Generics

HadjshellAbout 1 minNoteLanguageJava

One of the most significant change in Java SE5
The concept of parameterized types: you tell what type you want to use, and it takes care of the details

// generic class or interface
public class ClassName<T> {}
public interface InterfaceName<A, B, C, D> {}

// generic methods
public <T> void func(T x) {}

If you don't specify the type explicitly when declaring the instance, it will be an Object type
Generics class, generics interface, generics method
Static generic method cannot use the generic type declared by its class because that type is only accessible after instantiation

Bounds

Allow you to place constraints on the parameter types that can be used with generics

class A {}
class B extends A {}

// always extends whenever A is a class or an interface
class Test<T extends A> {}

public class Main {
  public static void main(String[] args) {
    // both allowed
    Test<A> t = new Test<>();
    Test<B> t = new Test<>();
  }
}

Wildcard

Type arguments of parameterized types

// upper bound
? extends B
// lower bound
? super B

class Test {
    static void printCollection(Collection<?> c) {
                                // a wildcard collection
        for (Object o : c) {
            System.out.println(o);
        }
    }

    public static void main(String[] args) {
        Collection<String> cs = new ArrayList<String>();
        cs.add("hello");
        cs.add("world");
        printCollection(cs);
    }
}

If the type parameter is only used in method signature, in the absence of such interdependency between the type(s) of the argument(s), the return type and/or throws type, generic methods are considered bad style, and wildcards are preferred

Java generics are implemented using erasure, which means that any specific type information is erased when you use a generic
- ```
// This is not allowed
T[] t = new T[size]; // compile error

T[] t = (T[]) new Object[size]; // no error
```
Erasure is a compromise in the implementation of Java generics, reification would be a better choice if generics had been part of Java 1.0
Erasure ensures migration compatibility