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Prevent anonymously subclassing

+3
−0

Is it possible to somehow prohibit anonymously subclassing of a specific class? For instance, with a plain public parent class:

public class Parent {

}

Extending this class should not be possible with an unnamed, anonymous subclass:

var myClass = new Parent() { ... }

However, all other forms of subclassing or usage should work, such as:

public class Child extends Parent {
    public static Object feelingLucky(boolean really) { 
        return really ? new Child() : new Parent();
    }
}
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Sample use case? (3 comments)

1 answer

+5
−0

There is no keyword or compile-time construct to prevent this, but it can be accomplished through the use of a runtime exception:

class Parent {
    
    public Parent() {
        if (getClass().isAnonymousClass()) {
            throw new IllegalStateException("You violated my secret rule!");
        }
    }
}

For actual use in production code, one should probably reconsider the reasons for which one would like to do this. Limiting the creation of anonymous classes is a possibility, but generally a bad idea.

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