How to enforce constructor in Java

Question

Is there any way to enforce a specific constructor in Java classes?

For example I want all classes that inherit from a Class to have a constructor like -

public classname(string s1,string s2){....}

I know it should not be avoided because it can lead to problems with multiple inheritance. But is there a way to do it anyway?

"_I know it should not be done_" They why do it? Why on earth would you practice doing things the wrong way? — takendarkk, Aug 19 '15 at 12:40
Constructors cannot be overridden. It is to create instance of class. — Nitesh Virani, Aug 19 '15 at 12:41
possible duplicate of [How do you force constructor signatures and static methods?](http://stackoverflow.com/questions/161231/how-do-you-force-constructor-signatures-and-static-methods) — Alex Salauyou, Aug 19 '15 at 12:41
_"I know it should not be avoided because it can lead to problems with multiple inheritance."_ Well, java doesn't support multiple inheritance. — BackSlash, Aug 19 '15 at 12:41
@BackSlash Not exactly. It supports multiple inheritance of contract, not of implementation. — RealSkeptic, Aug 19 '15 at 12:42
if you have a custom constructor and no no-arg constructor in your super class your subclasses must implement at least a super() call to it as the implicit super() call would lead to a compiler error. Its really no good practise though. — dly, Aug 19 '15 at 12:45
I need it because I load classes at runtime with the reflection API and I want all the classes I load to have a specific constructor. All the classes I load inherit from another class so i was wondering if I can just enforce it. — ju ho, Aug 19 '15 at 12:46
@juho all you can do here is to check presence of constructor with required signature when loading these classes. — Alex Salauyou, Aug 19 '15 at 12:54
@juho Ok, got it. Maybe the factory design pattern can be helpful for you? This may help getting rid of the reflection API too. — Lyubomyr Shaydariv, Aug 19 '15 at 13:21

score 4 · Accepted Answer · answered Aug 19 '15 at 12:57

There are no facilities in Java to do that directly.

However, it can be enforced to some extent usin an abstract method.

abstract class Base {

    Base(String s1, String s2) {
        init(s1, s2);
    }

    protected abstract void init(String s1, String s2);
}

class MyClass extends Base {

    // Forced to do this.
    MyClass() {
        super("One", "Two");
    }

    // Forced to do this.
    @Override
    protected void init(String s1, String s2) {
    }

}

score 3 · Answer 2 · answered Aug 19 '15 at 12:56

You want that there is only one constructor, and that with the same signature. That could in a costly way done with reflection, at run-time.

public BaseClass(String s, String t, int n) {
    Class<?> cl = getClass();
    do {
        check(cl);
        cl = cl.getSuperclass();
    } while (cl != BaseClass.class);
}

private void check(Class<?> cl) {
    if (cl.getConstructors().length != 1) {
        throw new IllegalStateException("Needs only 1 constructor in: " + cl.getName());
    }
    try {
        cl.getConstructor(String.class, String.class, int.class);
    } catch (NoSuchMethodException e) {
        throw new IllegalStateException("Constructor should have parameter types (String, String, int) in: " + cl.getName());
    }
}

Not advisable

However you could make a factory to be used that hides class hierarchies. Or in fact use a single class that delegates to your class hierarchy (has a member of your class).

@SashaSalauyou you are right, however the check must be done in non-static code, here the constructor. However one could make a `static Set>` of already checked classes. — Joop Eggen, Aug 19 '15 at 13:15

Keith · Answer 3 · 2015-08-19T13:20:17.517

-1

Sorry, but no, you cannot force classes to only implement a specific constructor.

edited Aug 19 '15 at 13:20

answered Aug 19 '15 at 12:52

Keith

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Yes you can. Make an abstract class with a protected constructor. Everything which is an implementation of that class must call the constructor – Mark Aug 19 '15 at 13:18
2

@Mark must call, but must not have the same constructor. – Alex Salauyou Aug 19 '15 at 13:20
Oops -- I missed an operative word "... to *only* implement..." in my response response. Thanks for catching that. :) – Keith Aug 19 '15 at 13:20
@SashaSalauyou I get what he's saying. We're probably debating semantics of my poorly worded answer Yes, you must define an explicit constructor, at least by calling super(...) in the child class – Keith Aug 19 '15 at 13:23
That's what I meant. With the abstract method you must define a constructor but indeed your implementing class could implement its own however it likes. – Mark Aug 19 '15 at 15:25

How to enforce constructor in Java

3 Answers3