How to restrict calling of methods by clients?

Question

I have below class with few public methods.

public class Sample{

  public void method1(){ }

  public void method2(){ }

  public void method3(){ }

  public void method4(){ }
}

I need to expose only first three methods to clients and not method4(). How can I create one more class exposing only the first 3 methods?

possible duplicate of http://stackoverflow.com/questions/215497/in-java-whats-the-difference-between-public-default-protected-and-private — Matteo Gatto, Jan 29 '14 at 10:43
I cannot change to private. Because when i instantiate the same class, i should be able to call the method4. — user755806, Jan 29 '14 at 10:43
Do you want to create another class, e.g. `RestrictedSample`, that only offers access to methods 1-3? — Duncan Jones, Jan 29 '14 at 10:52

score 2 · Answer 1 · answered Jan 29 '14 at 10:56

2

create an interface which expose only 3 methods:

public interface ISample{
  public void method1();
  public void method2();
  public void method3();
}

Let the class implement it:

public class Sample implement ISample {
 // all method definitions are not changed
}

When you need full access, use type Sample, when restricted, then ISample:

Sample sample=new Sample();
sample.method4();
passToUser((ISample)sample);

answered Jan 29 '14 at 10:56

Alexei Kaigorodov

13,189
1
21
38

I'm guessing `passToUser` takes `ISample` instances? If so, the cast is unnecessary. Note also that the `I` prefix is very common in .NET, but generally not used in Java. – Duncan Jones Jan 29 '14 at 10:58
This approach is also flawed, because the code receiving the `ISample` instance can cast it to `Sample` and access the hidden method. – Duncan Jones Jan 29 '14 at 11:07
@Duncan malicious client can read RestrictedSample.sample from your variant using reflection, Unsafe, or JNI. We don't know required defense level - maybe my variant is OK, maybe your variant is insecure. – Alexei Kaigorodov Jan 29 '14 at 11:21
That's a fair point. Perhaps I was too strong with my wording (r.e 'flawed'). However, I think it's worth noting that a simple cast overcomes this approach. – Duncan Jones Jan 29 '14 at 11:23

Duncan Jones · Accepted Answer · 2014-01-29T11:03:54.147

1

To make a public class that offers methods 1-3 only, wrap Sample in another class like this:

public class RestrictedSample {

  private Sample sample = new Sample();

  public void method1(){ sample.method1(); }

  public void method2(){ sample.method2(); }

  public void method3(){ sample.method3(); }
}

This is known as the façade pattern, or simply delegation.

Be warned: as long as Sample remains public, other code can construct instances of that class. That may not be a problem for you, but it's worth remembering.

edited Jan 29 '14 at 11:03

answered Jan 29 '14 at 10:55

Duncan Jones

67,400
29
193
254

Duncan, the idea is, through RestrictedSample class clients can set the data into sample object. Now how can i get the same sample object ? If i provide getter method then clients can access the sample object which i dont want to do. – user755806 Jan 29 '14 at 10:58
@user755806 Provide setter methods in `RestrictedSample` that adjust the inner `sample` instance. – Duncan Jones Jan 29 '14 at 10:59
then how can i get the data which is in sample instance? – user755806 Jan 29 '14 at 11:00
@user755806 Provide getter methods! – Duncan Jones Jan 29 '14 at 11:02

score 0 · Answer 3 · answered Jan 29 '14 at 10:53

Better control over scope of visibility is one of the major things OSGi adds to Java. If this is a serious project, you may want to consider investing in learning OSGi.

If this is just something you're doing for yourself, or for class, you may want to consider something weaker but simpler such as marking the method deprecated or just renaming it to INTERNAL_USE_ONLY_method4.

Or leave the method as "package" access.

How to restrict calling of methods by clients?

3 Answers3