NullPointerException error when using ModelDriven

Question

I have two classes as following

Address
  int ID
  int unit
  String street 

User 
    int ID 
    String Name
    Address address

my user class

public class user{
    .....
    private Address address;
     ...getters setters...
}

As shown above User class has an object of Address class in it. My code is expected to receive the values of a form and show them on console.

But when I try to access the unit attribute of address class it shows the following error.

"SEVERE: Exception occurred during processing request: null java.lang.NullPointerException"

my controller

@Action
class myclass implements ModelDriven{

   private User user = new User();

   public Register(){
       System.out.println("User's Unit" + user.getAddress().getUnit()); // error is on this line
   }
     @Override
    public Object getModel() {
       return user;
    }

jsp file

<s:form action="Register">
             <s:textfield name="name" label="Name"/>
             <s:textfield name="unit" label="Unit"/>
             <s:textfield name="block" label="Block"/>
</s:form>

Answer 1

do the following mate

<s:textfield name="address.unit" label="Unit"/>

Answer 2

try this action class this will definitly work

public class myclass extends ActionSupport implements ModelDriven<User>, Preparable{

    private User user = new User();

@Override
public void prepare() throws Exception {
    user=new User();
}

@Override
public User getModel() {
    return user;
}
public Register(){
   System.out.println("User's Unit" + user.getAddress().getUnit()); 
   }
}

and change the jsp

<s:textfield name="address.unit" label="Unit"/>

for setting only unit property

Answer 3

 <s:textfield name="name" label="Name"/>

Here struts2 will search for name="name" property in the related DTO's in your respective action, but value-stack will failed to find the property named "name" in your action, because you have created a different bean class Address which is having member "name". So you have to tell your form-element that "name" instance resides in different POJO i.e. Adderess . So make changes like this:

<s:textfield name="address.name" label="name"/>

Answer 4

the problem as I see it is, you've not newd address object

address = new Address(); in the constructor of User class.

Answer 5

Looks like when you created the user object using "private User user = new User()" the address object of the user is not initialized. As user.getAddress() is null, when you try to invoke user.getAddress().getUnit() it will result in null pointer exception