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Constants and how they can be misleading
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Sample code for ways Java Constants may not work as you would expect |
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public class App { | |
public static void main(String[] args) { | |
System.out.println(MyAnnotatedClass.class.getAnnotation(MyAnnotation.class).value()); | |
System.out.println(Constants.OUTPUT_DIRECTORY); | |
} | |
} |
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public class Constants { | |
// if you're wondering where the drive letter is, you have more reading to do :) | |
public static final String OUTPUT_DIRECTORY = "/opt/application-output"; | |
} |
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@MyAnnotation(Constants.OUTPUT_DIRECTORY) | |
public class MyAnnotatedClass { | |
} |
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import java.lang.annotation.ElementType; | |
import java.lang.annotation.Retention; | |
import java.lang.annotation.RetentionPolicy; | |
import java.lang.annotation.Target; | |
@Target(ElementType.TYPE) | |
@Retention(RetentionPolicy.RUNTIME) | |
public @interface MyAnnotation { | |
String value(); | |
} |
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$ tree | |
. | |
|____App.java | |
|____Constants.java | |
$ javac -cp . *.java | |
$ tree | |
. | |
|____App.class | |
|____App.java | |
|____Constants.class | |
|____Constants.java | |
$ java -cp . SimpleApp | |
/opt/application-output | |
$ rm Constants.class | |
$ java -cp . SimpleApp | |
/opt/application-output |
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public class SimpleApp { | |
public static void main(String[] args) { | |
System.out.println(Constants.OUTPUT_DIRECTORY); | |
} | |
} |
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