Accessing implemented class methods through interface reference without constructor initialization

Question

I have a class CommonDaoImpl that implements an interface CommonDao. Now i am trying to access the getRegisterData() of CommonDaoImpl through interface CommonDao reference like this

public class CommonServiceImpl implements CommonService 
{
    CommonDao commonDao
    public boolean insertRegisterData(CommonBean objCommonBean)  {
        return commonDao.getRegisterData(objCommonBean);
    }

but it is not working and thow an NullPointerException

So i slightly change my code and initialize interface reference with the constructor of implemented class CommonDao impl like this

public class CommonServiceImpl implements CommonService 
{
    CommonDao commonDao=new CommonDaoImpl();

    public boolean getRegisterData(CommonBean objCommonBean)  {
        return commonDao.insertRegisterData(objCommonBean);
    }

But i could not understand why it happens.

Jigar Joshi · Accepted Answer · 2012-07-02 14:30:28Z

3

In first case

CommonDao commonDao

it is a member field of your class, which is un initilized which has default value null so invoking method on null results into NullPointerException

while in your second case you are initializing it

CommonDao commonDao=new CommonDaoImpl();

answered Jul 2, 2012 at 14:30

Jigar Joshi

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Comments

Vladimir Ivanov · Accepted Answer · 2012-07-02 14:31:50Z

1

Why happens what? NullPointerException? It happens, because once you get CommonServiceImpl instantiated, all the fields receives default values. commonDao is a field which default value is null.

answered Jul 2, 2012 at 14:31

Vladimir Ivanov

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Comments

Heiko Schmitz · Accepted Answer · 2012-07-02 14:39:15Z

0

It's because you need an instance of CommonDao to call the insertRegisterDatamethod. The statement commonDao=new CommonDaoImpl() creates an instance calling the constructor and assigns it to commonDao, so it's not null any more.

answered Jul 2, 2012 at 14:39

Heiko Schmitz

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Comments

Chris Okyen · Accepted Answer · 2012-11-17 23:31:20Z

First commonDao is a reference to an implemented class

Sicne commonDao is an interface, it does not implement any methods

CommonDao commonDao

commonDao does not referencing an initialized object that implements the interface, so when you call commonDao.insertRegisterData(objCommonBean); comonDao is Null as no memory address is assigned to it currently and it throws a NullException ( See Diagram )

CommonDao commonDao=new CommonDaoImpl(); < --- This initializes right here

Now commonDao is referencing an initalized object that implements the interface. So when you call commonDao.insertRegisterData(objCommonBean), comonDao interface. commonDao has an address assigned to it which is of an initalized class. So now there's an existing object to reference that has the method.

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