JDK 5 Notes (continue)

繼續...

Varargs

• Limitation:

  //below are compile error
  public void varargsMustBeLastArguments(String... strings, String illegal) ; 
  public void onlyOneVarargs(String... strings, String... illegals) ; 
  

• Compiler treat varargs as Array, and allow zero length:

  //declare:
  public void variableStrings(String... strings) ;
  
  //invoking:
  variableStrings();  //zero length is valid;
  variableStrings(new String[] {"A","B"}) ; //pass array
  

Annontation

• 3 policy of @Retention annotation: SOURCE, CLASS (default), RUNTIME
• Annotation type is just interface, no implementation at all. The implementation for custom annotation type is almost done via reflection. Thus, for most case we need to apply: @Retention(RetentionPolicy.RUNTIME) to ensure reflection working at runtime. Hibernate Annotation use RUNTIME too.
• some odd syntax:

  public @interface AnnotationX {   // declare annotation type
      String foo() default "bar" ;  // default value
  }
  

• default annotation type: @Deprecated, @Override, @SuppressWarnings
• meta-annotation type: @Target, @Retention, @Documented, @Inherited
• Subclass only inherit class-level annotation, not method-level.

foreach loop

• foreach loop can apply to non-generic Iterable, but only for Object

    List list = new ArrayList() ; // non-generic Iterable
    for(Object o: list) { // Object only
       //....
    }
  

• eclipse built in code template: type "foreach" and press alt+/
• Authough new foreach is good, we still use old format when we need Iterator. In Eclipse, we can add new code template for Generaric Iterator: Select Menu -> Window -> Java -> Editor -> Templates -> New... and add following template:

  for (Iterator<${iterable_type}> ${iterator} = ${collection}.iterator(); ${iterator}.hasNext(); ) {
  	${iterable_type} ${element} = ${iterator}.next();
  	${cursor}
  }
  

  ( I don't understand why Eclipse 3.1 not built in this template... )

Static import

• local variable first:

  import static java.lang.System.out;
  public class Test {
      public void shadow(PrintStream out) {        
          out.println("xyz") ; //local variable first
      }
  }
  

Formatter

• formatter for date:

  //format as -- 2004/01/02 23:10:59
  System.out.printf("%tY/%<tm/%<td %<tk:%<tM:%<tS", System.currentTimeMillis());
  
  //format as -- 20040102 231059
  System.out.printf("%tY%<tm%<td %<tk%<tM%<tS", System.currentTimeMillis());
  
  //format as -- 2004/01/02 
  System.out.printf("%tY/%<tm/%<td", System.currentTimeMillis());
  

Annotation 還真難...