Question

can someone explain me the difference between the 2 programs, basically the difference between generic and object.
also why
System.out.println(“adding 1 to the set: ” + vs.add(new String(“Hello”)));
works only for the first program

First Vector set program

import java.util.Vector;

class VectorSet1 {
boolean add(Object obj) {
        if (contains(obj)) return false;
    v.add(obj);
    return true;
}

boolean contains(Object obj) {return v.contains(obj);}

public String toString() {return v.toString();}

void clear() {v.clear();}

int size() {return v.size();}

boolean isEmpty() {return v.isEmpty();}

Vector v = new Vector();

public static void main(String [] args) {
    VectorSet1 vs = new VectorSet1();

    System.out.println("set: " + vs);

    System.out.println("adding 1 to the set: " + vs.add(1));
    System.out.println("adding 5 to the set: " + vs.add(5));
    System.out.println("adding 17 to the set: " + vs.add(17));
    System.out.println("adding 1 to the set: " + vs.add(1));
    //System.out.println("adding 1 to the set: " + vs.add(new   String("Hello")));

    System.out.println("set: " + vs);

    System.out.println("testing if 1 s in the set: " + vs.contains(1));
    System.out.println("testing if 17 is in the set: " + vs.contains(17));
    System.out.println("testing if 6 is in the set: " + vs.contains(6));

    System.out.println("set is empty: " + vs.isEmpty());
    System.out.println("size of set: " + vs.size());


    vs.clear();

    System.out.println("after invoking clear");

    System.out.println("set: " + vs);

    System.out.println("set is empty: " + vs.isEmpty());
    System.out.println("size of set: " + vs.size());
}
}

Second Vector set program

import java.util.Vector;
import java.util.Iterator;

class VectorSet2<E> {
boolean add(E e) {
        if (contains(e)) return false;
    v.add(e);
    return true;
}

boolean contains(E e) {return v.contains(e);}

public String toString() {return v.toString();}

void clear() {v.clear();}

int size() {return v.size();}

boolean isEmpty() {return v.isEmpty();}

Vector<E> v = new Vector<E>();

public static void main(String [] args) {
    VectorSet2<Integer> vs = new VectorSet2<Integer>();

    System.out.println("set: " + vs);

    System.out.println("adding 1 to the set: " + vs.add(1));
    System.out.println("adding 5 to the set: " + vs.add(5));
    System.out.println("adding 17 to the set: " + vs.add(17));
    System.out.println("adding 1 to the set: " + vs.add(1));
    //System.out.println("adding Hello to the set: " + vs.add("Hello"));

    System.out.println("set: " + vs);

    System.out.println("testing if 1 s in the set: " + vs.contains(1));
    System.out.println("testing if 17 is in the set: " + vs.contains(17));
    System.out.println("testing if 6 is in the set: " + vs.contains(6));

    System.out.println("set is empty: " + vs.isEmpty());
    System.out.println("size of set: " + vs.size());


    vs.clear();

    System.out.println("after invoking clear");

    System.out.println("set: " + vs);

    System.out.println("set is empty: " + vs.isEmpty());
    System.out.println("size of set: " + vs.size());
}
}

Answer 1

When you create a VectorSet2, you specify a type for its elements. In the case of the second program, that type is Integer. A String is not an Integer, so you can’t add a String to a VectorSet2.

A VectorSet1, though, can have elements of any type (anything that can be treated as an Object). So if you have a VectorSet1, you can add any object to it, whether it’s a String, Integer, or any other kind of object.