can the following code produces Null pointer exception?‏

[khan gul]

Hi,
I have a question on the following code. Is it possible that the following code can throw Null Pointer Exception?

public class Test {
    private String name;
    private String city;
    public String getCity() {
        return city;
    }
    public Test(String name, String city){
        this.name = name;
        this.city = city;
    }
    public void setCity(String city) {
        this.city = city;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }

}

This Test class is been added to ByValue class as a static variable.

public class ByValue {

    private String myValue;
    [b]private static Test test;[/b]
   
    public ByValue(){
       
    }
    public String getMyValue() {
        return myValue;
    }
    public void setMyValue(String myValue) {
        this.myValue = myValue;
    }
    public ByValue(String str){
        myValue = str;
    }
    public  void foo(ByValue d) {
        d = new ByValue("Fifi");
    }
   

    public static Test getTest() {
        return test;
    }
    public static void setTest(Test test) {
        ByValue.test = test;
    }

}

// a thread to initialize ByValue and Test being set

public class Thread3 extends Thread{
    private String name;
    public Thread3(String name){
    this.name = name;
    }
    public void run(){
       
        try {
            Class<?> theClass = Class.forName("com.test.ByValue");
            ByValue req = (ByValue) theClass.newInstance();
            Test test = new Test("arslan111","toronto");
            req.setTest(test);
            System.out.println(req.getTest().getName());
        } catch (Exception e) {
            e.getClass();
        }       
       
    }

// a thread to initialize ByValue and Test being set

public class Thread4 extends Thread{
    private String name;
    public Thread4(String name){
    this.name = name;
    }
    public void run(){
       
        try {
            Class<?> theClass = Class.forName("com.test.ByValue");
            ByValue byValue= (ByValue) theClass.newInstance();
            Test test = new Test("abcd","xyz");
            req.setTest(test);
            System.out.println(byValue.getTest().getName());
        } catch (Exception e) {
            e.getClass();
        }       
       
    }

}

}

Since Test class is being added dynamically to ByValue class and Test is a static field in the ByValue, is it possible that when one Thread RESET this static Test field the other thread sees a NULL value?

From Time line point of view, say Thread T3 run() first and instantiate ByValue first and then set Test object to static Test field. But before calling byValue.getTest(), Thread T4 run() and reset a new Test object in the ByValue. Question I have is it possible that T3 can get Null Pointer Exception when it calls byValue.getTest().getName();

Thanks,

[Edit - added code tags - MB]

[Jeff Verdegan]

Please edit your post and UseCodeTags.

[khan gul]

I checked the code and it has [code:java] tag. Is it fine now?

[Jeff Verdegan]

No. The static Test variable cannot go back to null. It is null when the class is first initialized. Then you set it to some non-null value. It will never be null again unless you explicitly set it to null, which, as far as I can see, you're not doing.

[Mike Simmons]

Agreed. The code is not thread-safe and it's difficult to predict exactly what it will do. But getting a NullPointerException from the ByValue.test field does not appear to be possible.

By the way, you're making it very difficult to debug any problems, if they occur:

        } catch (Exception e) {  
            e.getClass();  
        }  

Calling getClass() has no observable effect. Nothing is printed. You're effectively hiding any exception, if it occurs, such that you will have no idea what the problem is. I strongly suggest printing the stack trace, or at least the toString() value of the exception. There's very useful information in there, which you are currently cut off from.

[Nomaan Butt]

Since Test class is being added dynamically to ByValue class and Test is a static field in the ByValue, is it possible that when one Thread RESET this static Test field the other thread sees a NULL value?

dynamic initiation has nothing to do with your query.