Question

I have come across some java classes used in the spring framework. First, there is the beans in the applicationContext.xml

<bean id="someBean" parent="txProxyTemplate">
    <property name="target">
        <bean class="path.to.bean.impl.SomeBeanImpl">
            ...
        </bean>
...
</bean>

And I have the interface ISomeBean, and its implementation SomeBeanImpl
Then, I have another class which uses ISomeBean.

public class SomeOtherClass {
  ...
    public function doStuff() {
        ...
        ApplicationContext ctx;
        SomeBean theBean = (SomeBean) ctx.getBean; 
   }
}

I want to know why do we cast to an interface instead of casting to the class.

Answer 1

Why would you want to tie SomeOtherClass to a specific implementation? Using the interface, you get loose coupling – you can test against a fake implementation, or switch to a different implementation later.

This is a large part of the benefit of inversion of control – you aren’t as tightly coupled to your dependencies as if you instantiate them directly within the class.