Annotation

Reference from oracle tutorial

Annotations, a form of metadata, provide data about a program that is not part of the program itself. Annotations have no direct effect on the operation of the code they annotate.

Annotations have a number of uses, among them:

• Information for the compiler — Annotations can be used by the compiler to detect errors or suppress warnings.
• Compile-time and deployment-time processing — Software tools can process annotation information to generate code, XML files, and so forth.
• Runtime processing — Some annotations are available to be examined at runtime.

Used on

• package
• class
• method
• field

@Override  //mark as override function from super classes
public String toString(){
}

Take a look at declaration

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.SOURCE)
public @interface Override {
}

@Deprecated   //out of date, not suggested use
Take a look at declaration

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(value={CONSTRUCTOR, FIELD, LOCAL_VARIABLE, METHOD, PACKAGE, PARAMETER, TYPE})
public @interface Deprecated {
}

@SuppressWarnings  //ignore warning when compile

public static void test(){

List list = new ArrayList();

}

Take a look at declaration

@Target({TYPE, FIELD, METHOD, PARAMETER, CONSTRUCTOR, LOCAL_VARIABLE})
@Retention(RetentionPolicy.SOURCE)
public @interface SuppressWarnings {
    /**
     * The set of warnings that are to be suppressed by the compiler in the
     * annotated element.  Duplicate names are permitted.  The second and
     * successive occurrences of a name are ignored.  The presence of
     * unrecognized warning names is not an error: Compilers must
     * ignore any warning names they do not recognize.  They are, however,
     * free to emit a warning if an annotation contains an unrecognized
     * warning name.
     *
     * Compiler vendors should document the warning names they support in
     * conjunction with this annotation type. They are encouraged to cooperate
     * to ensure that the same names work across multiple compilers.
     */
    String[] value();
}

See usage of Annotation:
Declare Annotation

Full Example of declare and use an annotation