Strategy to make a mutable class immutable

Hi,
i posted this topic in the wrong forum:
http://forum.java.sun.com/thread.jspa?threadID=5129395&messageID=9463016#9463016
Could you help me?
(I mentioned it already to the moderator)

class Immutable {
final Mutable delegatee;
public Immutable(Mutable m) {
delegatee = m; // <---- You must create adefensive copy of m first.
ImmutableChild getChild() {
return new ImmutableChild(delegatee.getChild());
Note that you must create a defensive copy of the
Mutable object in your Immutable constructor.
Otherwise the caller can modify the supposedly
Immutable through the Mutable reference it passed to
the constructor.Like Stefan, i don't see your point. In this example, the only caller who can change the original mutable object, is the wrapper class. But that's no problem because the wrapper is designed by the creator of the mutable object.
If you look to my specific problem, and you create a class SourceUnit, who has a reference to the class ImmutableAST, which is an immutable wrapper of my AST node class, i don't see how the client, that calls the method instanceOfSourceUnit.getImmutableAST() can modify the original abstract syntax tree (under condition that the the wrapper class does not delegate to original AST methodes that can change the state of the tree and that the fields it returns are also immutable (wrapper) classes)

Similar Messages

How can 1 make an object of user defined class immutable?

Hi All,
How can one make an object of user defined class immutable?
Whats the implementation logic with strings as immutable?
Regards,

Hi All,
How can one make an object of user defined class
immutable?The simple answer is you can't. That is, you can't make the object itself immutable, but what you can do is make a wrapper so that the client never sees the object to begin with.
A classic example of a mutable class:
class MutableX {
    private String name = "None";
    public String getName() {
        return name;
    public void setName(String name) {
        this.name = name;
}I don't think it's possible to make this immutable, but you can create a wrapper that is:
class ImmutableX {
    private final MutableX wrappedInstance;
    public ImmutableX (String name) {
        wrappedInstance = new MutableX();
        wrappedInstance.setName(name);
    public String getName() {
        return wrappedInstance.getName();
    // Don't give them a way to set the name and never expose wrappedInstance.
}Of course, if you're asking how you can make your own class immutable then the simple answer is to not make any public or protected methods that can mutate it and don't expose any mutable members.
Whats the implementation logic with strings as
immutable?
Regards,I don't understand the question.

Immutable Vs Mutable Classes

Hi.
I want to know the following:
1. What are immutable classes/objects?
2. How to implement an immutable class?
3. Is Math class immutable? I know it's final but is it immutable? what's the difference?
4. Difference between Immutable/Mutable class - Implementation wise...
Thanks

Hi.
I want to know the following:
1. What are immutable classes/objects?An immutable object is one whose internal state (its fields or instance variables) cannot be changed once it has been created. An immutable class... I guess that would be a class whose objects are immutable.
2. How to implement an immutable class?Make all instance variables private and do not provide any methods that change them.
3. Is Math class immutable? I know it's final but is
it immutable? what's the difference?The question doesn't apply, because you cannot create an object of class Math. The modifier "final" means that you cannot declare a subclass of the class; it has nothing to do with whether its objects are immutable.
4. Difference between Immutable/Mutable class -
Implementation wise...?
>
Thanks

Making a class immutable

Ho w to make a class immutable??
Thanx in advance

Example
import java.util.Date;
* Planet is an immutable class, since there is no way to change
* its state after construction.
* @is.Immutable
public final class Planet {
public Planet (double aMass, String aName, Date aDateOfDiscovery) {
     fMass = aMass;
     fName = aName;
     //make a private copy of aDateOfDiscovery
     //this is the only way to keep the fDateOfDiscovery
     //field private, and shields this class from any changes
     //to the original aDateOfDiscovery object
     fDateOfDiscovery = new Date(aDateOfDiscovery.getTime());
//gets but no sets, and no methods which change state
public double getMass() {
    return fMass;
public String getName() {
    return fName;
* Returns a defensive copy of the field.
* The caller of this method can do anything they want with the
* returned Date object, without affecting the internals of this
* class in any way.
public Date getDateOfDiscovery() {
    return new Date(fDateOfDiscovery.getTime());
// PRIVATE //
* Primitive data is always immutable.
private final double fMass;
* An immutable object field. (String objects never change state.)
private final String fName;
* A mutable object field. In this case, the state of this mutable field
* is to be changed only by this class. (In other cases, it makes perfect
* sense to allow the state of a field to be changed outside the native
* class; this is the case when a field acts as a "pointer" to an object
* created elsewhere.)
private final Date fDateOfDiscovery;

How to make a class immutable?

I hav attend the interview where they ask this question.
I know little bit is Make the class final and declare all variables as static final .
Can any Help me.
This question

Thi is just my opinion;
An immutable object is an object that once created, it's values can no longer be changed.
Thus an immutable object could be an object that allows it's values to be changed only when created with the use of the constructor, and only has get methods to retrieve those values (no set methods).
@Peetzore
Defining them as final is something I never did, however it makes sense :) and will start doing so as well
Regards,
Sim085

I want to make this class immutable. How to optimize performance?

Hi,
I designed an immutable class for other people to use. And I'm wondering if there's any way to optimize it for performance.
ie.
class MyImmutableList
final List myList;
public MyImmutableList(List list)
    myList = list;
public MyImmutableList append(Item item)
    List copy = myList.copy();
    copy.append(item);
    return new MyImmutableList(copy);
}But I find that many times, other people use the class like this:
someList = someList.append(item);So in a case like this, append() is unnecessarily making a copy of the list.
Is there anyway to optimize this?
Thanks
-Cuppo
Edited by: CuppoJava on 20-Jul-2008 5:44 AM
Edited by: CuppoJava on 20-Jul-2008 5:44 AM

DrClap wrote:
Well, of course, what else should they do when the method returns an ImmutableList?
What I would do with that append() method would be to remove it from the API entirely. The class is supposed to be immutable? Then it doesn't need an append() method. Unless, of course, the append() method isn't meant to change the state of the object, which seems a bit, let's say, unintuitive. Returning a copy of the list with the object appended then wouldn't change the state of the object, but it would return a new object. Which in fact is what the code does. But why? I'm like those other people, I would do that too. I don't understand the purpose of the method.The Defence calls java.math.BigDecimal
>
BigDecimal, would it be fair to say, you are an immutable class?
It would.
Would it also be true to say that you have an add() method?
I do
And would it be fair to say that adding could be construed as a mutating operation?
I suppose so
And what does this method do, given that you claim to be immutable?
It returns a new object which is the result of the add() operation
Thankyou

Trying to make a square class

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import java.awt.Rectangle;
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private int width;
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You realize that by definition a square has equal sides? You don't need a width or a height, you just need a length. My "Square" class would only have a getLength() method and possibly setLength() if you want it to be mutable. If this is intended for use with AWT you could subclass Rectangle and guarantee that all sides are equal. Given your current design it would be trivially easy for me to create a Square that is in fact not a square at all.
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* Returns a square rectangle. The returned Rectangle is mutable
* and as such it could be mutated to no longer be a square. It is the
* responsibility of the client to ensure this does not happen. Each
* returned Rectangle is unique and mutations in one will not affect
* the others.
* @return A Rectangle that is a perfect square.
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FATAL ERROR WHILE LOADING SQL DRIVER: java.lang.ClassNotFoundException: com.mysql.jdbc.Driver
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It looks like you are putting the mysql jar file inside the dassimul jar. That won't work unless you write custom code. You should use the Class-Path attribute of the manifest to define a relative path(s) to the other jar(s) that you need to use.
[http://java.sun.com/docs/books/tutorial/deployment/jar/index.html]

Best strategy to make commits by group of 100/200 insert/update

Hi,
I have a Java batch written in Java and i want to improve performances when writing to database.
For 6000 lines of inserts, i 'd like to group insert by 100/200 in a Unit Of Work then commit.
- i should use the methode useBatchWriting to group the inserts
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I applied what you suggested yesterday , but stll have problems
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commit etc...
Doen't work... Java freeze .
I'm working with TL 9.0.3.4 in jdk 13
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Strategy for mapping existing Java classes to db uing toplink

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Assuming your schema is as follows
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Is it possible to make a local class accessible in SE24 subclasses?

G'Day,
The scenario is simple: I have a global class created in SE24 with a local helper class defined in the local definitions/implementations sections. The issue is anything declared here is private; however I would like my local class to be available in the subclasses of the global class as well. In other words I want a protected local class.
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    |
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I've messed about with different definitions and FRIENDS but no joy.
I can think of two alternatives:
1. Write a protected wrapper method in zcl_super to expose each lcl_helper method to the subclasses. Annoying, cumbersome, defeats the object of using a local class in first place.
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Hi Naimesh,
Thanks for the input. Interesting idea about interfaces, I need to play around with this a little.
I don't agree what I'm trying to do would violate encapsulation. I'm after the same visibility level as other class component type. i.e. methods can be private, protected or public. Classes can be local or global - a protected class visible to subclasses would fit the principles quite well.
Consider a global class GADGET which has a protected attribute WIDGET, type ref to a local class. The widget's methods and attributes should really be separate because it is it's own entity, but at the same time it has no business in the wider SAP system. ==> Perfectly sensible use of a local class.
Now, this widget can turn, open, close, light up. So in a GADGET method I can say WIDGET->TURN( ). Create a subclass of GADGET and boom, all such code working with WIDGETs is broken. In some ways it makes local classes a little un-future-proof.
The bothersome thing about the workarounds is that they de-object-orient the two objects by forcing us to create protected methods TURN_WIDGET LIGHT_WIDGET OPEN_WIDGET etc. on the GADGET class. Which goes back to my original point that there's little benefit of creating a local class if we need to do this....
Or am I missing something?
Cheers,
Mike

Keeping a queue class immutable

Hi,
I recently got hooked on the concept of immutable classes. I'm not really sure why, but anyway...
I have a class that queues up tasks to a worker thread for execution, which is currently immutable. However, I never implemented a way to stop the worker thread if I wanted the queue to shutdown, so I'm trying to come up with a way to do so while retaining the immutability of the class.
Normally, I'd have a boolean value, and then set the boolean to false or true or whatever, and then the run() method in the worker thread would return. However, If I have to set a boolean, it would break the concept of immutability.
So while this question may seem somewhat ridiculous, I'm more curious than anything to see if there's a way to solve this problem.
Here's my queue class, with an incomplete shutdown() method, which I'd like to, in the future, "stop" the ThreadWorker thread:
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          } catch (InterruptedException ignored) {
     public void shutdown() {
     private final class ThreadWorker implements Runnable {
          public void run() {
               for (;;) {
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}If I'm just being pedantic with the whole immutability thing, so be it; I'm really curious.
Any advice will be greatly appreciated.
Thanks,
Dan

Well it looks as if I need to look up the definition of immutable again. I didn't think that adding things to a list would constitute changing the state of the object, but now that I think about it, that obviously does.
Anyway, my question is answered; the question was a bit ridiculous like I had thought, but oh well.
> >
It seems to me tho that while shutdownNow could potentially stop the pool (which it seems is just 1 thread), it won't stop the other threads... BTW, all the threads seem to do is to loop and tell the single executor to run the next job... Are they actually running the jobs? Otherwise, why do you need more than 1 of them?
As to this, shutting down the ExecutorService stops that thread, and it won't stop the thread running from ThreadWorker (which is why I changed my code as I'll show later). Anyway, the point of ThreadWorker is to execute each task in a "background" thread. Actually, since the ExecutorService runs in its own thread, it seems as if ThreadWorker is pretty worthless. If I added pool.execute(queue.take());it doesn't seem like it'll matter. Looks like I'll have to try that out and see what happens.
Anyway, here's the most recent update of my code, with the new shutdownQueue() method and a new loop in ThreadWorker:
public final class ThreadQueue {
     private final ExecutorService pool;
     private final BlockingQueue<Runnable> queue = new LinkedBlockingQueue<Runnable>(5);
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     public ThreadQueue() {
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          stopThreadWorker = false;
          (new Thread(new ThreadWorker())).start();
     public synchronized void addToQueue(Runnable task) {
          try {
               queue.put(task);
          } catch (InterruptedException ignored) {
     public void shutdownQueue() {
          stopThreadWorker = true;
          pool.shutdownNow();
     private final class ThreadWorker implements Runnable {
          public void run() {
               while(stopThreadWorker) {
                    try {
                         pool.execute(queue.take());
                    } catch (InterruptedException ignored) {
}Thanks for all the help!

When is the next Strategy Management (SSM) Technical Overview class?

The next SSM class is February 4-8 in Chicago. To register:
1. Go to http://www50.sap.com/useducation/index.asp
2. In the Course Search Field type in CPM.
3. On the list select TZCPM1 SAP Strategy Management Technical Overview (Chicago).
Any questions?

The next SSM class is February 4-8 in Chicago. To register:
1. Go to http://www50.sap.com/useducation/index.asp
2. In the Course Search Field type in CPM.
3. On the list select TZCPM1 SAP Strategy Management Technical Overview (Chicago).
Any questions?

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Dear Gurus!
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I think are you saying that the raw material for the finished product which the customers of your client are needing can also be used by your own client, right.
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"IF my client wants to use this raw material for add-hoc case. They must be accepted by customer and buy to full fill." what did you meant by this line ? Please elaborate on this....
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Strategy to make a mutable class immutable

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