Broken multithreaded version. OK, why don't we just make the Helper field volatile?

DoubleFoo.java

class DoubleFoo {
    private volatile Helper d_helper;
    public Helper getHelper() {
        Helper result = d_helper;
        if (result == null) {
            synchronized(this) {
                result = d_helper;
                if (result == null) {
                    helper = result = new Helper();
                }
            }
        }
        return result;
    }

    // other functions and members...
}

Foo.java

// Broken multithreaded version
// "Double-Checked Locking" idiom
class Foo {
    private Helper helper;
    public Helper getHelper() {
        if (helper == null) {
            synchronized(this) {
                if (helper == null) {
                    helper = new Helper();
                }
            }
        }
        return helper;
    }

    // other functions and members...
}

Because we don't want to enter the critical section if helper has been initialized