Constructors

1 You cannot invoke constructors directly except from within other constructors.

2  Constructors invoking each other - by either super( ) or this(...) -  must be the first statement in the constructor code body.  Explicitly calling super( ) is entirely optional.  The JVM will do it anyway.



3  You will not get a parameter-less default constructor automatically created for you if you create another constructor with parameters. Doing so gives a compile error if anyone invokes the (now-nonexistant) default constructor via a new. i.e.

class Test {
        public Test(int i ) {   }
}

class Two extends Test {                                             // simply extending Test will trigger the error
        public static void main(String[] args) {
        Test T = new Test( );  }                                          // any explicit new will trigger the error
}

4  You cannot return any values from a constructor. Adding a return type to what looks like a constructor just makes it a legal method. i.e.

class Test {
int i;
        public int Test( return i; ) {   }              // Test is now just another legal method
}

class Two extends Test {                                          
        public static void main(String[] args) {
        Test T = new Test( );  }                                      
}

5  You cannot mark constructors void or static.  They can only be public, private, or protected.

6  You cannot invoke your own same constructor again, from within the constructor, or the compiler detects a circular reference. i.e.

         this( yoursameparmshere );    won't work inside the same constructor.
You can call another constructor using this with    this(differentparms);