CODEWITHSUNDEEP
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user542719
user542719

Reputation: 307

static variable lose its value

I have helper class with this static variable that is used for passing data between two classes.

public class Helper{
    public static  String paramDriveMod;//this is the static variable in first calss
}

this variable is used in following second class mathod

    public void USB_HandleMessage(char []USB_RXBuffer){
    int type=USB_RXBuffer[2];
    MESSAGES ms=MESSAGES.values()[type];
    switch(ms){
        case READ_PARAMETER_VALUE:  // read parameter values
            switch(prm){
                case PARAMETER_DRIVE_MODE:  // paramet drive mode
                    Helper.paramDriveMod =(Integer.toString(((USB_RXBuffer[4]<< 8)&0xff00)));
                    System.out.println(Helper.paramDriveMod+"drive mode is selectd ");
                    //here it shows the value that I need...........
            }
    }
    //let say end switch and method
}

and the following is an third class method use the above class method

public void buttonSwitch(int value) throws InterruptedException{
    boolean bool=true;
    int c=0;
    int delay=(int) Math.random();
    while(bool){
        int param=3;
        PARAMETERS prm=PARAMETERS.values()[param];
        switch(value){
            case 0: 
                value=1;
                while(c<5){
                    Thread.sleep(delay);                
                    protocol.onSending(3,prm.PARAMETER_DRIVE_MODE.ordinal(),dataToRead,dataToRead.length);//read drive mode
                    System.out.println(Helper.paramDriveMod+"  drive mode is ..........in wile loop");//here it shows null value
                }
                //break; ?
        }
    }
    //let say end switch and method
}

what is the reason that this variable lose its value?

Upvotes: 0

Views: 4467

Answers (6)

Bill K
Bill K

Reputation: 62769

As I understand it, a "Class" (Not just an instance but the entire class object) Can be garbage collected just like any other unreferenced object--a static variable in that class instance won't prevent the GC from collecting your class.

I just came here because I think I'm seeing this behavior in a singleton and I wanted to see if anyone else noticed it (I've never had to research the problem before-and this knowledge is like a decade old from the back of my brain so I'm unsure of it's reliability at this point).

Going to go continue research now.

Just found this question, check the accepted answer--looks like it's unlikely that a static will be lost due to GC, but possible.

Are static fields open for garbage collection?

Upvotes: 1

Visalakshi
Visalakshi

Reputation: 1

The execution flow not shown - may be the 3rd code:

while(c<5){
    Thread.sleep(delay);                
    protocol.onSending(3,prm.PARAMETER_DRIVE_MODE.ordinal(),dataToRead,dataToRead.length);//read drive mode
    System.out.println(Helper.paramDriveMod+"  drive mode is ..........in wile loop");//here it shows null value  "

is executed before the second code 

switch(ms)
{
    case READ_PARAMETER_VALUE:  // read parameter values
        switch(prm){
            case PARAMETER_DRIVE_MODE:  // paramet drive mode
                Helper.paramDriveMod =(Integer.toString(((USB_RXBuffer[4]<< 8)&0xff00)));

Upvotes: -1

Costis Aivalis
Costis Aivalis

Reputation: 13728

Could it be that you may be confusing static with final? Static variables' values can change. Final variables' values can not.

Upvotes: 0

RATTLESNAKE
RATTLESNAKE

Reputation: 3228

Don't use static variable until you are in some critical situation. You can use getter setter instead

Upvotes: 0

InsertNickHere
InsertNickHere

Reputation: 3666

A Variable never "loses" its value. You set it to "null" somewhere, but your code here is not enough to tell whats going on. The only place here where you set it is this line:

Helper.paramDriveMod =(Integer.toString(((USB_RXBuffer[4]<< 8)&0xff00)));

But if you pass "null" to toString() you get some null pointer exception...so I would assume that this line never gets hit and so you get the "null" value as you dont initialize paramDriveMod with some other value.

Upvotes: 0

Jon Skeet
Jon Skeet

Reputation: 1500675

Could I suggest that to pass data between classes, you use separate objects instead of a global variable?

It's not at all clear how you expect the code in protocolImpl to get executed - as templatetypedef mentions, you haven't shown valid Java code in either that or the param class (neither of which follows Java naming conventions).

A short but complete example would really help, but in general I would suggest you avoid using this pattern in the first place. Think in terms of objects, not global variables.

Upvotes: 1

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