Trying to understand Static methods

Question

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?

Answer 1

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.
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