Java generics

Preview

1. What is the purpose of Java generics and how do they differ from traditional Java programming techniques?
2. How can you declare and use generic classes and methods in your Java code?
3. What are the benefits of using Java generics, and how can they help improve the readability and maintainability
   of your code?
4. What are some common mistakes to avoid when working with Java generics, and how can you ensure that your
   code is type-safe and exception-free?
5. Are there any limitations or trade-offs associated with using Java generics, and how can you determine when it is
   appropriate to use them in your code?

Explain

Java generics is a language feature that enables the creation of classes, interfaces, and methods that work with different types, which are specified at compile time. It is a way of allowing developers to write reusable and type-safe code.

Java generics are defined using angle brackets <>, which encapsulate a type parameter. For example, a generic class to represent a box that can hold any type of object would be defined as follows:

public class Box<T> {
    private T item;

    public void put(T item) {
        this.item = item;
    }

    public T get() {
        return item;
    }
}

In this example, the type parameter T can be any type, and the variables and methods in the class can work with any type of object that is specified at the time of instantiation.

Using this class, we can create instances of the Box class that are specific to a particular type:

Box<String> stringBox = new Box<>();
stringBox.put("Hello World!");
String message = stringBox.get(); // "Hello World!"

Box<Integer> integerBox = new Box<>();
integerBox.put(42);
int value = integerBox.get(); // 42

In this way, Java generics enable developers to write reusable and type-safe code, reducing the likelihood of errors and making the code more robust and easier to maintain.

Keypoint

1. Java generics were introduced in Java 5 to provide type safety and parameterized types.
2. They enable classes, interfaces, and methods to work with any data type.
3. They enable compile-time type checking which helps in identifying and fixing errors during the development phase.
4. Generics use type parameter, which is a placeholder for the actual type that will be used at runtime.
5. Usage of generics reduces the need for explicit casts and enhances code readability.
6. Generics can be used with collections, interfaces, classes, methods, and constructors.
7. Wildcards are used in generics to provide more flexibility- Upper Bounded, Lower Bounded, and Unbounded.
8. Generic types retain their type information at runtime through a mechanism called Type Erasure.
9. Typed Collection classes like ArrayList, LinkedList, Stack, and Vector are prime examples that use generics to ensure type safety.

Review

1. What is the difference between ClassCastException and TypeCastException in Java generics?
   Answer: ClassCastException occurs when we try to cast a non-compatible object to a generic type, whereas TypeCastException occurs when we try to cast a generic type to a non-compatible type.

2. Can we use primitive data types with Java generics?
   Answer: No, we cannot use primitive data types with Java generics. We need to use their corresponding object types instead.

3. What is the difference between wildcard and bounded type parameters in Java generics?
   Answer: Wildcard type parameters allow us to use any type as an argument, while bounded type parameters restrict the argument to a specific type or its subtypes.

4. Can we create an instance of a generic type in Java?
   Answer: No, we cannot create an instance of a generic type directly in Java. We need to create an instance of a concrete type and then cast it to the generic type.

5. What is the purpose of type erasure in Java generics?
   Answer: Type erasure is a process of replacing generic type parameters with their upper bounds during compilation. This is done to ensure backward compatibility with pre-generics code and to make the code more efficient.