Java知识点总结（注解-内置注解）

摘要：知识点总结注解内置注解知识点总结注解定义在中，此注释只适用于修饰方法，表示一个方法声明打算重写父类的另一个方法声明。此注释可用于修饰方法属性类，表示不鼓励程序员使用这样的元素，通常是因为它很危险或存在更好的选择。

@(Java知识点总结)[Java, 注解]

定义在java.lang.Override 中，此注释只适用于修饰方法，表示一个方法声明打算重写父类的另一个方法声明。

public class Demo01 {
  
  @Override
  public String toString() {
    return "";
  }
 
}

源码

import java.lang.annotation.*;
 
/**
 * Indicates that a method declaration is intended to override a
 * method declaration in a supertype. If a method is annotated with
 * this annotation type compilers are required to generate an error
 * message unless at least one of the following conditions hold:
 *
 * 

 * The method does override or implement a method declared in a
 * supertype.
 * 

 * The method has a signature that is override-equivalent to that of
 * any public method declared in {@linkplain Object}.
 * 
 *
 * @author  Peter von der Ahé
 * @author  Joshua Bloch
 * @jls 9.6.1.4 @Override
 * @since 1.5
 */
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.SOURCE)
public @interface Override {
}

定义在java.lang.Deprecated中，遗弃、废弃，不建议使用。此注释可用于修饰方法、属性、类，表示不鼓励程序员使用这样的元素，通常是因为它很危险或存在更好的选择。

public class Demo01 {
  @Deprecated
  public static void test1(){
   
  }
   
  public static void main(String[] args) {
   test1();
  }
}

源码

import java.lang.annotation.*;
import static java.lang.annotation.ElementType.*;
 
/**
 * A program element annotated @Deprecated is one that programmers
 * are discouraged from using, typically because it is dangerous,
 * or because a better alternative exists.  Compilers warn when a
 * deprecated program element is used or overridden in non-deprecated code.
 *
 * @author  Neal Gafter
 * @since 1.5
 * @jls 9.6.3.6 @Deprecated
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(value={CONSTRUCTOR, FIELD, LOCAL_VARIABLE, METHOD, PACKAGE, PARAMETER, TYPE})
public @interface Deprecated {
}

定义在java.lang.SuppressWarnings中，用来抑制编译时的警告信息。

与前两个注释有所不同， 你需要添加一个参数才能正确使用 ，这些参数值是已经定义好了的，我们选择性的使用就好了，参数如下：

@SuppressWarnings("unchecked")
@SuppressWarnings(value={"unchecked","deprecation"})

import java.util.ArrayList;
import java.util.List;
 
public class Demo01 {
  @SuppressWarnings("all")
  public static void test2(){
   List list = new ArrayList();
 }

源码

package java.lang;
 
import java.lang.annotation.*;
import static java.lang.annotation.ElementType.*;
 
/**
 * Indicates that the named compiler warnings should be suppressed in the
 * annotated element (and in all program elements contained in the annotated
 * element).  Note that the set of warnings suppressed in a given element is
 * a superset of the warnings suppressed in all containing elements.  For
 * example, if you annotate a class to suppress one warning and annotate a
 * method to suppress another, both warnings will be suppressed in the method.
 *
 * 
As a matter of style, programmers should always use this annotation
 * on the most deeply nested element where it is effective.  If you want to
 * suppress a warning in a particular method, you should annotate that
 * method rather than its class.
 *
 * @author Josh Bloch
 * @since 1.5
 * @jls 4.8 Raw Types
 * @jls 4.12.2 Variables of Reference Type
 * @jls 5.1.9 Unchecked Conversion
 * @jls 5.5.2 Checked Casts and Unchecked Casts
 * @jls 9.6.3.5 @SuppressWarnings
 */
@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.
     *
     * 
 The string {@code "unchecked"} is used to suppress
     * unchecked warnings. Compiler vendors should document the
     * additional 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.
     * @return the set of warnings to be suppressed
     */
    String[] value();
}