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 am fairly sure (those familiar with the Java Specification chime in) that it is to prevent you from (a) being allowed to use a partially-constructed object, and (b), forcing the parent class's constructor to construct on a "fresh" object.

Some examples of a "bad" thing would be:

class Thing
{
    final int x;
    Thing(int x) { this.x = x; }
}

class Bad1 extends Thing
{
    final int z;
    Bad1(int x, int y)
    {
        this.z = this.x + this.y; // WHOOPS! x hasn't been set yet
        super(x);
    }        
}

class Bad2 extends Thing
{
    final int y;
    Bad2(int x, int y)
    {
        this.x = 33;
        this.y = y; 
        super(x); // WHOOPS! x is supposed to be final
    }        
}