ) that act as placeholders for the actual types used in your code.

How to Use Generics:

Type Parameters: You define type parameters in angle brackets (<>) after the class name.
Type Arguments: When you create an instance of a generic class, you specify the type argument in angle brackets.
Bounded Type Parameters: You can restrict the types that can be used with a generic class by using bounded type parameters.

Example in Java

   1:  import java.util.ArrayList;

   2:  import java.util.List;

3:

   4:  public class GenericExample {

5:

   6:      public static void main(String[] args) {

   7:          // Generic List of Integers

   8:          List<Integer> intList = new ArrayList<>();

   9:          intList.add(10);

  10:          intList.add(20);

  11:          intList.add(30);

12:

  13:          // Generic List of Strings

  14:          List<String> stringList = new ArrayList<>();

  15:          stringList.add("Hello");

  16:          stringList.add("World");

  17:          stringList.add("!");

18:

  19:          // Generic method to print any type

  20:          printList(intList);

  21:          printList(stringList);

  22:      }

23:

  24:      public static <T> void printList(List<T> list) {

  25:          for (T item : list) {

  26:              System.out.print(item + " ");

  27:          }

  28:          System.out.println();

  29:      }

  30:  }

Example in C#

   1:  using System;

   2:  using System.Collections.Generic;

3:

   4:  public class GenericExample {

   5:      public static void Main(string[] args) {

   6:          // Generic List of Integers

   7:          List<int> intList = new List<int>();

   8:          intList.Add(10);

   9:          intList.Add(20);

  10:          intList.Add(30);

11:

  12:          // Generic List of Strings

  13:          List<string> stringList = new List<string>();

  14:          stringList.Add("Hello");

  15:          stringList.Add("World");

  16:          stringList.Add("!");

17:

  18:          // Generic method to print any type

  19:          PrintList(intList);PrintList(stringList);

  20:      }

21:

  22:      public static void PrintList<T>(List<T> list) {

  23:          foreach (T item in list) {

  24:              Console.Write(item + " ");

  25:          }

  26:          Console.WriteLine();

  27:      }

  28:  }

Generics in JavaScript:

JavaScript doesn't have true generics in the same way that Java and C# do. However, TypeScript, a superset of JavaScript, introduces generics.

In JavaScript, you can achieve similar results using dynamic typing and functions that accept any type of argument.

How to Use Generics:

Type Parameters: You define type parameters in angle brackets (<>) after the class name.
Type Arguments: When you create an instance of a generic class, you specify the type argument in angle brackets.
Bounded Type Parameters: You can restrict the types that can be used with a generic class by using bounded type parameters.

   1:  class GenericExample {

   2:      private items: T[] = [];

   3:      addItem(item: T): void {

   4:          this.items.push(item);

   5:      }

   6:      getItems(): T[] {

   7:          return this.items;

   8:      }

   9:  }

Summary:

Java:

Type Safety: Java's generics enforce type safety at compile time.
Type Erasure: Java's generics use type erasure, which means that the type information is removed at compile time.
Bounded Type Parameters: Java's generics allow you to restrict the types that can be used with a generic class by using bounded type parameters.

Type Safety: C#'s generics enforce type safety at compile time.
Reified Generics: C#'s generics use reified generics, which means that the type information is preserved at runtime.
Bounded Type Parameters: C#'s generics allow you to restrict the types that can be used with a generic class by using bounded type parameters.

JavaScript:

Dynamic Typing: JavaScript uses dynamic typing, which means that the type of a variable is not known until runtime.
No True Generics: JavaScript does not have true generics in the same way that Java and C# do.
TypeScript: TypeScript, a superset of JavaScript, introduces generics.
Type Inference: TypeScript uses type inference, which means that the type of a variable is inferred from its value.

2. Type Conversions in General: Java vs. C#

Java is more strict about type conversions. It generally requires explicit casting when converting between incompatible types. Java distinguishes between primitive types (like int, float, boolean) and reference types (like String, custom classes). Conversions between these categories are handled differently. Java has built-in methods for some conversions (e.g., Integer.parseInt(), String.valueOf()) when dealing with primitive wrappers or strings.	C# is more flexible with type conversions. It supports both implicit and explicit conversions. C# also distinguishes between value types (structs) and reference types (classes). C# provides a wider range of built-in conversion methods (e.g., Convert.ToInt32(), ToString()). C# allows user-defined conversions using the implicit and explicit keywords, which can be very powerful. Narrowing and Widening Conversions Widening Conversion (Implicit Conversion): Definition: Converting a smaller data type to a larger data type without loss of information.
Java automatically performs widening conversions between compatible primitive types. For example, an int can be implicitly converted to a long because long can hold all the values of an int. Widening conversions also occur between primitive types and their corresponding wrapper classes. Widening conversions can also occur between reference types, but this is less common.	C# also automatically performs widening conversions. For example, an int can be implicitly converted to a long. C# also allows user-defined widening conversions using the implicit keyword. Widening conversions can occur between reference types, but this is less common. Narrowing Conversion (Explicit Conversion): Definition: Converting a larger data type to a smaller data type, potentially losing information.
Java requires explicit casting for narrowing conversions. For example, you must use (int) to convert a long to an int. Narrowing conversions can also occur between primitive types and their corresponding wrapper classes. Narrowing conversions can occur between reference types, but this is less common.	C# requires explicit casting for narrowing conversions. For example, you must use (int) to convert a long to an int. C# also allows user-defined narrowing conversions using the explicit keyword. Narrowing conversions can occur between reference types, but this is less common.

So, Key Differences Summarized

Implicit vs. Explicit: C# is more flexible with implicit conversions, while Java generally requires explicit casting.
Primitive vs. Reference: Both languages handle conversions between primitive and reference types, but the mechanisms differ slightly.
User-Defined Conversions: C# allows user-defined conversions with implicit and explicit, offering more flexibility. Java has fewer options for user-defined conversions.
Built-in Methods: C# has a broader range of built-in conversion methods compared to Java.

Here are some simple examples to illustrate the concepts:

Java (Widening)

   1:  int myInt = 10;

   2:  long myLong = myInt; // Implicit widening conversion

   3:  System.out.println(myLong); // Output: 10

Java (Narrowing)

   1:  int myInt = 10;

   2:  long myLong = myInt; // Implicit widening conversion

   3:  System.out.println(myLong); // Output: 10

   1:  long myLong = 100L;

   2:  int myInt = (int) myLong; // Explicit narrowing conversion

   3:  System.out.println(myInt); // Output: 100

C# (Widening)

   1:  int myInt = 10;

   2:  long myLong = myInt; // Implicit widening conversion

   3:  Console.WriteLine(myLong); // Output: 10

C# (Narrowing)

   1:  long myLong = 100L;

   2:  int myInt = (int)myLong; // Explicit narrowing conversion

   3:  Console.WriteLine(myInt); // Output: 100

C# (User-Defined)

Summary, C# offers more flexibility in type conversions, including implicit conversions and user-defined conversions. Java is more strict, generally requiring explicit casting. Both languages support widening and narrowing conversions, but the specific mechanisms differ. C# provides a broader range of built-in conversion methods.

3. C# generic constraints

In C#, when you define a generic method or class, you can specify constraints on the type parameters. These constraints allow you to restrict the types that can be used as type arguments. When you want to ensure that different types passed to a generic function share a common interface, you can use constraints to enforce that.

1. Defining Constraints:

You use the where keyword to specify constraints on type parameters.

Constraints can include:

   1:  where T : class (T must be a reference type)

   2:  where T : struct (T must be a value type)

   3:  where T : SomeInterface (T must implement SomeInterface)

   4:  where T : BaseClass (T must inherit from BaseClass)

   5:  where T : new() (T must have a parameterless constructor)

2. Examples:

   1:  // Define an interface

   2:  public interface IPrintable

   3:  {

   4:  void Print();

   5:  }

   1:  // Define a generic method with a constraint

   2:  public static void PrintItem<T>(T item) where T : IPrintable

   3:  {

   4:  item.Print();

   5:  }

6:

   1:  // Example usage

   2:  public class Book : IPrintable

   3:  {

   4:  public void Print()

   5:  {

   6:  Console.WriteLine("Printing a book");

   7:  }

   8:  }

9:

  10:  public class Document : IPrintable

  11:  {

  12:  public void Print()

  13:  {

  14:  Console.WriteLine("Printing a document");

  15:  }

  16:  }

17:

  18:  public class Program

  19:  {

  20:  public static void Main(string[] args)

  21:  {

  22:  Book myBook = new Book();

  23:  Document myDocument = new Document();

24:

  25:  PrintItem(myBook); // Output: Printing a book

  26:  PrintItem(myDocument); // Output: Printing a document

  27:  }

  28:  }

3. Key Concepts:

Interface: An interface is a set of methods and properties that a type must implement.
Generic Type: A generic type is a type that can be used to create a variety of types.
Constraint: A constraint is a way to ensure that a type is compatible with a generic type.
Type Argument: A type argument is a type that is used to create a generic type.
Type Parameter: A type parameter is a type that is used to create a generic type.

4. Benefits of Constraints:

Type Safety: Ensures that the type is compatible with the generic type.
Reduced Code Duplication: Allows you to write code that works with different types.
Improved Readability: Makes it easier to understand the code.

5. Common Use Cases:

Implementing Interfaces: You can use constraints to ensure that a type implements an interface.
Implementing Base Class: You can use constraints to ensure that a type inherits from a base class.
Implementing Constructor: You can use constraints to ensure that a type has a parameterless constructor.

Summary:

Generic constraints are a way to ensure that different types passed to a generic function share a common interface.
Constraints are defined using the where keyword.
Constraints are useful for ensuring type safety and reducing code duplication.

4. Java generic constraints

In Java, the mechanism for achieving something similar to C#'s generic constraints is called bounded type parameters. Here's a breakdown of how it works in Java:

1. Bounded Type Parameters

Concept: Instead of using a where clause like in C#, Java uses the extends keyword in the type parameter declaration to specify a bound. This bound can be a class or an interface.

   1:  public class MyGenericClass<T extends MyBound> {

   2:  // ...

   3:  }

In this example, `T` is the type parameter, and `MyBound` is the upper bound. This means that any type used as a type argument for `T` must be a subtype of `MyBound` (either a class that extends `MyBound` or an interface that implements `MyBound`).

2. Example to Java

   1:  // Define an interface

   2:  interface Printable {

   3:  void print();

   4:  }

   1:  // Define a generic method with a bounded type parameter

   2:  class Printer {

   3:  public static <T extends Printable> void printItem(T item) {

   4:      item.print();

   5:      }

   6:      }

   1:  // Example usage

   2:  class Book implements Printable {

   3:  public void print() {

   4:  System.out.println("Printing a book");

   5:  }

   6:  }

7:

   8:  class Document implements Printable {

   9:  public void print() {

  10:  System.out.println("Printing a document");

  11:  }

  12:  }

13:

  14:  public class Main {

  15:  public static void main(String[] args) {

  16:  Book myBook = new Book();

  17:  Document myDocument = new Document();

18:

  19:  Printer.printItem(myBook); // Output: Printing a book

  20:  Printer.printItem(myDocument); // Output: Printing a document

  21:  }

  22:  }

3. Key Differences from C#

extends Keyword: Java uses extends to specify the upper bound of a type parameter, while C# uses where.
Multiple Bounds: Java allows you to specify multiple bounds for a type parameter using the & operator (intersection type). For example:

   1:  public class MyGenericClass<T extends MyBound1 & MyBound2> {

   2:      // ...

   3:      }

This means that `T` must be a subtype of both `MyBound1` and `MyBound2`.
super Keyword: Java also supports lower bounds using the super keyword. This is useful when you want to work with a type that is a supertype of the type parameter.
Wildcards: Java has a powerful feature called wildcards (?), which allow you to work with generic types without knowing the exact type.

4. Benefits of Bounded Type Parameters

Type Safety: Ensures that the type is compatible with the generic type.
Reduced Code Duplication: Allows you to write code that works with different types.
Improved Readability: Makes it easier to understand the code.

5. Common Use Cases

Implementing Interfaces: You can use bounded type parameters to ensure that a type implements an interface.
Implementing Base Class: You can use bounded type parameters to ensure that a type inherits from a base class.
Bounded Wildcards: You can use bounded wildcards to work with generic types without knowing the exact type.

Summary:

Bounded type parameters are defined using the extends keyword.
Bounded type parameters are useful for ensuring type safety and reducing code duplication.
Bounded wildcards are useful for working with generic types without knowing the exact type.
Bounded type parameters are the Java equivalent of generic constraints.
Bounded Type Parameters: Java's way of constraining generic types.
extends Keyword: Used to specify the upper bound.
Multiple Bounds: Java supports multiple bounds using &.
super Keyword: Used for lower bounds.
Wildcards: A powerful feature for working with generic types.

Java context:

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