Generics

Generics are a complex type which can contain multiple type arguments, which could be non-generic or generic.

Instantiation

Once a generic type is instantiated, it is called a parameterized type.

Generally, in the declaration of the class or method, <T> or any name should be written to indicate the generic nature of the code.

Example

public void <T> foo(T .....)

Bounded Type Parameters

We can bound the type parameters to declare generic classes while ensuring the type passed in fits the requirements of the class.

Example

If we know that <T> should be a type of Circle, we should declare it as public void <T extends Circle> ... instead.

Type Erasure

At compilation, the type parameters of the generics are erased.

Example

Pair<String, Integer> is transformed to Pair… Pair<T extends String, S extends Integer> is transformed to String, Integer

Since arrays are reifiable, this causes an issue as type information of generics are not readily available at run-time.

Reifiable types are types where full type information is available during run-time.

Generic array declaration T[] array is allowed, but generic array instantiation new T[]{} is not.

Unchecked Warnings

Type erasures can cause a run-time error that a compiler cannot prevent due to the lack of type information available.

If the type safety is assured, we can use the annotation @SuppressWarning("unchecked") to assure compiler.

Raw Types

Raw types are generic types without type arguments. This causes a warning in the compiler as well.