问为什么this（）和super（）必须是构造函数中的第一个语句？

public class MyClass {

    public MyClass(int x) {}

}

public class MySubClass extends MyClass {

    public MySubClass(int a, int b) {

        int c = a + b;

        super(c);  // COMPILE ERROR

    }

}

public class MySubClass extends MyClass {

    public MySubClass(int a, int b) {

        super(a + b);  // OK

    }

}

public class MyClass {

    public MyClass(List list) {}

}

public class MySubClassA extends MyClass {

    public MySubClassA(Object item) {

        // Create a list that contains the item, and pass the list to supe

        List list = new ArrayList();

        list.add(item);

        super(list);  // COMPILE ERROR

    }

}

public class MySubClassB extends MyClass {

    public MySubClassB(Object item) {

        // Create a list that contains the item, and pass the list to supe

        super(Arrays.asList(new Object[] { item }));  // OK

    }

}