Annotations offer an alternative to the use of XML descriptors and marker interfaces. Theyre a powerful part of Java that was added in JDK5. Suppose we wanted to annotate it with an annotation, lets call it Immutable. and then make your code carefully treat Optional.NULL as if it were really null. Java annotations are a mechanism for adding metadata information to our source code. To motivate the type parameter use case, consider Guavas class ImmutableList.Public static final NULL = "THIS IS A SPECIAL NULL VALUE - DO NOT USE" The only thing you can do is workaround it, like this: Annotations in Java are used to provide additional information, so it is an alternative option for XML and Java marker interfaces. attached with class, interface, methods or fields to indicate some additional information which can be used by java compiler and JVM. In the previous examples, Override and SuppressWarnings are predefined Java annotations. Java Annotation is a tag that represents the metadata i.e. The annotation type can be one of the types that are defined in the java.lang or packages of the Java SE API. For more information, see Repeating Annotations. In this article, we will discuss how to change annotation value at runtime using Reflection. These annotations can be processed at compile time and embedded to class files or can be retained and accessed at runtime using Reflection. You can never pass null as a Java annotation parameter value, because, uh, null isn't a ConstantExpression. Repeating annotations are supported as of the Java SE 8 release. Annotations, a form of metadata which you can add to Java code. Notice anything missing? That's right, null. "The integer " + Long.MAX_VALUE + " is mighty big." JSR-175, which defined annotations for Java 5, just says "If member type is a primitive type or String, the ConditionalExpression must be a constant expression ( JLS 15.28)." JLS 15.28, in turn, says that constant expressions can be, for example, any of these: Why is this? The specifications are opaque. javac says "attribute value must be constant " Eclipse says "The value for annotation attribute Optional.value must be a constant expression." In fact, it's not that surprising, because even if you didn't set a default value, writing this would also be illegal (same the error is saying is that you can't set a Java annotation parameter to null. Suppose you want to write a Java annotation that has a parameter value, but you want its default value to be null. I don't update here often enough, but here's a tidbit I wish I'd found on Google earlier. The ElementType.TYPEPARAMETER target indicates that the annotation can be written on the declaration of a type variable (e.g., class Målass.
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