Using null to replace values

Question

I'm a newbie! Can you tell me how can replace existing values in a variable. My plot in the program is to have the price of products in my right hand then transfer that to the cart and then to cashier lady. I have to make sure and know that when I transfer the price from my right hand to the value cart the right hand should no value, then when from cart to the cashier the cart should have no value. The code below is not yet finish. I'm just testing it on one product. I just to know the transferring concept. Can I use null?
This my are codes:
the customer class
public class customer{
     String fullName,walletName;
     double wallet,money;
     public void sayName(){
          System.out.println(fullName + " is inside the grocery");
     public String giveFullName(){
          return fullName;
     public void sayWalletBrand(){
          System.out.println("Let me see how much money I got from my " + walletName + " wallet\n");
     public void contentWallet(double money){
          wallet = money;
     public void lookWallet(){
          System.out.println("Show that wallet has the value " + wallet);
          System.out.println();
the product class:
public class product{
     String description;
     double price,rightHand;
     public double containRightHand(){
          rightHand = price;
          System.out.println(rightHand);
          System.out.println();
          return rightHand;
     public void showDescription(){
          System.out.println("Show product description " + description);
     public void showPrice(){
          System.out.println("Show the product price value " + price);
          System.out.println();
the cart class:
public class cart extends product{
     double contents[] = new double[2];
     product doritos,apple;
     //Movement method from rightHand to cart
     public double getProduct1(product doritos){
          contents[0] = doritos.containRightHand();
          return contents[0];
     public double getProduct2(product apple){
          contents[1] = apple.containRightHand();
          return contents[1];
     public void showProduct1(){
          System.out.println("Show that contents[0] has the value "+ contents[0]);
     public void showProduct2(){
          System.out.println("Show that contents[1] has the value "+ contents[1]);
the cashier class:
public class cashier{
     customer cashierLady;
     public void tellNameCashier(customer cashierLady){
          System.out.println("Shows the name of the cashier lady named " + cashierLady.giveFullName());
the main:
public class grocery{
     public static void main(String []arg){
          customer Peter, Lady;
          Peter = new customer();
          Peter.fullName = "Raymond Malicdem";
          Peter.walletName = "Seiko";
          Peter.wallet = 1000.0;
          Peter.sayName();
          Peter.sayWalletBrand();
          Peter.contentWallet(1000.0);
          Peter.lookWallet();
          product Doritos = new product();
          Doritos.description = "Junk Food";
          Doritos.price = 60.0;
          Doritos.showDescription();
          Doritos.showPrice();
          Doritos.containRightHand();
          cart product1 = new cart();
          product1.getProduct1(Doritos);
          product1.showProduct1();
          Lady = new customer();
          Lady.fullName = "Christine";
          cashier lady = new cashier();
          lady.tellNameCashier(Lady);

Answer 1

Variables of object (class) types may be set to null, meaning they point to no object.
Variables of fundamental types (int, byte, char, long, double, float, boolean, etc.) can not be set to null. Maybe you can set your numeric types to 0 and boolean to false, to indicate nothingness.