Trouble understanding static objects

Hello,
I have trouble understanding static objects.
1)
class TestA
public static HashMap h = new HashMap();
So if I add to TestA.h from within a Servlet, this is not a good idea, right?
But if I just read from it, that is ok, right?
2)
class TestB
public static SimpleDateFormat df = new SimpleDateFormat();
What about TestB.df from within a Servlet? Is this ok?
example: TestB.df.format(new Date(1980, 1, 20));

There is exactly one instance of a static member for every instance of a class. It is okay to use static members in a servlet if they are final, or will not change. If they may change, it is a bad idea. Every call to a servlet launches a new thread. When there are multiple calls, there are multiple threads. If each of these threads are trying to make changes to the static memeber, or use the static memeber while changes are being made, the servelt could give incorrect results.
I hope that helped..
Thanks
Cardwell

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Trouble understanding static

Like the topic says, I do use it (actually I have to for the GUIs or they don't compile), but I really don't understand too much about them (what they allow, what they limit) and the definition on java.sun is not very clear to me. Could someone please give me a brief explanation on this subject and maybe an example?

public class Test {
    private String myName = "My name is . . .!"; //private variable of class Test
public static void main(String[] argv)   //is static because the runtime environment needs to call this first, before making any objects, so it's not essential for your class, just gives you a way of running your code from within your class itself
    String theNameMainGets = ""; //main will receive the name in this
    Test testThis = new Test(); //make an instance without a name
    Test testThisWithAName = new Test("nogoodatcoding"); //pass a cool name to the constructor :D
    //theNameMainGets = whatIsMyName(); //will not work since main is static, so even though it is part of the same class, it cannot call a non-static member, whatIsMyName, because a copy of that is with every instance, and when main calls it, the instance may not exist
    theNameMainGets = testThis.whatIsMyName(); //correct way to access the non-static member, by creating an instance and then using that to access it
    System.out.println(theNameMainGets);
    theNameMainGets = testThisWithAName.whatIsMyName();
    System.out.println(theNameMainGets);
    //note, you can't say, testThis.myName; since myName is private to testThis.
Test()
      myName = "No one named me";
Test( String newName )
      myName = newName;
    public String whatIsMyName()
      return myName;
}Hope this helps

Have Trouble understanding the runnable interface

Hi
I am new to java programming. I have trouble understanding threading concepts.When I run the below code,Thread3 runs first. I need a small clarification here- I created two thread objects
Thread thread1=new Thread(new RunnableThread(),"thread1");
Thread thread2=new Thread(new RunnableThread(),"thread2");
As soon as I create these objects -Will the constructor with the string argument in the "RunnableThread" class gets invoked. If so why doesn't System.out.println(runner.getName()); get invoked for the above objects. what is the sequence of execution for the below program.
/*************CODE****************************/
class RunnableThread implements Runnable
     Thread runner;
     public RunnableThread()
     public RunnableThread(String threadName)
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          runner.start();          //Start Thread
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          //Display info about this particular Thread
          System.out.println(Thread.currentThread());
public class RunnableExample
     public static void main(String argv[])
          Thread thread1=new Thread(new RunnableThread(),"thread1");
          Thread thread2=new Thread(new RunnableThread(),"thread2");
          RunnableThread thread3=new RunnableThread("thread3");
          //start the threads
          thread1.start();
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}

srinivasaditya wrote:
Hi
I am new to java programming. I have trouble understanding threading concepts.I'd consider it to be an advanced area.
When I run the below code,Thread3 runs first. I need a small clarification here- I created two thread objects
Thread thread1=new Thread(new RunnableThread(),"thread1");
Thread thread2=new Thread(new RunnableThread(),"thread2");No, this is wrong. You don't need your RunnableThread. Runnable and Thread are enough. Your wrapper offers nothing of value.
>
As soon as I create these objects -Will the constructor with the string argument in the "RunnableThread" class gets invoked. If so why doesn't System.out.println(runner.getName()); get invoked for the above objects. what is the sequence of execution for the below program.did you run it and see? Whatever happens, that's the truth.
%

Trying to understand Static methods

Hi all,
I am trying to learn a little bit about static methods and hope that someone here can help me understand it better. My understanding of static methods is that they exist only once (More like global methods). This tells me that usage of static methods should be used with care as they are likely to cause problems in cases where multiple users try to access a static object at the same time.
I am looking at a piece of code that had me thinking for a bit. I cant post the code itself but here is an example of how the code is structured
The first class declares a couple of non static arrays and makes them available via the getters and setters. It also has one method that calls a static method in another class.
package com.tests.statictest;
import java.util.ArrayList;
public class ClassA{
     ArrayList arrayList1 = new ArrayList();
     ArrayList arrayList2 = new ArrayList();
     public ClassA(){
          arrayList1.add("TEST1");
          arrayList1.add("TEST2");
          arrayList2.add("Test3");
          arrayList2.add("Test4");
     ArrayList getArrayList1(){
          return arrayList1;
     ArrayList getArrayList2(){
          return arrayList2;
     ArrayList getJoinedArrayList(){
          return ClassB.joinArrays(arrayList1,arrayList2);
}The second class contains the static method that is called by the above class to join the two arrays. This class is used by many other classes
package com.tests.statictest;
import java.util.ArrayList;
public class ClassB{
     public static ArrayList joinArrays(ArrayList list1, ArrayList list2){
          ArrayList list3 = new ArrayList();
          list1.addAll(list2);
          return list1;
}And here is my test class
package com.tests.statictest;
public class ClassC{
      public static void main(String args[]){
           ClassA classA = new ClassA();
          System.out.println(classA.getArrayList1());
          System.out.println(classA.getArrayList2());
          System.out.println(classA.getJoinedArrayList());
     }The output to the above program is shown below
[TEST1, TEST2]
[Test3, Test4]
[TEST1, TEST2, Test3, Test4]My question for the above is that i am wondering if the above is safe. Can you think of situations where the above is not recommended. What exactly would happen if ten instances of ClassA threads are executed at the same time? Woulnt the static method in ClassB corrupt the data?

ziggy wrote:
Hi all,
I am trying to learn a little bit about static methods and hope that someone here can help me understand it better. My understanding of static methods is that they exist only once (More like global methods). This tells me that usage of static methods should be used with care as they are likely to cause problems in cases where multiple users try to access a static object at the same time. There is no such thing as a "static object" in Java. The word "static" simply means "belonging to a class as a whole, rather than an individual instance."
My question for the above is that i am wondering if the above is safe. Can you think of situations where the above is not recommended. What exactly would happen if ten instances of ClassA threads are executed at the same time?ClassA isn't a thread, so it can't be "executed", per se.
Woulnt the static method in ClassB corrupt the data?"Staticness" doesn't have anything to do with it; the issue at hand is operations on a shared data structure, which is a concern whether you're dealing with static or non-static members. There is nothing inherent in ClassB that will "corrupt" anything, however.
~

Trouble Understanding Logic of Primes Program

I am reading a book called Beginning Java 7, by Ivor Horton. In his book he has a primes calculation program. The code is below. I am having trouble understanding the logic of the program. I've stared at it for the past several hours with no luck. What I can't understand is how both i and j are both initialized to 2 but the if statement:
if(i%j == 0) {
          continue OuterLoop;
        }then passes a 2 to the println() function. I am totally lost on this problem. The program below outputs all of the prime numbers from 2 to 50 without any errors. Please help!
Thank you.
public class Primes2 {
public static void main(String[] args) {
    int nValues = 50;                                                  // The maximum value to be checked
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      for(int j = 2 ; j < i ; ++j) {
        if(i%j == 0) {                                                 // This is true if j divides exactly
          continue OuterLoop;                                          // so exit the loop
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      System.out.println(i);                                           // so output the value
}Edited by: EJP on 27/03/2013 19:54: fixed all the bizarre formatting, spelling, and removed the even more bizarre comment marketers around the text. Please use standard English and its existing conventions here.

Hi. I did notice that. What I don't understand is how 2 is outputted as a prime number when the Net Beans debugger shows that i's value is 3 in the if statement. Where does the value 2 come from? Thanks.
Edited by: 996440 on Mar 27, 2013 3:07 PM

Static Object Vs. Lots of referece passing...

I am implementing a discrete event simulator for which there is only ever one "executor" object and one "output manager" object used to advance time and deal with output to files. However many objects in the simulator need access to these objects and there "security" is not an issue. I was wondering if anyone knows if it is gives better performance to make these classes with static methods or whether I should pass a reference to them to nearly every other object. Also do you know why whatever one is beter?
Cheers,
Mark

For this project performance really is the issue
though. My concern is that if I pass a reference of
an object to lots of other objects (in the order of
100,000) that is simply a waste of time and memory
So let's get this straight. You can have 100,000 Objects, but you can't add one reference varialble to each.
if making the objects access methods static may
remove this I will happily accept this over
management and testability issues. What do you think making the method static will save you, exactly?
What I really want to know is for example...
is the above less effective than...No, generally , static methods are less effective than instance methods.
if StaticObjectType in the later example is the same
as the OtherObjectType in the first example., but
with a static method, given that only one instance of
OtherObjectType will ever exist. This makes no sense. There is no such thing as a 'static' Object. Static only applies to references and methods and nested classes.
My worry is that passing this object address around
and storing it as a field will be far less effective
than having just a static method, although I am not
sure this is true.There is a slight performance hit when calling an instance method over a static method. Declaring a method final may remove this. When I say slight, I mean really slight. Both methods are called in the basically the same manner.
As this is happening thousands of times a second in
this program and storage and speed are key really
just need to choose the most efficient
implementation.Again I don't understand how you can afford o have thousands of Objects but can't afford an extra reference on them.

Using public static object for locking thread shared resources

Lets Consider the example,
I have two classes
1. Main_Reader -- Read from file
public class Main_Reader
public static object tloc=new object();
public void Readfile(object mydocpath1)
lock (tloc)
string mydocpath = (string)mydocpath1;
StringBuilder sb = new StringBuilder();
using (StreamReader sr = new StreamReader(mydocpath))
String line;
// Read and display lines from the file until the end of
// the file is reached.
while ((line = sr.ReadLine()) != null)
sb.AppendLine(line);
string allines = sb.ToString();
2. MainWriter -- Write the file
public class MainWriter
public void Writefile(object mydocpath1)
lock (Main_Reader.tloc)
string mydocpath = (string)mydocpath1;
// Compose a string that consists of three lines.
string lines = "First line.\r\nSecond line.\r\nThird line.";
// Write the string to a file.
System.IO.StreamWriter file = new System.IO.StreamWriter(mydocpath);
file.WriteLine(lines);
file.Close();
Thread.Sleep(10000);
In main have instatiated two function with two threads.
public string mydocpath = "E:\\testlist.txt"; //Here mydocpath is shared resorces
MainWriter mwr=new MainWriter();
Writefile wrt=new Writefile();
private void button1_Click(object sender, EventArgs e)
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t2.Start(mydocpath);
Thread t1 = new Thread(new ParameterizedThreadStart(mrw.Readfile));
t1.Start(mydocpath);
MessageBox.Show("Read kick off----------");
For making this shared resource thread safe, i am using a public static field,
public static object tloc=new object(); in class Main_Reader
My Question is ,is it a good approach.
Because i read in one of msdn forums, "avoid locking on a public type"
Is any other approach for making this thread safe.

Hi ,
Since they are easily accessed by all threads, thus you need to apply any kind of synchronization (via locks, signals, mutex, etc).
Mutex:https://msdn.microsoft.com/en-us/library/system.threading.mutex(v=vs.110).aspx
Thread signaling basics:http://stackoverflow.com/questions/2696052/thread-signaling-basics
Best regards,
Kristin
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I have application worked in cluster. Is some possibility to propagate change in static object to all application in cluster.

You can use application context (javax.naming.InitialContext) and bind (set), rebind (set new value), lookup (get) and unbind (remove) variables (must be serializable).
You must corect configure RMI replication.
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Third-party C++ Driver : Static object

Hello,
I'm writting a C++ driver for Labview RT.
I'm using Visual C++ to compile and link this driver.
I have some initialisation problem on the target.
It seems that static object are not created correctly. All members are not created correctly and it crashes as soon as I use these objects.
Could someone confirm this problem ?
Is there someone develop a C++ driver (based on VISA) for Labview RT ? Thanks for your responses !
David Chuet

David:
For starters, note that NI does not officially support MSVC-built DLLs on LabVIEW RT. I believe we only officially support DLLs built with LabVIEW or CVI. However, in most cases MSVC-built DLLs will work fine.
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Static Objects.

If an object were static, would its non-static members act like static? What kind of problems would happen if two threads try to use this object to call one of its non-static functions? Would any member of this object (either static or non-static) be part of a critical section?

jverd wrote:
wpafbuser1 wrote:
There's no such thing as a static object (since there's no instance).No, there's no such thing as a static object because the property of being static or not is not applicable to objects. Saying there's no instance is wrong, since, if there's an object, there's obviously an instance--the object.I wrote too fast, I meant there's no instantiation. Obviously there's an object/instance. But my thinking was flawed there anyway. :)
A class is considered static when it's fields and methods are static.No. A class is static when it's a nested class and it's declared static. A class is often incorrectly called static simply by virtue of it having only static members. Regardless of whether the class is static, all instances of that class are neither satic nor non-static.The term static, incorrectly used or not, I was referring too top-level classes not inner classes.
I guess what I'm saying is that if you include non-static methods or fields in your class, it's no longer a static class Wrong.
class Outer {
static class Nested {
int x;
void foo() {}
}Nested has a non-static member variable and a non-static method, but it's still a static class, because it's declared static.Again, we're talking about 2 different things now.

How to persistent static object

JDO always assigns a StateManager to a PersistenceCapable object when
creating instance. If an object is static, it'll keep its StateManager for
ever after instantiation. So If I excute the following code repeadly, kodo
will complain that persistence manager has been closed.
1) Get PersistenceManager
2) Begin transaction
3) Get object from datastore. If not exist, create a new object. This
object is a static.
4) Commit transaction
5) pm.close
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object. But for the second time, exception, "persistence manager has been
closed", will arise when excuting some of object's methods. Because each
StateManager object owns a PersistenceManager object. If this pm is
closed, according StateManager object is also useless.
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Thanks !

Dear Marc,
Thanks for your kind help. It's an effective advice to prevent
persistent object from being static.
However, what if the object to be persistent is an enumeration type
constant ? For example :
public class MyEnumType
private String name = null;
private int value = 0;
protected MyEnumType( String name, int value )
this.name = name;
this.value = value;
public static final TYPE_1 = new MyEnumType( "type 1", 1 );
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private MyEnumType type = null;
public void setType(..) {..}
public MyEnumType getType() {..}
public class TestClass
public void f()
PersistenceManager pm = null;
try
// Obtain pm
pm = ...;
pm.currentTransaction().begin();
EnumTypeUseClass use = new EnumTypeUseClass();
// Obtain myenumtype object from datastore
MyEnumType type = getJdoObject( pm, MyEnumType.class, "value ==" +
MyEnumType.TYPE_1.getValue() );
// Following line will cause "PersistenceManager has been closed"
exception
use.setType( type );
pm.makePersistent( use );
pm.currentTransaction().commit();
catch( .. )
finally
pm.close();
public static void main( String args[] )
for( int i = 0; i < 10; i ++ )
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Actually, exception is caused by MyEnumType.TYPE_1.getValue() because
this object's pm has been closed in the first loop time.
Now I solved this problem by not persistent MyEnumType objects, and
change type field in EnumTypeUseClass from MyEnumType to int.
But I still wonder how to persistent static objects.
Maybe your first advice is feasible, but I think it will make your
program tangly. Do you think so ? :)
Marc Prud'hommeaux wrote:
Liang-
You are correct that a persistent instance must be associated with a
PersistenceManager. You could always just leave the PersistenceManager
open (if using optimistic transaction, Kodo won't tie up database
resources in this case). Another option is to not have the singleston
instance be a static variable, but have it be obtained via a factory
method that takes a PersistenceManager argument.
If this doesn't help, perhaps you could help clarify the situation by
posting some code that shows that you would like to do?
In article <[email protected]>, Liang Zhilong wrote:
JDO always assigns a StateManager to a PersistenceCapable object when
creating instance. If an object is static, it'll keep its StateManager for
ever after instantiation. So If I excute the following code repeadly, kodo
will complain that persistence manager has been closed.
1) Get PersistenceManager
2) Begin transaction
3) Get object from datastore. If not exist, create a new object. This
object is a static.
4) Commit transaction
5) pm.close
For the first time, kodo will assign a new StateManager to this static
object. But for the second time, exception, "persistence manager has been
closed", will arise when excuting some of object's methods. Because each
StateManager object owns a PersistenceManager object. If this pm is
closed, according StateManager object is also useless.
After enhancing, kodo will change some getter/setter methods to
according jdo getter/setter ones. These methods may invoke
startManager.isLoaded(), which invokes pm.isActive(). Unfortunately, pm
has been closed.
Any other tricky methods to persistent static object ?
Thanks !
Marc Prud'hommeaux [email protected]
SolarMetric Inc. http://www.solarmetric.com

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