Hidden Features of Java

Question

After reading Hidden Features of C  I wondered  What are some of the hidden features of Java

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Not really a feature  but an amusing trick I discovered recently in some Web page   class Example     public static void main String   args          System out println  Hello World         http   Phi Lho free fr      System exit 0           is a valid Java program  although it generates a warning   If you don t see why  see Gregory s answer   -  Well  syntax highlighting here also gives a hint

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Java processing does a neat trick on variable definition if you do not use a default initializer         int x      if whatever        x 1      if x    1                This will give you an error at compile time that you have a path where X isn t properly defined   This has helped me a few times  and I ve taken to considering default initialization like these    int x 0  String s null    to be a bad pattern since it blocks this helpful checking   That said  sometimes it s difficult to get around--I have had to go back and edit in the  null when it made sense as a default  but I never put it in on the first pass any more

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Allowing methods and constructors in enums surprised me  For example   enum Cats     FELIX 2   SHEEBA 3   RUFUS 7      private int mAge    Cats int age        mAge   age        public int getAge         return mAge           You can even have a  constant specific class body  which allows a specific enum value to override methods   More documentation here

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Java 6  from Sun  comes with an embedded JavaScrip interpreter   http   java sun com javase 6 docs technotes guides scripting programmer guide index html jsengine

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final initialization can be postponed   It makes sure that even with a complex flow of logic return values are always set   It s too easy to miss a case and return null by accident   It doesn t make returning null impossible  just obvious that it s on purpose   public Object getElementAt int index        final Object element      if  index    0             element    Result 1         else if  index    1             element    Result 2         else            element    Result 3             return element

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Not really a feature  but it makes me chuckle that goto is a reserved word that does nothing except prompting javac to poke you in the eye   Just to remind you that you are in OO-land now

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For most people I interview for Java developer positions labeled blocks are very surprising  Here is an example      code goes here  getmeout       for  int i   0  i  lt  N    i            for  int j   i  j  lt  N    j                for  int k   j  k  lt  N    k                      do something here                 break getmeout                                    Who said goto in java is just a keyword

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Since no one else has said it yet  I Think  my favorite feature is Auto boxing   public class Example       public static void main String   Args                 int a   5           Integer b   a     Box           System out println  A       a            System out println  B       b

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I was surprised when I first noticed the Ternary-Operator which equals a simple if-then-else statement   minVal    a  lt  b    a   b

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As of Java 1 5  Java now has a much cleaner syntax for writing functions of variable arity  So  instead of just passing an array  now you can do the following  public void foo String    bars       for  String bar  bars        System out println bar       bars is automatically converted to array of the specified type  Not a huge win  but a win nonetheless

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const  is a keyword  but you can t use it   int const   1        not a statement  const int i   1      illegal start of expression    I guess the compiler writers thought it might be used in the future and they d better keep it reserved

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The power you can have over the garbage collector and how it manages object collection is very powerful  especially for long-running and time-sensitive applications  It starts with weak  soft  and phantom references in the java lang ref package  Take a look at those  especially for building caches  there is a java util WeakHashMap already   Now dig a little deeper into the ReferenceQueue and you ll start having even more control  Finally grab the docs on the garbage collector itself and you ll be able to control how often it runs  sizes of different collection areas  and the types of algorithms used  for Java 5 see http   java sun com docs hotspot gc5 0 gc tuning 5 html

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final for instance variables    Really useful for multi-threading code and it makes it a lot easier to argue about the instance state and correctness  Haven t seen it a lot in industry context and often not thought in java classes     static  something      Used to initialize static members  also I prefer a static method to do it  because it has a name   Not thought

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Some control-flow tricks  finally around a return statement   int getCount        try   return 1      finally   System out println  Bye           The rules for definite assignment will check that a final variable is always assigned through a simple control-flow analysis   final int foo  if        foo   1  else   throw new Exception    foo 1

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You can add runtime checks of generic types using a Class lt T gt  object  this comes in handy when a class is being created in a configuration file somewhere and there is no way to add a compile time check for the generic type of the class   You dont want the class to blow up at runtime if the app happens to be configured wrong and you dont want all you classes riddled with instance of checks   public interface SomeInterface     void doSomething Object o     public abstract class RuntimeCheckingTemplate lt T gt      private Class lt T gt  clazz    protected RuntimeChecking Class lt T gt  clazz        this clazz   clazz         public void doSomething Object o        if  clazz isInstance o           doSomethingWithGeneric clazz cast o          else            log it  do something by default  throw an exception  etc               protected abstract void doSomethingWithGeneric T t      public class ClassThatWorksWithStrings extends RuntimeCheckingTemplate lt String gt      public ClassThatWorksWithStrings          super String class          protected abstract void doSomethingWithGeneric T t           Do something with the generic and know that a runtime exception won t occur         because of a wrong type

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static imports to  enhance  the language  so you can do nice literal things in type safe ways    List lt String gt  ls   List  a    b    c       can also do with maps  arrays  sets     http   gleichmann wordpress com 2008 01 13 building-your-own-literals-in-java-lists-and-arrays   Taking it further    List lt Map lt String  String gt  gt  data   List Map  o  name    michael    o  sex    male

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Perhaps the most surprising hidden feature is the sun misc Unsafe class   http   www docjar com html api ClassLib Common sun misc Unsafe java html  You can    Create an object without calling a constructor  Throw any exception even Exception without worrying about throws clauses on methods   There are other way to do this I know  Get set randomly accessed fields in an object without using reflection  allocate free copy resize a block of memory which can be long  64-bit  in size  Obtain the location of fields in an object or static fields in a class  independently lock and unlock an object lock   like synchronize without a block  define a class from provided byte codes  Rather than the classloader determining what the byte code should be   You can do this with reflection as well    BTW  Incorrect use of this class will kill the JVM  I don t know which JVMs support this class so its not portable

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An optimization trick that makes your code easier to maintain and less susceptible to a concurrency bug   public class Slow         Loop counter  initialized to 0       private long i     public static void main  String args           Slow slow   new Slow         slow run           private void run         while  i    lt  10000000000L                     time java Slow real    0m15 397s   time java Slow real    0m20 012s   time java Slow real    0m18 645s  Average  18 018s  public class Fast         Loop counter  initialized to 0       private long i     public static void main  String args           Fast fast   new Fast         fast run           private void run         long i   getI         while  i    lt  10000000000L                setI  i           private long setI  long i         this i   i         private long getI         return this i            time java Fast real    0m12 003s   time java Fast real    0m9 840s   time java Fast real    0m9 686s  Average  10 509s  It requires more bytecodes to reference a class-scope variable than a method-scope variable  The addition of a method call prior to the critical loop adds little overhead  and the call might be inlined by the compiler anyway    Another advantage to this technique  always using accessors  is that it eliminates a potential bug in the Slow class  If a second thread were to continually reset the value of i to 0  by calling slow setI  0    for example   the Slow class could never end its loop  Calling the accessor and using a local variable eliminates that possibility   Tested using J2SE 1 6 0 13 on Linux 2 6 27-14

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Dynamic proxies  added in 1 3  allow you to define a new type at runtime that conforms to an interface   It s come in handy a surprising number of times

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It took them long enough to add support for this   System Tray

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The addition of the for-each loop construct in 1 5   I  lt 3 it      For each Object  instantiated as foo  in myCollection for Object foo  myCollection      System out println foo toString         And can be used in nested instances   for  Suit suit   suits    for  Rank rank   ranks      sortedDeck add new Card suit  rank      The for-each construct is also applicable to arrays  where it hides the index variable rather than the iterator  The following method returns the sum of the values in an int array       Returns the sum of the elements of a int sum int   a      int result   0    for  int i   a      result    i    return result      Link to the Sun documentation

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The next-generation Java plugin found in Java 1 6 Update 10 and later has some very neat features    Pass java arguments parameter to pass arguments to the JVM that is created  This allows you to control the amount of memory given to the applet  Create separate class loaders or even separate JVM s for each applet  Specify the JVM version to use  Install partial Java kernels in cases where you only need a subset of the full Java libraries  functionality  Better Vista support  Support  experimental  to drag an applet out of the browser and have it keep running when you navigate away    Many other things that are documented here  http   jdk6 dev java net plugin2   More from this release here  http   jdk6 dev java net 6u10ea html

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I think another  overlooked  feature of java is the JVM itself  It is probably the best VM available  And it supports lots of interesting and useful languages  Jython  JRuby  Scala  Groovy   All those languages can easily and seamlessly cooperate   If you design a new language  like in the scala-case  you immediately have all the existing libraries available and your language is therefore  useful  from the very beginning   All those languages make use of the HotSpot optimizations  The VM is very well monitor and debuggable

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A couple of people have posted about instance initializers  here s a good use for it   Map map   new HashMap          put  a key    a value        put  another key    another value          Is a quick way to initialize maps if you re just doing something quick and simple   Or using it to  create a   quick swing frame prototype   JFrame frame   new JFrame     JPanel panel   new JPanel      panel add  new JLabel  Hey there          setBackground Color black       setForeground  Color white         panel add  new JButton  Ok         addActionListener  new ActionListener            public void actionPerformed  ActionEvent ae                System out println  Button pushed                                frame add  panel      Of course it can be abused       JFrame frame   new JFrame              add  new JPanel                    add  new JLabel  Hey there                          setBackground Color black                       setForeground  Color white                                         add  new JButton  Ok                         addActionListener  new ActionListener                            public void actionPerformed  ActionEvent ae                                System out println  Button pushed

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People are sometimes a bit surprised when they realize that it s possible to call private methods and access change private fields using reflection     Consider the following class   public class Foo       private int bar       public Foo             setBar 17              private void setBar int bar            this bar bar             public int getBar             return bar             public String toString             return  Foo bar   bar                Executing this program     import java lang reflect     public class AccessibleExample       public static void main String   args          throws NoSuchMethodException IllegalAccessException  InvocationTargetException  NoSuchFieldException           Foo foo new Foo            System out println foo            Method method Foo class getDeclaredMethod  setBar   int class           method setAccessible true           method invoke foo  42            System out println foo           Field field Foo class getDeclaredField  bar            field setAccessible true           field set foo  23           System out println foo                will yield the following output   Foo bar 17  Foo bar 42  Foo bar 23

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Language-level assert keyword

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SwingWorker for easily managing user interface callbacks from background threads

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I was aware that Java 6 included scripting support  but I just recently discovered jrunscript   which can interpret and run JavaScript  and  one presumes  other scripting languages such as Groovy  interactively  sort of like the Python shell or irb in Ruby

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Classpath wild cards since Java 6   java -classpath   lib   so Main   Instead of  java -classpath   lib log4j jar   lib commons-codec jar   lib commons-httpclient jar   lib commons-collections jar   lib myApp jar so Main   See http   java sun com javase 6 docs technotes tools windows classpath html

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Use StringBuilder instead of StringBuffer when you don t need synchronized management included in StringBuilder  It will increase the performance of your application   Improvements for Java 7 would be even better than any hidden Java features    Diamond syntax  Link   Don t use those infinite  lt   syntax at instanciation   Map lt String  List lt String gt  gt  anagrams   new HashMap lt String  List lt String gt  gt         Can now be replaced with this   Map lt String  List lt String gt  gt  anagrams   new HashMap lt  gt        Strings in switch  Link   Use String in switch  instead of old-C int   String s    something   switch s     case  quux       processQuux s          fall-through    case  foo     case  bar       processFooOrBar s       break     case  baz        processBaz s          fall-through    default      processDefault s       break       Automatic Resource Management Link   This old code   static void copy String src  String dest  throws IOException       InputStream in   new FileInputStream src       try           OutputStream out   new FileOutputStream dest           try               byte   buf   new byte 8   1024               int n              while   n   in read buf    gt   0                  out write buf  0  n             finally               out close                    finally           in close              can now be replaced by this much simpler code   static void copy String src  String dest  throws IOException       try  InputStream in   new FileInputStream src               OutputStream out   new FileOutputStream dest             byte   buf   new byte 8192           int n          while   n   in read buf    gt   0              out write buf  0  n

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The value of   new URL  http   www yahoo com   equals new URL  http   209 191 93 52      is true    From Java Puzzlers

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How about Properties files in your choice of encodings   Used to be  when you loaded your Properties  you provided an InputStream and the load   method decoded it as ISO-8859-1   You could actually store the file in some other encoding  but you had to use a disgusting hack like this after loading to properly decode the data    String realProp   new String prop getBytes  ISO-8859-1     UTF-8      But  as of JDK 1 6  there s a load   method that takes a Reader instead of an InputStream  which means you can use the correct encoding from the beginning  there s also a store   method that takes a Writer    This seems like a pretty big deal to me  but it appears to have been snuck into the JDK with no fanfare at all   I only stumbled upon it a few weeks ago  and a quick Google search turned up just one passing mention of it

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I was surprised by instance initializers the other day   I was deleting some code-folded methods and ended up creating multiple instance initializers    public class App       public App String name    System out println name     s constructor called           static   System out println  static initializer called             System out println  instance initializer called           static   System out println  static initializer2 called             System out println  instance initializer2 called           public static void main  String   args             new App  one            new App  two            Executing the main method will display    static initializer called static initializer2 called instance initializer called instance initializer2 called one s constructor called instance initializer called instance initializer2 called two s constructor called   I guess these would be useful if you had multiple constructors and needed common code  They also provide syntactic sugar for initializing your classes   List lt Integer gt  numbers   new ArrayList lt Integer gt      add 1   add 2        Map lt String String gt  codes   new HashMap lt String String gt         put  1   one       put  2   two

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This is not exactly  hidden features  and not very useful  but can be extremely interesting in some cases  Class sun misc Unsafe - will allow you to implement direct memory management in Java  you can even write self-modifying Java code with this if you try a lot    public class UnsafeUtil        public static Unsafe unsafe      private static long fieldOffset      private static UnsafeUtil instance   new UnsafeUtil         private Object obj       static           try               Field f   Unsafe class getDeclaredField  theUnsafe                f setAccessible true                unsafe    Unsafe f get null               fieldOffset   unsafe objectFieldOffset UnsafeUtil class getDeclaredField  obj               catch  Exception e                throw new RuntimeException e

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My vote goes to java util concurrent with its concurrent collections and flexible executors allowing among others thread pools  scheduled tasks and coordinated tasks  The DelayQueue is my personal favorite  where elements are made available after a specified delay   java util Timer and TimerTask may safely be put to rest   Also  not exactly hidden but in a different package from the other classes related to date and time  java util concurrent TimeUnit is useful when converting between nanoseconds  microseconds  milliseconds and seconds   It reads a lot better than the usual someValue   1000 or someValue   1000

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The C-Style printf       System out printf   d  f   4f   3 Math E Math E     Output  3 2 718282 2 7183  Binary Search  and it s return value   int   q   new int     1 3 4 5   int position   Arrays binarySearch q  2     Similar to C   if  2  is not found in the array  it returns a negative value but if you take the 1 s Complement of the returned value you actually get the position where  2  can be inserted   In the above example  position   -2   position   1 which is the position where 2 should be inserted   it also lets you find the  closest  match in the array   I thinks its pretty nifty

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Intersection types allow you to  kinda sorta  do enums that have an inheritance hierarchy  You can t inherit implementation  but you can delegate it to a helper class   enum Foo1 implements Bar    enum Foo2 implements Bar     class HelperClass      static  lt T extends Enum lt T gt   amp  Bar gt  void fooBar T the enum         This is useful when you have a number of different enums that implement some sort of pattern  For instance  a number of pairs of enums that have a parent-child relationship   enum PrimaryColor  Red  Green  Blue   enum PastelColor  Pink  HotPink  Rockmelon  SkyBlue  BabyBlue    enum TransportMedium  Land  Sea  Air   enum Vehicle  Car  Truck  BigBoat  LittleBoat  JetFighter  HotAirBaloon     You can write generic methods that say  Ok  given an enum value thats a parent of some other enum values  what percentage of all the possible child enums of the child type have this particular parent value as their parent    and have it all typesafe and done without casting   eg  that  Sea  is 33  of all possible vehicles  and  Green  20  of all possible Pastels    The code look like this  It s pretty nasty  but there are ways to make it better  Note in particuar that the  leaf  classes themselves are quite neat - the generic classes have declarations that are horribly ugly  but you only write them onece  Once the generic classes are there  then using them is easy   import java util EnumSet   import javax swing JComponent   public class zz extends JComponent        public static void main String   args            System out println PrimaryColor Green         ParentUtil pctOf PrimaryColor Green                  System out println TransportMedium Air         ParentUtil pctOf TransportMedium Air                     class ParentUtil       private ParentUtil         static  lt P extends Enum lt P gt   amp  Parent lt P  C gt   C extends Enum lt C gt   amp  Child lt P  C gt  gt         float pctOf P parent            return  float  parent getChildren   size                         float  EnumSet allOf parent getChildClass    size                        100f            public static  lt P extends Enum lt P gt   amp  Parent lt P  C gt   C extends Enum lt C gt   amp  Child lt P  C gt  gt         EnumSet lt C gt  loadChildrenOf P p            EnumSet lt C gt  cc   EnumSet noneOf p getChildClass             for C c  EnumSet allOf p getChildClass                   if c getParent      p                    cc add c                                   return cc           interface Parent lt P extends Enum lt P gt   amp  Parent lt P  C gt   C extends Enum lt C gt   amp  Child lt P  C gt  gt        Class lt C gt  getChildClass         EnumSet lt C gt  getChildren       interface Child lt P extends Enum lt P gt   amp  Parent lt P  C gt   C extends Enum lt C gt   amp  Child lt P  C gt  gt        Class lt P gt  getParentClass         P getParent       enum PrimaryColor implements Parent lt PrimaryColor  PastelColor gt        Red  Green  Blue       private EnumSet lt PastelColor gt     children       public Class lt PastelColor gt  getChildClass             return PastelColor class             public EnumSet lt PastelColor gt  getChildren             if children    null  children ParentUtil loadChildrenOf this           return children           enum PastelColor implements Child lt PrimaryColor  PastelColor gt        Pink PrimaryColor Red   HotPink PrimaryColor Red          Rockmelon PrimaryColor Green          SkyBlue PrimaryColor Blue   BabyBlue PrimaryColor Blue        final PrimaryColor  parent       private PastelColor PrimaryColor parent            this parent   parent             public Class lt PrimaryColor gt  getParentClass             return PrimaryColor class             public PrimaryColor getParent             return parent           enum TransportMedium implements Parent lt TransportMedium  Vehicle gt        Land  Sea  Air       private EnumSet lt Vehicle gt     children       public Class lt Vehicle gt  getChildClass             return Vehicle class             public EnumSet lt Vehicle gt  getChildren             if children    null  children ParentUtil loadChildrenOf this           return children           enum Vehicle implements Child lt TransportMedium  Vehicle gt        Car TransportMedium Land   Truck TransportMedium Land          BigBoat TransportMedium Sea   LittleBoat TransportMedium Sea          JetFighter TransportMedium Air   HotAirBaloon TransportMedium Air        private final TransportMedium   parent       private Vehicle TransportMedium parent            this parent   parent             public Class lt TransportMedium gt  getParentClass             return TransportMedium class             public TransportMedium getParent             return parent

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Didn t read about this  Integer a   1  Integer b   1  Integer c   new Integer 1   Integer d   new Integer 1    Integer e   128  Integer f   128   assertTrue  a    b        again  this is true  assertFalse e    f      again  this is false  assertFalse c    d        again  this is false    read more about this by searching java s pool of integer  internal  cache  from -128 to 127 for autoboxing  or look into Integer valueOf

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Transfer of control in a finally block throws away any exception   The following code does not throw RuntimeException -- it is lost   public static void doSomething         try           Normally you would have code that doesn t explicitly appear          to throw exceptions so it would be harder to see the problem        throw new RuntimeException          finally         return              From http   jamesjava blogspot com 2006 03 dont-return-in-finally-clause html

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I enjoyed   javadoc s taglet and doclet that enable us to customize javadoc output  JDK tools  jstat  jstack etc

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Read  Java Puzzlers  by Joshua Bloch and you will be both enlightened and horrified

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Something that really surprised me was the custom serialization mechanism   While these methods are private    they are  mysteriously  called by the JVM during object serialization   private void writeObject ObjectOutputStream out  throws IOException  private void readObject ObjectInputStream in  throws IOException  ClassNotFoundException    This way you can create your own custom serialization to make it more  whatever   safe  fast  rare  easy etc      This is something that really should be considering if a lot of information has to be passed through nodes  The serialization mechanism may be changed to send the half of data   There are many times when the bottlenecks are not in the platform  but in the amount of that sent trough the wire  may save you thousands of dlls in hardware   Here is an article  http   java sun com developer technicalArticles Programming serialization

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Source code URLs  E g  here is some legal java source code   http   google com    Yes  it was in Java Puzzlers  I laughed

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Apparently with some debug builds there is an option which dumps the native  JIT  assembly code from HotSpot  http   weblogs java net blog kohsuke archive 2008 03 deep dive into html  Unfortunately I wasn t able to find the build via the link in that post  if anyone can find a more precise URL  I d love to play with it

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Joshua Bloch s new Effective Java is a good resource

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Using this keyword for accessing fields methods of containing class from an inner class  In below  rather contrived example  we want to use sortAscending field of container class from the anonymous inner class  Using ContainerClass this sortAscending instead of this sortAscending does the trick   import java util Comparator   public class ContainerClass   boolean sortAscending  public Comparator createComparator final boolean sortAscending       Comparator comparator   new Comparator lt Integer gt               public int compare Integer o1  Integer o2                if  sortAscending    ContainerClass this sortAscending                    return o1 - o2                else                   return o2 - o1                                      return comparator

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Instances of the same class can access private members of other instances   class Thing     private int x     public int addThings Thing t2        return this x   t2 x      Can access t2 s private value

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You can access final local variables and parameters in initialization blocks and methods of local classes  Consider this       final String foo    42       new Thread             public void run                  dowhatever foo                   start      A bit like a closure  isn t it

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Haven t seen anyone mention instanceof being implemented in such a way that checking for null is not necessary   Instead of   if  null    aObject  amp  amp  aObject instanceof String                 just use   if  aObject instanceof String

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Local classes  Instantiating Java inner-classes from outside of the containing class

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Double Brace Initialization took me by surprise a few months ago when I first discovered it  never heard of it before   ThreadLocals are typically not so widely known as a way to store per-thread state   Since JDK 1 5 Java has had extremely well implemented and robust concurrency tools beyond just locks  they live in java util concurrent and a specifically interesting example is the java util concurrent atomic subpackage that contains thread-safe primitives that implement the compare-and-swap operation and can map to actual native hardware-supported versions of these operations

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Some years ago when I had to do Java  1 4 x  I wanted an eval   method and Suns javac is  was   written in Java so it was just to link tools jar and use that with some glue-code around it

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If you do a lot of JavaBean development and work with property change support  you generally wind up writing a lot of setters like this   public void setFoo Foo aFoo     Foo old   this foo    this foo   aFoo    changeSupport firePropertyChange  foo   old  aFoo       I recently stumbled across a blog that suggested a more terse implementation of this that makes the code a lot easier to write   public void setFoo Foo aFoo     changeSupport firePropertyChange  foo   this foo  this foo   aFoo       It actually simplified things to the point where I was able to adjust the setter template in Eclipse so the method gets created automatically

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Oh  I almost forgot this little gem  Try this on any running java process   jmap -histo live PID  You will get a histogram of live heap objects in the given VM  Invaluable as a quick way to figure certain kinds of memory leaks  Another technique I use to prevent them is to create and use size-bounded subclasses of all the collections classes  This causes quick failures in out-of-control collections that are easy to identify

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Identifiers can contain foreign language chars like umlauts   instead of writing   String title       someone could write   String   berschrift

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Javadoc - when written properly  not always the case with some developers unfortunately   it gives you a clear  coherent description of what code is supposed to do  as opposed to what it actually does  It can then be turned into a nice browsable set of HTML documentation  If you use continuous integration etc it can be generated regularly so all developers can see the latest updates

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I just  re learned today that   is a legal name for a method or variable in Java   Combined with static imports it can make for some slightly more readable code  depending on your view of readable   http   garbagecollected org 2008 04 06 dollarmaps

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Here s my list   My favourite  and scariest  hidden feature is that you can throw checked exceptions from methods that are not declaring to throw anything   import java rmi RemoteException   class Thrower       public static void spit final Throwable exception            class EvilThrower lt T extends Throwable gt                 SuppressWarnings  unchecked               private void sneakyThrow Throwable exception  throws T                   throw  T  exception                                  new EvilThrower lt RuntimeException gt    sneakyThrow exception            public class ThrowerSample       public static void main  String   args             Thrower spit new RemoteException  go unchecked                  Also you may like to know you can throw  null      public static void main String   args         throw null        Guess what this prints   Long value   new Long 0   System out println value equals 0        And  guess what this returns    public int returnSomething         try           throw new RuntimeException  foo           finally           return 0            the above should not surprise good developers     In Java you can declare an array in following valid ways   String   strings   new String      foo    bar        the above is equivalent to the following  String   strings      foo    bar       So following Java code is perfectly valid   public class Foo       public void doSomething String   arg          public void example             String   strings      foo    bar             doSomething strings             Is there any valid reason why  instead  the following code shouldn t be valid   public class Foo        public void doSomething String   arg          public void example             doSomething    foo    bar                I think  that the above syntax would have been a valid substitute to the varargs introduced in Java 5  And  more coherent with the previously allowed array declarations

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Not so hidden  but interesting   You can have a  Hello  world  without main method   it throws NoSuchMethodError thought     Originally posted by  RusselW on Strangest language feature  public class WithoutMain       static           System out println  Look ma  no main              System exit 0              java WithoutMain Look ma  no main

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Surprises me that an interface can extend multiple interfaces but class can extend only one class

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JDK 1 6 07  contains an app called VisualVM  bin jvisualvm exe  that is a nice GUI on top of many of the tools   It seems more comprehensive than JConsole

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Most people does not know they can clone an array   int   arr    1  2  3   int   arr2   arr clone

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I can add Scanner object  It is the best for parsing     String input    1 fish 2 fish red fish blue fish   Scanner s   new Scanner input  useDelimiter    s fish  s     System out println s nextInt     System out println s nextInt     System out println s next     System out println s next     s close

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I know this was added in release 1 5 but the new enum type is a great feature   Not having to use the old  int enum pattern  has greatly helped a bunch of my code   Check out JLS 8 9 for the sweet gravy on your potatoes

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JVisualVM from the bin directory in the JDK distribution  Monitoring and even profiling any java application  even one you didn t launch with any special parameters  Only in recent versions of the Java 6SE JDK

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The strictfp keyword   I never saw it used in a real application though     You can get the class for primitive types by using the following notation  int class   float class  etc  Very useful when doing reflection   Final arrays can be used to  return  values from anonymous inner classes  warning  useless example below    final boolean   result   new boolean 1   SwingUtilities invokeAndWait new Runnable       public void run     result 0    true

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Annotation Processing API from Java 6 looks very perspective for code generation and static code verification

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The type params for generic methods can be specified explicitly like so   Collections  lt String Integer gt emptyMap

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How about covariant return types which have been in place since JDK 1 5   It is pretty poorly publicised  as it is an unsexy addition  but as I understand it  is absolutely necessary for generics to work     Essentially  the compiler now allows a subclass to narrow the return type of an overridden method to be a subclass of the original method s return type   So this is allowed   class Souper       Collection lt String gt  values                          class ThreadSafeSortedSub extends Souper        Override     ConcurrentSkipListSet lt String gt  values                           You can call the subclass s values method and obtain a sorted thread safe Set of Strings without having to down cast to the ConcurrentSkipListSet

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Comma  amp  array  It is legal syntax  String s         123       234

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You can build a string sprintf-style using String format     String w    world   String s   String format  Hello  s  d   w  3     You can of course also use special specifiers to modify the output   More here  http   java sun com j2se 1 5 0 docs api java util Formatter html syntax

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You can use enums to implement an interface   public interface Room      public Room north       public Room south       public Room east       public Room west       public enum Rooms implements Room      FIRST         public Room north              return SECOND                   SECOND         public Room south              return FIRST                   public Room north     return null       public Room south     return null       public Room east     return null       public Room west     return null          EDIT  Years later      I use this feature here  public enum AffinityStrategies implements AffinityStrategy     https   github com peter-lawrey Java-Thread-Affinity blob master src main java vanilla java affinity AffinityStrategies java  By using an interface  developers can define their own strategies  Using an enum means I can define a collection  of five  built in ones

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Part feature  part bother  Java s String handling to make it  appear  a native Type  use of operators on them          Being able to write   String s    A   s      String      so s     A String    is very convenient  but is simply syntactic sugar for  ie gets compiled to    String s   new String  A    s   new StringBuffer s  append   String   toString      ergo an Object instantiation and 2 method invocations for a simple concatenation  Imagine Building a long String inside a loop in this manner   AND all of StringBuffer s methods are declared synchronized  Thankfully in  I think  Java 5 they introduced StringBuilder which is identical to StringBuffer without the syncronization   A loop such as   String s       for  int i   0   i  lt  1000     i    s          i     Really an Object instantiation  amp  3 method invocations    can  should  be rewritten in your code as   StringBuilder buf   new StringBuilder       Empty buffer for  int i   0   i  lt  1000     i    buf append      append i      Cut out the object instantiation  amp  reduce to 2 method invocations String s   buf toString      and will run approximately 80   faster than the original loop   up to 180  on some benchmarks I have run

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A feature with which you can display splash screens for your Java Console Based Applications   Use the command line tool java or javaw with the option -splash  eg   java -splash C  myfolder myimage png -classpath myjarfile jar com my package MyClass   the content of C  myfolder myimage png will be displayed at the center of your screen  whenever you execute the class  com my package MyClass

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When working in Swing I like the hidden Ctrl - Shift - F1 feature   It dumps the component tree of the current window   Assuming you have not bound that keystroke to something else

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You can define an anonymous subclass and directly call a method on it even if it implements no interfaces   new Object       void foo String s        System out println s         foo  Hello

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As a starter I really appreciate the JConsole monitoring software in Java 6  it has solved a couple of problems for me already and I keep on finding new uses for it   Apparently the JConsole was there already in Java 5 but I reckon it is improved now and at least working much more stable as of now   JConsole in Java 5  JConsole in Java 5  JConsole in Java 6  JConsole in Java 6  And while you are at it  have a good look at the other tools in the series  Java 6 troubleshooting tools

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List subList returns a view on the original list A documented but little known feature of lists  This allows you to work with parts of a list with changes mirrored in the original list  List subList int fromIndex  int toIndex    quot This method eliminates the need for explicit range operations  of the sort that commonly exist for arrays   Any operation that expects a list can be used as a range operation by passing a subList view instead of a whole list  For example  the following idiom removes a range of elements from a list         list subList from  to  clear     Similar idioms may be constructed for indexOf and lastIndexOf  and all of the algorithms in the Collections class can be applied to a subList  quot

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Self-bound generics   class SelfBounded lt T extends SelfBounded lt T gt  gt        http   www artima com weblogs viewpost jsp thread 136394

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i personally discovered java lang Void very late -- improves code readability in conjunction with generics  e g  Callable lt Void gt

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Java Bean property accessor methods do not have to start with  get  and  set      Even Josh Bloch gets this wrong in Effective Java

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It s not exactly hidden  but reflection is incredibly useful and powerful   It is great to use a simple Class forName        newInstance   where the class type is configurable   It s easy to write this sort of factory implementation

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Actually  what I love about Java is how few hidden tricks there are  It s a very obvious language  So much so that after 15 years  almost every one I can think of is already listed on these few pages   Perhaps most people know that Collections synchronizedList   adds synchronization to a list  What you can t know unless you read the documentation is that you can safely iterate on the elements of that list by synchronizing on the list object itself   CopyOnWriteArrayList might be unknown to some  and Future represents an interesting way to abstract multithreaded result access   You can attach to VMs  local or remote   get information on GC activity  memory use  file descriptors and even object sizes through the various management  agent and attach APIs   Although TimeUnit is perhaps better than long  I prefer Wicket s Duration class

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You can switch this  inside method definitions of enum classes   Made me shout  whut   loudly when I discovered that this actually works

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Not really part of the Java language  but the javap disassembler which comes with Sun s JDK is not widely known or used

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I really like the rewritten Threading API from Java 1 6  Callables are great  They are basically threads with a return value

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This is not really a hidden feature but it did give me a big surprise when I saw this compiled fine   public int aMethod        http   www google com     return 1      the reason why it compiles is that the line http   www google com the  http   part is treated by the compiler as a label and the rest of the line is a comment   So  if you want to write some bizzare code   or obfuscated code   just put alot of http addresses there   -

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Joint union in type parameter variance   public class Baz lt T extends Foo  amp  Bar gt       For example  if you wanted to take a parameter that s both Comparable and a Collection   public static  lt A  B extends Collection lt A gt   amp  Comparable lt B gt  gt  boolean foo B b1  B b2  A a       return  b1 compareTo b2     0     b1 contains a     b2 contains a       This contrived method returns true if the two given collections are equal or if either one of them contains the given element  otherwise false  The point to notice is that you can invoke methods of both Comparable and Collection on the arguments b1 and b2

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My favorite  dump all thread stack traces to standard out   windows  CTRL-Break in your java cmd console window  unix   kill -3 PID

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The asList method in java util Arrays allows a nice combination of varargs  generic methods and autoboxing   List lt Integer gt  ints   Arrays asList 1 2 3

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Every class file starts with the hex value 0xCAFEBABE to identify it as valid JVM bytecode    Explanation

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with static imports you can do cool stuff like    List lt String gt  myList   list  foo    bar    Set lt String gt  mySet   set  foo    bar    Map lt String  String gt  myMap   map v  foo    2    v  bar    3

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Shutdown Hooks  This allows to register a thread that will be created immediatly but started only when the JVM ends   So it is some kind of  global jvm finalizer   and you can make useful stuff in this thread  for example shutting down java ressources like an embedded hsqldb server   This works with System exit    or with CTRL-C   kill -15  but not with kill -9 on unix  of course    Moreover it s pretty easy to set up               Runtime getRuntime   addShutdownHook new Thread                       public void run                           endApp

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Functors are pretty cool  They are pretty close to a function pointer  which everyone is usually quick to say is impossible in Java   Functors in Java

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It has already been mentioned that a final array can be used to pass a variable out of the anonymous inner classes   Another  arguably better and less ugly approach though is to use AtomicReference  or AtomicBoolean AtomicInteger      class from java util concurrent atomic package   One of the benefits in doing so is that these classes also provide such methods as compareAndSet  which may be useful if you re creating several threads which can modify the same variable     Another useful related pattern   final AtomicBoolean dataMsgReceived   new AtomicBoolean false   final AtomicReference lt Message gt  message   new AtomicReference lt Message gt     withMessageHandler new MessageHandler         public void handleMessage Message msg             if  msg isData                   synchronized  dataMsgReceived                     message set msg                    dataMsgReceived set true                    dataMsgReceived notifyAll                                       new Interruptible         public void run   throws InterruptedException           synchronized  dataMsgReceived                while   dataMsgReceived get                      dataMsgReceived wait                                        In this particular example we could have simply waited on message for it to become non-null  however null may often be a valid value and then you need to use a separate flag to finish the wait   waitMessageHandler      above is yet another useful pattern  it sets up a handler somewhere  then starts executing the Interruptible which may throw an exception  and then removes the handler in the finally block  like so   private final AtomicReference lt MessageHandler gt  messageHandler   new AtomicReference lt MessageHandler gt     public void withMessageHandler MessageHandler handler  Interruptible logic  throws InterruptedException       synchronized  messageHandler            try               messageHandler set handler               logic run              finally               messageHandler set null                       Here I assume that the messageHandler s  if it s not null  handleMessage      method is called by another thread when a message is received  messageHandler must not be simply of MessageHandler type  that way you will synchronize on a changing variable  which is clearly a bug   Of course  it doesn t need to be InterruptedException  it could be something like IOException  or whatever makes sense in a particular piece of code

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String Parameterised Class Factory   Class forName  className   newInstance      Load a resource  property file  xml  xslt  image etc  from deployment jar file   this getClass   getClassLoader   getResourceAsStream

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I like the static import of methods   For example create the following util class   package package name   public class util         private static void doStuff1              the end              private static String doStuff2            return  the end               Then use it like this   import static package name util     public class main        public static void main String   args             doStuff1       wee no more typing util doStuff1             System out print doStuff2        or util doStuff2               Static Imports works with any class  even Math     import static java lang Math    import static java lang System out  public class HelloWorld       public static void main String   args            out println  Hello World             out println  Considering a circle with a diameter of 5 cm  it has             out println  A circumference of      PI   5     cm            out println  And an area of      PI   pow 5 2      sq  cm

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You can override a method and have the superclass constructor call it  this may come as a surprise to C   programmers    Example

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You can define and invoke methods on anonymous inner classes   Well they re not that hidden  but very few people know they can be used to define a new method in a class and invoke it  like this    new Object         public String someMethod             return  some value            someMethod      Probably is not very common because it not very useful either  you can call the method it only when you define it   or via reflection

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You can declare a class in a method   public Foo foo String in        class FooFormat extends Format           public Object parse String s  ParsePosition pp       parse stuff             return  Foo  new FooFormat   parse in

[java] Hidden Features of Java

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