Java - 多线程

Question

I got this question in an interview, can some one clarify it..

**/**
 * Assuming a threaded environment, and without knowing anything else,
 * what is the potential problem with myMethod()?
 * Fix it in the simplest way.
 */
*/
public class ClumsyTest {
    private static final String FUBAR = "fubar";

    public boolean myMethod(final MyObject bar) {
        if (bar.getFoo() != null) {
            return bar.getFoo().equals(FUBAR);
        } else {
            return false;
        }
    }

    public interface MyObject {
        String getFoo();
        void setFoo(String o);
    }
}**

in 'myMethod', bar.getFoo() was verified twice. In multi thread environment, it is very well possible that value can change. So, we need to verify it only once, and if we write as below,

 **if (bar.getFoo().equals(FUBAR)) {
        return true;
    } else {
        return false;
    }**

it will throw null pointer exception if bar.getFoo() is null.

How can we fix this method to check only once and by avoiding null pointer exception

Answer 1

It is quite simple actually.

Rather than doing bar.getFoo().equals(FUBAR) which risks NullPointerException , you gotta check it other way around as FUBAR.equals(bar.getFoo()) . And that's all there is to it.

So simply

    public boolean myMethod(final MyObject bar) {
        
        return FUBAR.equals(bar.getFoo());
        
    }

Answer 2

There's nothing wrong with the accepted answer. However, it works only for this specific case where equals() is used. Had any other method of bar.getFoo() been used, we would not have been able to avoid the possible NullPointerException in the same way.

The more generic answer is to store the result in a local variable.

public boolean myMethod(final MyObject bar) {
    
    Object temp = bar.getFoo(); // temp can obviously be a more specific type
    if (temp != null) {
        return temp.toString().equals(FUBAR);
    } else {
        return false;
    }
}