How to Create a Generic Instance of a Type in Java?

How can I create a generic instance of a type in Java?

List<String> stringList = new ArrayList<>(); // Example of creating a generic list

In Java, generics allow you to define classes, interfaces, and methods with a placeholder for types, enabling type safety and code reusability. This powers the creation of instances of generic types, providing you with flexibility in using different data types without sacrificing type checking during compile time.

// Defining a generic class
class Box<T> {
    private T item;
    
    public void setItem(T item) {
        this.item = item;
    }

    public T getItem() {
        return item;
    }
}

// Creating an instance of a generic class
Box<String> stringBox = new Box<>(); 
stringBox.setItem("Hello Generics!");  // Storing a String
System.out.println(stringBox.getItem());  // Output: Hello Generics!

Causes

• Lack of understanding of generics syntax and concepts.
• Trying to create an instance of a generic type directly, which is not allowed in Java.

Solutions

• Use type parameters when defining your class, interface, or method.
• Instantiate generic classes with concrete type arguments only when you create variables.
• Utilize the diamond operator ("><") for type inference in many cases.

Mistake: Trying to instantiate a generic type directly, like 'new T()'.

Solution: Instead, use a concrete type when creating instances.

Mistake: Forgetting to specify type parameters when using generic classes.

Solution: Always specify the type when declaring a generic variable.

Mistake: Overusing generics leading to complex and unreadable code.

Solution: Use generics judiciously to maintain code clarity.