Why do this() and super() have to be the first statement in a constructor?

Question

Java requires that if you call this() or super() in a constructor, it must be the first statement. Why?

For example:

public class MyClass {
    publi         


        

Answer 1

I totally agree, the restrictions are too strong. Using a static helper method (as Tom Hawtin - tackline suggested) or shoving all "pre-super() computations" into a single expression in the parameter is not always possible, e.g.:

class Sup {
    public Sup(final int x_) { 
        //cheap constructor 
    }
    public Sup(final Sup sup_) { 
        //expensive copy constructor 
    }
}

class Sub extends Sup {
    private int x;
    public Sub(final Sub aSub) {
        /* for aSub with aSub.x == 0, 
         * the expensive copy constructor is unnecessary:
         */

         /* if (aSub.x == 0) { 
          *    super(0);
          * } else {
          *    super(aSub);
          * } 
          * above gives error since if-construct before super() is not allowed.
          */

        /* super((aSub.x == 0) ? 0 : aSub); 
         * above gives error since the ?-operator's type is Object
         */

        super(aSub); // much slower :(  

        // further initialization of aSub
    }
}

Using an "object not yet constructed" exception, as Carson Myers suggested, would help, but checking this during each object construction would slow down execution. I would favor a Java compiler that makes a better differentiation (instead of inconsequently forbidding an if-statement but allowing the ?-operator within the parameter), even if this complicates the language spec.