Java Generics Lecture Notes

Introduction to Generics

• Presenter: John, Lead Java Software Engineer
• Purpose: Explain generics in Java, clarifying their utility and application.
• Generics can appear confusing with terms like T, K, V, and special characters, but will be simplified.
• A full Java course is available via the link in the description.

Problems Before Generics

• Example Class: IntegerPrinter

  • Holds an integer value and prints it.
  • Simple setup with integer field and constructor.
  • Method to print the integer (e.g., System.out.println(thingToPrint);).

• **Issues:

  • Code Duplication**
  • To create similar functionality for other types (e.g., double, string), separate classes would need to be created (e.g., DoublePrinter, StringPrinter).
  • Leads to significant code duplication, especially for more complex classes.

Introduction of Generics

• Solution: Generics allow a single class to handle multiple types.
• Generic Class Implementation
  • Rename IntegerPrinter to Printer.
  • Define a type parameter (conventionally T) in angle brackets.
  • Change field type to T, making it flexible for any type.

How to Use Generics

• Create a printer instance using a specific type: Printer<Integer> intPrinter = new Printer<>(23);
• Generics eliminate the need for multiple classes for different types, reducing code duplication.
• Generics do not work with primitive types; use wrapper classes instead (e.g., Integer, Double).

Generics in Java Collections

• Example with ArrayList: ArrayList<Cat> cats = new ArrayList<>();
• This ensures type safety; errors occur at compile time if a wrong type is added (e.g., adding a dog will result in a compile error).

Benefits of Generics

• Generics provide:
  • Type safety (compile-time checks).
  • Flexibility (single class for multiple types).

Bounded Generics

• Specialization: Limit the type to certain bounds (e.g., T extends Animal).
• Example:
  • Create a printer that only accepts types that extend Animal.
• This allows access to methods defined in the Animal class (e.g., thingToPrint.eat()).

Advanced Generics Concepts

• Multiple Bounds:
  • Specify bounds with &, e.g., T extends Animal & Serializable.
• Generic Methods:
  • Define methods that accept generic types. Example:
  private static <T> void shout(T thingToShout) { System.out.println(thingToShout + "!"); }
• Wildcards:
  • Use wildcards for unknown types (e.g., List<?>).
  • Allows passing lists of any type without knowing what it is.
  • E.g., printList(List<?> list).

Conclusion

• Generics are essential for effective Java programming, providing code reusability, type safety, and flexibility.
• Encouraged to explore further and practice using generics in Java.
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• Thank You: Appreciate the audience for attending the lecture.