使用接口隐藏实施细节

Question

I have a project with 1 interface and 2 classes:-

public interface Account {
    int add();
}

public class AccountImpl implements Account{
    @Override
    public int add() {
         return 0;
    }
}

and 1 class with main method

public class Testing {
    Account account;

    public static void main(String[] args) {
        Testing t = new Testing();
        t.call();
    }

    public void call() {
        int a = account.add();
    }
}

I am getting Null pointer exception in the line int a = account.add(); as account value is null.

I am new to java, can you please help to remove this?

Answer 1

When call is being called in the main function, the private variable account is not initialized. That means that you never gave it a value; it's not pointing to an object (it's a “null pointer” pointing to nothing). As such, you cannot call a method of that object.

To fix this, you will need to initialize the variable first. For example in the constructor of your Testing class:

public Testing () {
    account = new AccountImpl();
}

Answer 2

You haven't instantiated an instance of AccountImpl to call; the exception you are getting could commonly be referred to as 'you didn't make one of those yet'.

public class Testing {
     Account account;
     public static void main(String[] args) {
        Testing t = new Testing();
        t.call();
     }

     public void call() {
         account = new AccountImpl();
         int a = account.add();
     }
}

Answer 3

You have not initialized the Account. You would better do something like this.

Account account = new AccountImpl();

at the very first line of the Test class.