CODEWITHSUNDEEP
javaoopinheritancemultiple-inheritance
ju ho
ju ho

Reputation: 328

How to enforce constructor in Java

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?

Upvotes: 1

Views: 487

Answers (3)

Keith
Keith

Reputation: 3151

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

Upvotes: -1

OldCurmudgeon
OldCurmudgeon

Reputation: 65813

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) {
    }

}

Upvotes: 4

Joop Eggen
Joop Eggen

Reputation: 109547

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).

Upvotes: 3

Related Questions