When working with Java Generics, it is essential to grasp the concept of upper and lower type bounds. These bounds determine the limits of the types that can be used as type arguments in generic classes, methods, and interfaces.
Let's dive deeper into these two types of bounds and explore how they can be used in practice.
An upper type bound is used to define a constraint on the type argument of a generic class, method, or interface. It ensures that the type argument is a subtype of the specified upper bound type.
To set an upper type bound, you use the extends keyword followed by the desired upper bound type. For example, consider the following declaration:
noob to masterclass MyClass<T extends Number> {
// ...
}In this case, the type T can be any subclass of the Number class. This means you can use Integer, Double, Float, or any other class that extends Number as the type argument when creating an instance of MyClass.
An upper type bound restricts the possible types that can be used. It allows you to make assumptions about the capabilities of the type argument. In the example above, you can safely assume that the methods and properties provided by the Number class will be available for the type T.
On the other hand, lower type bounds are used to set a constraint on the type argument by specifying a supertype. In this case, the type argument must be the specified supertype or any of its subclasses.
To define a lower type bound, you use the super keyword followed by the desired lower bound type. Let's look at an example:
class MyClass<T super Number> {
// ...
}Here, the type T must be either Number or a superclass of Number. This enables you to use Number, Object, or any other superclass of Number as the type argument.
Using a lower type bound allows you to make assumptions about how the type argument will be used. It guarantees that the methods and properties provided by Number or its supertypes will be accessible.
To make things more concrete, let's consider a practical example where upper and lower type bounds can be applied effectively.
Imagine you are designing a generic method that calculates the average of a collection of numbers. In this case, you might want to restrict the input to only accept types that are numeric, such as Integer, Double, or Float, but not other non-numeric types.
To achieve this, you can use an upper type bound to ensure that the type argument extends Number. Here's an example:
public <T extends Number> double calculateAverage(List<T> numbers) {
// implementation
}This method can now accept a list of any class that is a subclass of Number. Thus, you can pass a List<Integer>, List<Double>, or any other subtype of Number as an argument.
By utilizing the upper type bound, you can guarantee that the generic method will only work with numeric types, preventing unexpected runtime errors.
Understanding and using upper and lower type bounds is crucial when working with Java Generics. They allow you to express constraints on the types that can be used as type arguments, providing compile-time safety and enabling assumptions about the properties and methods available for those types.
By leveraging upper and lower type bounds effectively, you can design more flexible and robust generic classes, methods, and interfaces.
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