The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
A set of annotation types are predefined in the Java SE API. Some annotation types are used by the Java compiler, and some apply to other annotations.
The predefined annotation types defined in java.lang
are @Deprecated
, @Override
, and @SuppressWarnings
.
@Deprecated
@Deprecated
annotation indicates that the marked element is deprecated and should no longer be used. The compiler generates a warning whenever a program uses a method, class, or field with the @Deprecated
annotation. When an element is deprecated, it should also be documented using the Javadoc @deprecated
tag, as shown in the following example. The use of the at sign (@
) in both Javadoc comments and in annotations is not coincidental: they are related conceptually. Also, note that the Javadoc tag starts with a lowercase d and the annotation starts with an uppercase D.
// Javadoc comment follows /** * @deprecated * explanation of why it was deprecated */ @Deprecated static void deprecatedMethod() { } }
@Override
@Override
annotation informs the compiler that the element is meant to override an element declared in a superclass. Overriding methods will be discussed in
Interfaces and Inheritance.
// mark method as a superclass method // that has been overridden @Override int overriddenMethod() { }
While it is not required to use this annotation when overriding a method, it helps to prevent errors. If a method marked with @Override
fails to correctly override a method in one of its superclasses, the compiler generates an error.
@SuppressWarnings
@SuppressWarnings
annotation tells the compiler to suppress specific warnings that it would otherwise generate. In the following example, a deprecated method is used, and the compiler usually generates a warning. In this case, however, the annotation causes the warning to be suppressed.
// use a deprecated method and tell // compiler not to generate a warning @SuppressWarnings("deprecation") void useDeprecatedMethod() { // deprecation warning // - suppressed objectOne.deprecatedMethod(); }
Every compiler warning belongs to a category. The Java Language Specification lists two categories: deprecation
and unchecked
. The unchecked
warning can occur when interfacing with legacy code written before the advent of
generics. To suppress multiple categories of warnings, use the following syntax:
@SuppressWarnings({"unchecked", "deprecation"})
@SafeVarargs
@SafeVarargs
annotation, when applied to a method or constructor, asserts that the code does not perform potentially unsafe operations on its varargs
parameter. When this annotation type is used, unchecked warnings relating to varargs
usage are suppressed.
@FunctionalInterface
@FunctionalInterface
annotation, introduced in Java SE 8, indicates that the type declaration is intended to be a functional interface, as defined by the Java Language Specification.
Annotations that apply to other annotations are called meta-annotations. There are several meta-annotation types defined in java.lang.annotation
.
@Retention
@Retention
annotation specifies how the marked annotation is stored:
RetentionPolicy.SOURCE
– The marked annotation is retained only in the source level and is ignored by the compiler.RetentionPolicy.CLASS
– The marked annotation is retained by the compiler at compile time, but is ignored by the Java Virtual Machine (JVM).RetentionPolicy.RUNTIME
– The marked annotation is retained by the JVM so it can be used by the runtime environment.@Documented
@Documented
annotation indicates that whenever the specified annotation is used those elements should be documented using the Javadoc tool. (By default, annotations are not included in Javadoc.) For more information, see the
Javadoc tools page.
@Target
@Target
annotation marks another annotation to restrict what kind of Java elements the annotation can be applied to. A target annotation specifies one of the following element types as its value:
ElementType.ANNOTATION_TYPE
can be applied to an annotation type.ElementType.CONSTRUCTOR
can be applied to a constructor.ElementType.FIELD
can be applied to a field or property.ElementType.LOCAL_VARIABLE
can be applied to a local variable.ElementType.METHOD
can be applied to a method-level annotation.ElementType.PACKAGE
can be applied to a package declaration.ElementType.PARAMETER
can be applied to the parameters of a method.ElementType.TYPE
can be applied to any element of a class.@Inherited
@Inherited
annotation indicates that the annotation type can be inherited from the super class. (This is not true by default.) When the user queries the annotation type and the class has no annotation for this type, the class' superclass is queried for the annotation type. This annotation applies only to class declarations.
@Repeatable
@Repeatable
annotation, introduced in Java SE 8, indicates that the marked annotation can be applied more than once to the same declaration or type use. For more information, see
Repeating Annotations.