Java 8 Type Annotations

Before Java 8, we can use annotations at many places but only on declarations e.g. on classes, interfaces, fields, method declaration etc. In Java 8 we can generally use annotations on any use of a type, , e.g. with generics, with method return types, with object initiation using 'new', with 'implements' and 'extends' clauses, with type casts and with throws clauses. etc.

New Target ElementType in Java 8

The enum constants provided by java.lang.annotation.ElementType are used in java.lang.annotation.Target meta-annotations to specify where it is legal to write annotations of a given type.

Java 8 introduced two new Element types

• ElementType.TYPE_USE: allows an annotation to be applied at any type use.
• ElementType.TYPE_PARAMETER: allows an annotation to be applied at type variables (e.g. T in MyClass<T>).

For example we can now create a new annotation like:

@Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
public @interface MyAnnotation {
}

Usage Examples

public class MyClass extends @MyAnnotation OtherClass { }

public class MyClass implements @MyAnnotation someInterface { }

public class MyClass<@MyAnnotation T> {}

List<@MyAnnotation String> myList;
List<@MyAnnotation T> myList;
List<@MyAnnotation ? extends OtherClass> myList;
List<@MyAnnotation ? super T> myList
List<@MyAnnotation ?> myList;

private @MyAnnotation String str;
private @MyAnnotation List<String> mylist

private @MyAnnotation char[] chars; //on char
private @MyAnnotation char[][] chars2;//on char
private char @MyAnnotation [] chars3;//on char array, char[]
private char[] @MyAnnotation [] chars4; //on char array, char[] which is component of char[][]

MyClass.@MyAnnotation NestedClass nestedClass;//NestedClass is nested class of MyClass

List<String> list = new @MyAnnotation ArrayList<>();
MyClass myClass = new @MyAnnotation MyClass();

MyClass myClass = new @MyAnnotation MyClass();
MyClass.NestedClass nestedClass = myClass.new @MyAnnotation NestedClass();

MyClass myClass = (@MyAnnotation MyClass) otherClassInstance;

boolean b = myObject instanceof @MyAnnotation OtherClass;

public @MyAnnotation String getString(){.....}

public void showString(@MyAnnotation String str){.....}

myObject.<@MyAnnotation String>myMethod(.....)

public void myMethod(){
        @MyAnnotation List<String> str = new ArrayList<>();
}

Function<MyClass, String> f = @MyAnnotation MyClass::myMethod;

void myMethod() throws @MyAnnotation Exception{}

try {
     myObject.myMethod();
  } catch (@MyAnnotation Exception e) {
            .....
 }

In next tutorial we will explore some use cases of type annotations.