JDK 5 Notes

雜記...

Eclipse:

• Enable jdk 5 compliance mode: window -> preferences -> java -> compiler -> Compiler compliance Level -> 5.0
• refactoring old code to generic:

  //Old code:
  List l = new ArrayList();
  for (Iterator iter = l.iterator(); iter.hasNext();) {
    String element = (String) iter.next();
  }
  
  //change to:
  List<String> l = new ArrayList();
  for (Iterator iter = l.iterator(); iter.hasNext();) {
    String element = (String) iter.next();
  }
  

  then select eclipse menu: "Refactor -> Infer Generic Type Arguments", eclipse will do rest for you:

  List<String> l = new ArrayList<String>();
  for (Iterator<String> iter = l.iterator(); iter.hasNext();) {
     String element = iter.next();
  }
  

Ant:

• Enable jdk5 mode for javac task:

    <javac source="1.5" >
       ...
  

Generic:

• Erasure -- Generic type information will be "erased" after compile (ByteCode does not contain any type information, hence we can not do reflection on generic type)
• Due to above reason, we can not cast generic type:

  List<Integer> ints = new LinkedList<Integer>();
  List<Number> nums = ints; // compile error.
  

• Thus, we need to use wildcard <?> to support more types, however, <?> is a read-only type:

  List<?> list = new ArrayList(); // image ? is a "unknown" type 
  list.get() ; // return unknown type, here is Object 
  list.add("1"); // compile error, add() method can not process unknown type.
  

• Considering wildcard <?> is a kind of concrete type, just like <Integer>,<String>...etc. It's not type for declaration
• static variable is not allowed to declared with generic type

  //compile error, because static variable is shared for all instances. But every instance
  //has its own generic type.
  private static List<T> staticList = new ArrayList<T>();
  

Enum:

• Enum can not do subclass, but can implement interface.
• Always use .name() instead of .toString() to determine Enum.
• Mapping Enum as a value type in Hibernate: Enum CustomUserType
• Hibernate Customed Enum Type with join-table (many-to-many for Enum)

  <typedef name="Gender" class="GenericEnumUserType">
     <param name="enumClass">Gender</param>
  </typedef>
  
  <set name="genders" table="room_genders" >
      <key column="room_id" />
      <element type="Gender" column="gender"  />
  </set>
  

Mystery bug from autoboxing/unboxing:

• Null

  Integer i = null ;
  int j = i ; //throw NullPointException
  

• Object Identity

  Integer i1 = 256 ;
  Integer i2 = 256 ;
  assertFalse(i1 == i2) ; // not always true
  
  Integer i3 = 90 ;
  Integer i4 = 90 ;
  assertTrue(i3 == i4) ; // sun jdk implementation reuse Integer instance within -128~127
  

繼續學...