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class Foo {
  private Helper helper;

  public Helper getHelper() {
    return helper;
  }

  public void initializesetHelper(int num) {
    helper = new Helper(num);
  }
}

...

Because the Helper class is immutable, it cannot be changed after it is initialized setHelperd and is therefore thread-safe.

However, because the helper field of class Foo is not properly synchronized, it is possible that the Foo.getHelper() method will return a reference to a partially or incorrectly initialized setHelperd helper object, if invoked from a separate thread.

...

This compliant solution synchronizes the methods of class Foo to ensure that no thread sees a partially initialized setHelperd helper field.

class Foo {
  private Helper helper;

  public synchronized Helper getHelper() {
    return helper;
  }

  public synchronized void initializesetHelper(int num) {
    helper = new Helper(num);
  }
}

...

class Foo {
  private volatile Helper helper;

  public Helper getHelper() {
    return helper;
  }

  public void initializesetHelper(int num) {
    helper = new Helper(num);
  }
}

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