10년후의 미래는 현재가 말해준다

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Programmers sometimes overload a method when they mean to override it; a classic example of this is the equals() method which is defined in the Object class.  AnnotatedClass.java contains a simple example of this:

public boolean equals(String otherName) {     int comparisson = name.compareTo(otherName);     return (comparisson == 0); }

The programmer thinks that they are overriding the equals() method from the Object class, but since that takes an Object as a parameter the result (which is perfectly correct Java) is an overloaded method instead.  This could, potentially cause some very subtle and therefore hard to debug runtime errors.  Adding a standard annotation type to this can identify these problems at compile time.

(1.1) Override annotation

0. Start NetBeans IDE if you have not done so yet.
1. Create a new NetBeans project

• Select File->New Project (Ctrl+Shift+N). The New Project dialog box appears.
• Under Choose Project pane, select Java under Categories and Java Application under Projects.
  
• Click Next.

• Under Name and Location pane, for the Project Name field, type in OverrideAnnotation as project name.
• For Create Main Class field, type in OverrideAnnotation. 
  
• Click Finish.

• Observe that OverrideAnnotation project appears and IDE generated OverrideAnnotation.java is displayed in the source editor window of NetBeans IDE.

2. Modify the IDE generated OverrideAnnotation.java as shown in Code-1.11 below.  Study the code by paying special attention to the bold fonted parts. 

public class OverrideAnnotation {         private String name;     private int id;         /**      *  Constructor      **/     public OverrideAnnotation() {         name = "Java Passion!";     }         public String getName() {         return name;     }         public void setName(String name) {         this.name = name;     }     public int getId() {         return id;     }     public void setId(int id) {         this.id = id;     }         /**      *  Test of equals method overriding vs. overloading      *  For exercise 1 change this method      **/     public boolean equals(String otherName) {         int comparisson = name.compareTo(otherName);                 return (comparisson == 0);     }         public static void main(String[] args) {         // TODO code application logic here     }     }

Code-1.11:  equals() method without @Override annotation

3. Build and run the project

• Right click ExtendThreadClassTest0 project and select Run.
• Observe the compilation of the code succeeds. (Figure-1.13 below) This is not desirable.  What we want is compile time detection of a problem.

run: BUILD SUCCESSFUL (total time: 1 second)

Figure-1.13: Result of running ExtendThreadClassTest0 application

5. Modify the ExtendThreadClassTest0.java as shown in Code-1.15 below. The code fragments that need to be added are highlighted in bold and blue-colored font.

public class OverrideAnnotation {         private String name;     private int id;         /**      *  Constructor      **/     public OverrideAnnotation() {         name = "Java Passion!";     }         public String getName() {         return name;     }         public void setName(String name) {         this.name = name;     }     public int getId() {         return id;     }     public void setId(int id) {         this.id = id;     }         /**      *  Test of equals method overriding vs. overloading      *  For exercise 1 change this method      **/     @Override     public boolean equals(String otherName) {         int comparisson = name.compareTo(otherName);                 return (comparisson == 0);     }         public static void main(String[] args) {         // TODO code application logic here     }     }

Code-1.15: equals() method with @Override annotation

6. Observe that the compiler now detects a problem.  This is a desired behavior.


Figure-1.16: Result

7. Modify the ExtendThreadClassTest0.java as shown in Code-1.17 below. The code fragments that need to be added are highlighted in bold and blue-colored font.

public class OverrideAnnotation {         private String name;     private int id;         /**      *  Constructor      **/     public OverrideAnnotation() {         name = "Java Passion!";     }         public String getName() {         return name;     }         public void setName(String name) {         this.name = name;     }         public int getId() {         return id;     }         public void setId(int id) {         this.id = id;     }         /**      *  Test of equals method overriding vs. overloading      *  For exercise 1 change this method      **/     @Override     public boolean equals(Object otherName) {         String newName = (String) otherName;         int comparison = name.compareTo(newName);                 return (comparison == 0);     }         public static void main(String[] args) {         // TODO code application logic here     }     }

Code-1.17: Make it overriden method

8. Build the project.  Observe that there is no compile error anymore.

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javase5annotation/samples/OverrideAnnotation.  You can just open it and run it. 

Summary

In this exercise, you learned how to use a simple marker annotation called @Override to detect an unintentional overloading at the compile time.

                                                                                                     

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As explained earlier, it is possible to specify that annotations are maintained in the classfile and made available via the runtime environment of the JVM.  To access the runtime information you need to use the reflection APIs which have been modified in J2SE 5.0 to include support for Metadata.

The Class class now has two additional methods:

• getAnnotations() which returns an array of all annotations for the class
• getAnnotation(Class c) which returns the information about the annotation of type c passed as a parameter.
  

The Method, Constructor and Field classes also have two new methods:

• getAnnotation(Class c) which is the same as for Class
• getDeclaredAnnotations() which returns an array of annotations declared for the Method, Constructor or Field.
  

(6.1) Schedule one-time task

1. Create a new NetBeans project

• Select File->New Project (Ctrl+Shift+N). The New Project dialog box appears.
• Under Choose Project pane, select Java under Categories and Java Application under Projects.
  
• Click Next.

• Under Name and Location pane, for the Project Name field, type in RuntimeAnnotation as project name.
• For Create Main Class field, type in RuntimeAnnotation. 
  
• Click Finish.

• Observe that RuntimeAnnotation project appears and IDE generated RuntimeAnnotation.java is displayed in the source editor window of NetBeans IDE.

2. Modify the IDE generated RuntimeAnnotation.java as shown in Code-6.11 below.  Study the code by paying special attention to the bold fonted parts.

import java.lang.annotation.*; public class RuntimeAnnotation {         AnnotatedClass ac;         public RuntimeAnnotation() {         ac = new AnnotatedClass();     }         public void printAnnotations() {         Class c = ac.getClass();         Annotation[] annotations = c.getAnnotations();         int numberOfAnnotations = annotations.length;         System.out.println("Class " + c.getName() + " has " +                 numberOfAnnotations + " annotations");                 for (int i = 0 ; i < numberOfAnnotations; i++) {             System.out.println("Annotation " + i + ": " + annotations[i] +                     ", type" + annotations[i].annotationType().getName());         }     }         public static void main(String[] args) {         RuntimeAnnotation ar = new RuntimeAnnotation();         ar.printAnnotations();     } }

Code-6.11: RuntimeAnnotation.java

3.  Write AnnotatedClass.java.

@Reviewer(@Name(first = "James", last = "Gosling")) public class AnnotatedClass {         private String name;     private int id;         public AnnotatedClass() {         name = "Java Passion!";     }         @Accessor(variableName = "name")     public String getName() {         return name;     }         @Mutator(variable ="name")     public void setName(String name) {         this.name = name;     }         @Accessor(variableName = "name", variableType = "int")     public int getId() {         return id;     }         public void setId(int id) {         this.id = id;     }         public boolean equals(String otherName) {         int comparisson = name.compareTo(otherName);                 return (comparisson == 0);     } }

Code-6.12: AnnotatedClass.java.

4. Write Reviewer.java.  This is the same code you've written above.

import java.lang.annotation.*; @Retention(RetentionPolicy.RUNTIME) public @interface Reviewer {     Name value(); }

Code-6.13: Reviewer.java

5. Write Name.java.  This is the same code you've written above.

import java.lang.annotation.*; public @interface Name {   String first();   String last(); }

Code-6.14: Name.java

6. Write Mutator.java.  This is the same code you've written above.

import java.lang.annotation.*; @Retention(RetentionPolicy.RUNTIME) public @interface Mutator {     String variable(); }

Code-6.15: Name.java

7. Write Accessor.java.  This is the same code you've written above.

import java.lang.annotation.*; /**  *  Annotation definition for an accessor method.  This shows the use of  *  multiple members.  **/ @Retention(RetentionPolicy.RUNTIME) public @interface Accessor {     String variableName();     String variableType() default "String"; }

Code-6.16: Name.java

8. Build and run the project

• Right click RuntimeAnnotation project and select Build.
• Observe the result in the Output window. (Figure-6.17 below)

Class AnnotatedClass has 1 annotations Annotation 0: @Reviewer(value=@Name(first=James, last=Gosling)), typeReviewer

Figure-6.17: Result of running UnRuntimeAnnotation application

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javase5annotation/samples/RuntimeAnnotation.  You can just open it and run it. 


                                                                                      

Summary

In this exercise, you learned how to retrieve annotation information during runtime.

• Create a new annotation called RequestForEnhancement as following
  

• Use the above annotation in your MyOwnAnnotationExample project to a method of your choice - in other words, you are using the above annotation to annotate a method of your choice -  by providing appropriate values.
• Use Runtime annotation API to retrieve the values of the annotation (as you've done in Exercise 6 above) and display them.
  

• Zip file of the the MyOwnAnnotationExample NetBeans project.  (Someone else should be able to open and run it as a NetBeans project.)  You can use your favorite zip utility or you can use "jar" utility that comes with JDK as following.
  
  • cd <parent directory that contains MyOwnAnnotationExample directory> (assuming you named your project as MyOwnAnnotationExample)
    
  • jar cvf MyOwnAnnotationExample.zip MyOwnAnnotationExample (MyOwnAnnotationExample directory should contain nbproject directory)
• Captured output screen  - name it as JavaIntro-javase5annotation.gif orJavaIntro-javase5annotation.jpg (or JavaIntro-javase5annotation.<whatver graphics format>)
  • Any screen capture that shows that your program is working is good enough.  No cosmetic polishment is required.
• If you decide to use different IDE other than NetBeans, the zip file should contain all the files that are needed for rebuilding the project - war file with necessary source files is OK.