Constructor VS static method

can a constructor be compared to a static method because of the following reasons:
1)both static methods and contructors belong only to the class in whic they are defined.
2)both of them can be overloaded but cannot be overridden
Except for the difference that a constructor cannot be called explicitly while a static method can be????

new Object(); // explicit constructor invocationCertainly one can compare disparate artifacts by certain criteria, but that doesn't necessarily indicate that those artifacts are similar. Tigers and monkeys both have 2 legs - are they similar?

Similar Messages

  • Class constructor is actually "static" method, as Bruce Eckel says?

    <Thinking in java> chapter 7 :
    Even though constructors are not polymorphic (they?re actually static methods, but the static declaration is implicit), it?s important ...
    Are constructors actually static methods? I do not think so. static method can only manipulate static members, but constructor can also deal with non-static members. In other words, constructor always associate with a object, while static method always associate with a class.
    Why Mr. Eckel says that constructors are static?

    From the JLS ?8.8.3
    A constructor is always invoked with respect to an object, so it makes no sense for a constructor to be static, so no they are not.
    Are constructors actually static methods?Constructor declarations are not members. Members are defined as the set fields, methods, nested classes and interfaces(JLS ?8). So they are not methods.

  • Calling a non-static method from another Class

    Hello forum experts:
    Please excuse me for my poor Java vocabulary. I am a newbie and requesting for help. So please bear with me! I am listing below the program flow to enable the experts understand the problem and guide me towards a solution.
    1. ClassA instantiates ClassB to create an object instance, say ObjB1 that
        populates a JTable.
    2. User selects a row in the table and then clicks a button on the icon toolbar
        which is part of UIMenu class.
    3. This user action is to invoke a method UpdateDatabase() of object ObjB1. Now I want to call this method from UIMenu class.
    (a). I could create a new instance ObjB2 of ClassB and call UpdateDatabase(),
                                      == OR ==
    (b). I could declare UpdateDatabase() as static and call this method without
         creating a new instance of ClassB.With option (a), I will be looking at two different object instances.The UpdateDatabase() method manipulates
    object specific data.
    With option (b), if I declare the method as static, the variables used in the method would also have to be static.
    The variables, in which case, would not be object specific.
    Is there a way or technique in Java that will allow me to reference the UpdateDatabase() method of the existing
    object ObjB1 without requiring me to use static variables? In other words, call non-static methods in a static
    way?
    Any ideas or thoughts will be of tremendous help. Thanks in advance.

    Hello Forum:
    Danny_From_Tower, Encephalatic: Thank you both for your responses.
    Here is what I have done so far. I have a button called "btnAccept" created in the class MyMenu.
    and declared as public.
    public class MyMenu {
        public JButton btnAccept;
         //Constructor
         public MyMenu()     {
              btnAccept = new JButton("Accept");
    }     I instantiate an object for MyMenu class in the main application class MyApp.
    public class MyApp {
         private     MyMenu menu;
         //Constructor     
         public MyApp(){
              menu = new MyMenu();     
         public void openOrder(){
               MyGUI MyIntFrame = new MyGUI(menu.btnAccept);          
    }I pass this button all the way down to the class detail02. Now I want to set up a listener for this
    button in the class detail02. I am not able to do this.
    public class MyGUI {
         private JButton acceptButton;
         private detail02 dtl1 = new detail02(acceptButton);
         //Constructor
         public AppGUI(JButton iButton){
         acceptButton = iButton;
    public class detail02{
        private JButton acceptButton;
        //Constructor
        public detail02(JButton iButton){
          acceptButton = iButton;
          acceptButton.addActionListener(new acceptListener());               
       //method
        private void acceptListener_actionPerformed(ActionEvent e){
           System.out.println("Menu item [" + e.getActionCommand(  ) + "] was pressed.");
        class acceptListener implements ActionListener {       
            public void actionPerformed(ActionEvent e) {
                   acceptListener_actionPerformed(e);
    }  I am not able to get the button Listener to work. I get NullPointerException at this line
              acceptButton.addActionListener(new acceptListener());in the class detail02.
    Is this the right way? Or is there a better way of accomplishing my objective?
    Please help. Your inputs are precious! Thank you very much for your time!

  • Using static methods to instantiate an object

    Hi,
    I have a class A which has a static method which instantiates it. If I were to instantiate the class A twice with it's static method within class B then you would think class B would have two instances of class A, right?
    Example:
    class A
       static int i = 0;
       public A(int _i)
           i = _i;
       public static A newA(int _i)
            return new A(_i);
    class B
        public B()
            A.newA(5);
            A.newA(3);
            System.out.println(A.i);
        public static void main(String[] args)
            B b = new B();
    }Is there really two instances of class A or just one? I think it would be the same one because if you check the int i it would be 3 and not 5. Maybe static instantiated objects have the same reference within the JVM as long as you don't reference them yourself?
    -Bruce

    It
    seems like you are the only one having trouble
    understanding the question.Well I was only trying to help. If you don't want any help, its up to you, but if that's the case then I fail to see why you bothered posting in the first place...?
    Allow me to point out the confusing points in your posts:
    you would think class B would have two
    instances of class A, right?The above makes no sense. What do you mean by one class "having" instances of another class? Perhaps you mean that class B holds two static references to instances of Class A, but in your code that is not the case. Can you explain what you mean by this?
    Is there really two instances of class
    A or just one?You have created two instances.
    >I think it would be the same one because if you
    check the int i it would be 3 and not 5.
    I fail to see what that has to do with the number of instances that have been created...?
    Maybe static instantiated objects have the
    same reference within the JVM as long as you
    don't reference them yourself????? What is a "static instantiated object"? If you mean that it was created via some call to a static method, then how is it different to creating an object inside the main method, or any other method that was called by main? (i.e. any method)
    What do you mean by "the same reference within the JVM"? The same as what?
    What happened to the first call "A.newA(5)"?
    I think it was overidden by the second call
    "A.newA(3)".As i already asked, what do you mean by this?
    If you change the line for the constructor
    in A to i = i * i you will get "9" as the
    output, if you delete the call "A.newA(3)"
    you will get "25" as the output. Yes. What part of this is cauing a problem?

  • Force Derived Class to Implement Static Method C#

    So the situation is like, I have few classes, all of which have a standard CRUD methods but static. I want to create a base class which will be inherited so that it can force to implement this CRUD methods. But the problem is, the CRUD methods are static. So
    I'm unable to create virtual methods with static (for obvious reasons). Is there anyway I can implement this without compromising on static.
    Also, the signature of these CRUD methods are similar.
    E.g. ClassA will have CRUD methods with these type of Signatures
    public static List<ClassA> Get()
    public static ClassA Get(int ID)
    public static bool Insert(ClassA objA)
    public static bool Update(int ID)
    public static bool Delete(int ID)
    ClassB will have CRUD signatures like
    public static List<ClassB> Get()
    public static ClassB Get(int ID)
    public static bool Insert(ClassB objB)
    public static bool Update(int ID)
    public static bool Delete(int ID)
    So I want to create a base class with exact similar signature, so that inherited derived methods will implement their own version.
    For E.g. BaseClass will have CRUD methods like
    public virtual static List<BaseClass> Get()
    public virtual static BaseClassGet(int ID)
    public virtual static bool Insert(BaseClass objBase)
    public virtual static bool Update(int ID)
    public virtual static bool Delete(int ID)
    But the problem is I can't use virtual and static due to it's ovbious logic which will fail and have no meaning.
    So is there any way out?
    Also, I have few common variables (constants) which I want to declare in that base class so that I don't need to declare them on each derived class. That's why i can't go with interface also.
    Anything that can be done with Abstract class?

    Hi,
    With static methods, this is absolutely useless.
    Instead, you could use the "Singleton" pattern which restrict a class to have only one instance at a time.
    To implement a class which has the singleton pattern principle, you make a sealed class with a private constructor, and the main instance which is to be accessed is a readonly static member.
    For example :
    sealed class Singleton
    //Some methods
    void Method1() { }
    int Method2() { return 5; }
    //The private constructor
    private Singleton() { }
    //And, most importantly, the only instance to be accessed
    private static readonly _instance = new Singleton();
    //The corresponding property for public access
    public static Instance { get { return _instance; } }
    And then you can access it this way :
    Singleton.Instance.Method1();
    Now, to have a "mold" for this, you could make an interface with the methods you want, and then implement it in a singleton class :
    interface ICRUD<BaseClass>
    List<BaseClass> GetList();
    BaseClass Get(int ID);
    bool Insert(BaseClass objB);
    bool Update(int ID);
    bool Delete(int ID);
    And then an example of singleton class :
    sealed class CRUDClassA : ICRUD<ClassA>
    public List<ClassA> GetList()
    //Make your own impl.
    throw new NotImplementedException();
    public ClassA Get(int ID)
    //Make your own impl.
    throw new NotImplementedException();
    public bool Insert(ClassA objA)
    //Make your own impl.
    throw new NotImplementedException();
    public bool Update(int ID)
    //Make your own impl.
    throw new NotImplementedException();
    public bool Delete(int ID)
    //Make your own impl.
    throw new NotImplementedException();
    private CRUDClassA() { }
    private static readonly _instance = new CRUDClassA();
    public static Instance { get { return _instance; } }
    That should solve your problem, I think...
    Philippe

  • Static methods

    Is creating a static method frowned upon in OO development? Is it only a matter of convenience by not having to instantiate?
    Any insight is appreciated.
    --Gregory
    quote: Don't throw away the old bucket until you know whether the new one holds water.

    Regardless of how many objects you have created, there is only 1 copy of the static variables in the memory. Its not just a matter of convenience but is used in certain conditions based on some requirements. A good example would be Singleton. There already is a kind of Singleton class in the standard Java class libraries:
    the Math class. This is a class that is declared final and all methods are declared static, meaning that the class cannot be extended. The purpose of the Math class is to wrap a number of common mathematical functions such as sin and log in a class-like structure, since the Java language does not support functions that are not methods in a class. You can use the same approach to a Singleton pattern, making it a final class. You can�t create any instance of classes like Math, and can only call the static
    methods directly in the existing final class.
    Another approach, suggested by Design Patterns, is to create Singletons using a static method to issue and keep track of instances. To prevent instantiating the class more than once, we make the constructor private so an instance can only be created from within the static method of the class.
    Hope that helps
    &#9824

  • Parent constructor calls abstract method

    Hi everybody!
    I'm wondering if there is something wrong with java or if the idea is just too ill?
    Anyway, I think it would be great if this hierachy would work...
    Two classes A and B.
    Class A defines an astract method.
    In A's constructor this abstract method is called.
    Class B extends A and provides an implementation for the abstract method in A.
    Class B also defines a member variable that is set in B's implementation of the abstract method.
    In class' B constructor the parent constructor A() is called.
    example:
    public abstract class A {
      public A() {
        createComponents();
      public abstract void createComponents();
    public class B extends A {
      private String string = null;
      public B() {
        super();
        System.out.println("B::B() " + string);
      public void createComponents() {
        System.out.println("B::createComponents() begin");
        string = new String("test");
        System.out.println("B::createComponents() " + string);
      public void describe() {
        System.out.println("B::describe() " + string);
      public static void main(String[] args) {
        B b = new B();
        b.describe();
    }running the code above produces the following output:
    B::createComponents() begin
    B::createComponents() test
    B::B() null
    B::describe() null
    why is the string member variable null in B's constructor??
    thanks in advance
    Peter Bachl
    Polytechnic University of Upper Austria, Hagenberg
    [email protected]

    The answer is that the call of the super-constructor
    is allways done before the initialization
    of the member variable. That's all and that's the
    normal behavior.
    order :
    - initialization of static variables
    - call to the super-constructor
    - initialization of the instance variables
    - execution of the constructor
    Since this is the advanced forum it is relevant to point out that that is not exactly true.
    There is a step in java that 'initializes' member variables before any constructors are called, super or other wise.
    From the JLS 12.5...
    Otherwise, all the instance variables in the new object, including those declared in superclasses, are initialized to their default values (4.5.5)
    Using the following code as an example
      class MyClass
         int i1;
         int i2 = 1;
    .When creating an instance of the above there will be three 'initializations'
    // Part of object creation...
    i1 = 0; // The default value
    i2 = 0; // The default value
    // Part of construction...after super ctors called.
    i2 = 1; // The variable initializer (see step 4 of JLS 12.5)
    Unfortunately the descriptions are rather similar and so confusion can result as to when the assignment actually occurs.

  • Static method

    If I'm trying to write a method that accepts a String argument and prints it on the screen, but the method itself is supposed to return nothing, what exactly does that mean???
    This is the question itself:
    Write the definition of a class� Telephone . The class� has no constructors� and one static� method� printNumber . The method� accepts a String�argument� and prints it on the screen. The method� returns nothing.
    However, all I really am asking for is maybe some tips on understanding the concept itself. I've been looking through the tutorials online, but I just haven't found anything that explains this outright.
    To give an idea of where I am going completely, embarassingly wrong, here is what I tried to write (which I know!! it's probably far off!):
    import java.util.Scanner;
    public class Telephone
    void printNumber ()
    String lineIn;
    Scanner scan = new Scanner(System.in);
    lineIn = scan.nextLine();
    System.out.println(lineIn);
    Now please, laugh quietly to yourself and then help :P
    Thanks!

    the method itself is supposed to return nothing, what exactly
    does that mean???
    Now please, laugh quietly to yourself and then help
    :P
    Thanks!"return nothing" most probably means that the method has a void return type, and no return statement in the body of the method. Looks like you got that part right in your code.
    I would guess that the other part would be solved by adding the static key word to your method declaration. It should also probably be public, so that other classes can call it.
    So:
    public static void printNumber()
       // body of method goes here
    }Good luck,
    RD-R
    � {�
    Mon Mar 07 00:38:07 EST 2005
    Pseudo-random saying number 525 of 635
    A limerick packs laughs anatomical
    Into space that is quite economical.
            But the good ones I've seen
            So seldom are clean,
    And the clean ones so seldom are comical.

  • Non-static method cannot be referenced from a static context....again sry

    Hey, I know you guys have probably seen a lot of these, but its for an assignment and I need some help. The error I'm getting is: non-static method printHistory() cannot be referenced from a static context. Here are the classes effected
    public class BankAccount {
    private static int nextAccountNumber = 1000;
    //used to generate account numbers
    private String owner; //name of person who owns the account
    private int accountNumber; //a valid and unique account number;
    private double balance; //amount of money in the account
    private TransactionHistory transactions; //collection of past transactions
    private Transaction transaction;
    //constructor
    public BankAccount(String anOwnerName){
    owner = anOwnerName;
    accountNumber = nextAccountNumber++;
    balance = 0.0;
    transactions = new TransactionHistory();
    //public String getOwner() {
    public void deposit(double anAmount ){
         balance=balance+anAmount;
         transaction=new Transaction(TransactionType.DEPOSIT,accountNumber,anAmount,balance);
         transactions.add(transaction);
    //public void withdraw(double anAmount){
    //public String toString() {
    ***public void printHistory(){
         TransactionHistory.printHistory();
    AND
    public class TransactionHistory {
    final static int CAPACITY = 6; //maximum number of transactions that can be remembered
    //intentionally set low to make testing easier
    private Transaction[] transactions = new Transaction[CAPACITY];
    //array to store transaction objects
    private int size = 0;
    //the number of actual Transaction objects in the collection
    public void add(Transaction aTransaction){
         if (size>5){
         transactions[0]=transactions[1];
         transactions[1]=transactions[2];
         transactions[2]=transactions[3];
         transactions[3]=transactions[4];
         transactions[4]=transactions[5];
         transactions[5]=aTransaction;     
         transactions[size]=aTransaction;
         size=size++;
    public int size() {
         return size;
    ***public void printHistory() {
         for(int i=0;i<6;i++){
              System.out.println(transactions);
    //public void printHistory(int n){
    The project still isn't finished, so thats why some code is commented out. The line with *** infront on it are the methods directly effected, I think. Any help would be great.

    In Java, static means "something pertaining to an object class". Often, the term class is substituted for static, as in "class method" or "class variable." Non-static, on the other hand, means "something pertaining to an actual instance of an object. Similarly, the term +instance+ is often substituted for +non-static+, as in "instance method" or "instance variable."
    The error comes about because static members (methods, variables, classes, etc.) don't require an instance of the object to be accessed; they belong to the class. But a non-static member belongs to an instance -- an individual object. There's no way in a static context to know which instance's variable to use or method to call. Indeed, there may not be any instances at all! Thus, the compiler happily tells you that you can't access an instance member (non-static) from a class context (static).
    ~

  • Non-static method setUp() cannot be referenced  (error)

    Hi;
    I have this error
    "findContainer.java": non-static method setUp() cannot be referenced from a static context at line 10, column 26
    everytime i am trying to make this call
    jade.core.AddContTry.setUp();in
    public class findContainer {
      public findContainer() {}
        public void sUp(){
        System.out.println("please wait, this is first try to find he agents in the main container"+"\n");
        jade.core.AddContTry.setUp();
    this setup() method
    is written as follows
    public void AddContTry () {
    public void setUp(){
      AID[] list= null;
      try {
        MyMainContainer = myprofile.getMain();
        System.out.println("\n"+"This is the new container created in case of failure"+"\n");
        list = MyMainCont.agentNames();
        for (int i=0; i<list.length; i++){
          System.out.println("names = " + list.toString() + " \n");
    catch (ProfileException pe) { System.out.println("there is not main container");
    could you please help me?                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               

    My guess is that it is throwing a NullPointerException because you have not set 'a' to be anything.
    The sample code you provided does not make sense.
    AddContTry is a method in the code but you are using it like a class.
    You have to set the following before you can use the object 'a'
    AddContTry a = // something
    I am just guessing here but can you write
    AddContTry a = new AddContTry();
    a.setUp();
    Note, If this is the case, the code in setUp should be in the constructor.

  • Non-static method close() cannot be referenced from a static context

    Friends,
    I am having a little help with some static and not static issues.
    I created a JMenuBar, it's in the file: SlideViewMenu.java
    One of the operations is File->Close and another is File->Exit.
    The listener is in the SlideViewMenu.java file. The listener needs to reference two non-static methods within SlideView.java.
    Here's some of the code:
    SlideViewMenu.java
    public class SlideViewMenu {
        public JMenuBar createMenuBar() {
        final Action openAction = new OpenAction();
        Action aboutAction = new AboutAction();
        ActionListener menuListener = new MenuActionListener();
        JMenuBar menuBar = new JMenuBar();
         // All the menu stuff works fine and is taken care of here.
       // Listener for Menu
       class MenuActionListener implements ActionListener {
         public void actionPerformed (ActionEvent actionEvent) {
              String selection = (String)actionEvent.getActionCommand();
             if (selection.equals("Close"))
              SlideViewFrame.close();
             else  SlideViewFrame.exit();
    }SlideView.java
    // Driver class
    public class SlideView {
         public static void main(String[] args) {
              ExitableJFrame f = new SlideViewFrame("SlideView");
                    // Stuff here, works fine.
    // Frame class
    class SlideViewFrame extends ExitableJFrame {
            // some things here, work fine.
         private SlideViewMenu menuBar = new SlideViewMenu();
         public SlideViewFrame(String title) {
         // Set title, layout, and background color
         super(title);
         setJMenuBar(menuBar.createMenuBar());
            // Stuff here works fine.
         // Handles doing stuff once the file has been selected
         public void setFileName(File fullFileName, String simpleName) {
            // Stuff here works fine.     
         // File->Close. clean up everything.
         public void close() {
              setTitle("SlideView");
              textArea.setText("");
              scrollBar.setVisible(false);
              textArea.setVisible(false);
              statsPanel.setVisible(false);
         // File->Exit.     
         public void exit() {
              System.exit(0);
    }The error I'm getting is:
    SlideViewMenu.java:50: non-static method close() cannot be referenced from a static context
    I don't get it?
    Thanks for all help.

    Making close() and exit() static would not solve the problem because close() requires access to nonstatic member variables/functions.
    Fortunately, that is not necessary. The real reason you are having a problem is that you don't have any way in your listener to access the main frame window, which is what the listener trying to control. You made a stab at gaining access by prefixing the function with the class name, but, as the compiler has informed you, that is only valid for static methods. If you think about it, you should see the sense in that, because, what if you had a number of frames and you executed className.close()? Which one would you close? All of them?
    Fortunately, there is an easy way out that ties the listener to the frame.
    SlideViewMenu.java:public class SlideViewMenu
      // Here's where we keep the link to the parent.
      private SlideViewFrame parentFrame;
      // Here's where we link to the parent.
      public JMenuBar createMenuBar(SlideViewFrame linkParentFrame)
        parentFrame = linkParentFrame;
        final Action openAction = new OpenAction();
        Action aboutAction = new AboutAction();
        ActionListener menuListener = new MenuActionListener();
        JMenuBar menuBar = new JMenuBar();
        // All the menu stuff works fine and is taken care of here.
      // Listener for Menu --- It is assumed that this is a non-static nested
      // class in SlideViewMenu. All SlideViewMenu variables are accessible from
      // here. If this is not the case, simply add a similar member variable
      //  to this class, initialize it with a constructor parameter, and
      // pass the SlideViewMenu parentFrame when the listener is
      // constructed.
      class MenuActionListener implements ActionListener
        public void actionPerformed (ActionEvent actionEvent)
          String selection = (String)actionEvent.getActionCommand();
          // Use parentFrame instead of class name.
          if (selection.equals("Close"))
              parentFrame.close();
            else
              parentFrame.exit();
    }SlideView.java// Driver class
    public class SlideView
      public static void main(String[] args)
        ExitableJFrame f = new SlideViewFrame("SlideView");
        // Stuff here, works fine.
    // Frame class
    class SlideViewFrame extends ExitableJFrame
      // some things here, work fine.
      private SlideViewMenu menuBar = new SlideViewMenu();
      public SlideViewFrame(String title)
        // Set title, layout, and background color
        super(title);
        //Here's where we set up the link.
        setJMenuBar(menuBar.createMenuBar(this));
      // Stuff here works fine.
      // Handles doing stuff once the file has been selected
      public void setFileName(File fullFileName, String simpleName)
        // Stuff here works fine.     
      // File->Close. clean up everything.
      public void close()
        setTitle("SlideView");
        textArea.setText("");
        scrollBar.setVisible(false);
        textArea.setVisible(false);
        statsPanel.setVisible(false);
      // File->Exit.
      public void exit()
        System.exit(0);
    }

  • Export Internal table by calling static method

    Hi friends,
    Can u please tell me how to get internal table in export parameter by calling static method.
    OR  can u tell me how to declare internal table in method parameter.
    Thanks in advance

    Raja's method will work for all tables irrespective of the structure and is the best possible approach.  However, if your requirement is simple and you do not have to deal with a lot many tables,
    1. Create a Type-Pool, suppose ztypl
    2. Within the type pool, declare a Structure type,
    TYPES: BEGIN OF ztypl_struct1,
             "Place your fields here
           END OF ztypl_strict1,
           "Create a table of type ztypl_struct1 here
           ztypl_table1 TYPE TABLE OF ztypl_struct1.
    Use "ZTYPL_TABLE1" as the "Associated type" in your method's Parameter definition in SE24.
    eg.
    IT_TABLE      TYPE     ZTYPL_TABLE1
    If you are writing your class in an include,
        METHODS my_method1
          IMPORTING
            it_table1   TYPE ztypl_table1.
    Do remember to include
    TYPE-POOLS: ztypl.
    in your class's Constructor / Include program.
    Please award points if helpful.
    Regards,
    Ryan
    Message was edited by:
            Ryan Cannel

  • JNI static method call fails for the 6th time

    Hello,
    I have a JNI Method which calls the static method which gets reference to singleton class(getReference()), JVM crashes.
    what might be the problem?
    is it due to insufficient memory or any other reason?
    Here is my code.
    eScannerClass = gEnv->FindClass("com/elvista/jscaner/EScanner");
    eScannerContructId = gEnv->GetStaticMethodID(eScannerClass,"getReference","()Lcom/elvista/jscaner/EScanner;");
    eScannerUpdateMethodId = gEnv->GetMethodID(eScannerClass,"updateScanStatus","(Lcom/elvista/jscaner/EScanEvent;)V");
    eScannerObjectRef = gEnv->NewObject(eScannerClass,eScannerContructId);Thanx for any help on this.

    Hi,
    the eScannerContructId is refering a static method, not a constructor. Therefore you must not use gEnv->NewObject, which is only allowed for constructors. Instead you have to use gEnv->CallStaticObjectMethod to call getReference().
    Martin

  • Static methods, what for?

    What are static methods for? is there something that is not possible through methods associated with instances (non-static methods) ?

    There are many examples of static mehods in the Java API. For example the Math or the System class. All methods in this classes are static. So you dont have to instantiate the system class. In fact it is even not possible to instantiate the System class. System is final and all Constructors are private.
    Another use for static methods is the access to private static variables.

  • Apply static method requirements on class (Static interface methods)?

    I have a set of classes where is class is identified with a unique ID number. So each class has a public static int getId() method. Each class also has either a constructor that takes a byte array or a static factory method that takes a byte array.
    I would like to maintain a hashmap of these classes keyed to the ID's however I am not sure how to have some interface of abstract parent class that requires each of these classes to implement these static methods.
    What I was hoping to do was something like this>>>
         public interface MyClasses{
              static MyClasses createInstance(byte[] data);
         HashMap<Integer, MyClasses> map;
         public void init() {
              map = new HashMap<Integer, MyClasses>();
              map.put(Class1.getId(), Class1.class);
              map.put(Class2.getId(), Class2.class);
         public void createInstance (int id, byte[] data) {
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         }Is there some way I could do this?

    What about something like this?
    public interface Initializable
         public void initialize(byte[] data);
    public class Factory
         private final Map<Integer, Initializable> map = new HashMap<Integer, Initializable>();
            public void registerClass(int id, Class klass)
                    if (! Initializable.class.isAssignableFrom(klass))
                             // you may also want to ensure the class declares a parameterless constructor
                             throw new IllegalArgumentException("duff class");
                    if (this.map.keySet().contains(id))
                             throw new IllegalArgumentException("duff id");
              this.map.put(id, klass);
         public Initializable createInstance(int id, byte[] data)
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              Initializable i = map.get(id).newInstance();
                    i.initialize(data);
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