Constructor exception

Is it possible to create a class which throws an Exception in the constructor. Yes.

The following shows one example.

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public class Foo {

public Foo() {

throw new RuntimeException();

}

public void calculate() {

System.out.println("I am now calculating");

}

}

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The class above will always throw a run time exception whenever someone tries to instantiate it. Is there a way round it and call the calculate method? Yes there is. The class below actually succeeds. The solution is that Foo is not a final class. I will sub-class it and operate on that instance.

public class Bar extends Foo {

public static Foo foo;

@Override

protected void finalize() throws Throwable {

foo = this;

}

}

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Create a runner which will do the job.

public class Runner {

public static void main(String...args) {

try {

Foo foo = new Bar();

} catch (Exception e) {

System.out.println("Just to show exception caught " + e.getMessage());

}

System.gc();

System.runFinalization();

System.out.println("My instance " + Bar.foo);

Bar.foo.caluclate());

}

}

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This example shows that you should always use the final word when you do not want people to be able to create incomplete instances.