Interface as a type in Java?

Question

From The Java Tutorials:

In Java, a class can inherit from only one class but it can implement more than one interface. Therefore, objects can have multiple types: the type of their own class and the types of all the interfaces that they implement. This means that if a variable is declared to be the type of an interface, its value can reference any object that is instantiated from any class that implements the interface.

Can anyone provide me a basic pseudo type for this. I did not understand the bold lines.

Answer 1

Let's declare two interfaces and a class that implements them both:

interface I1 { }  interface I2 { }  class C implements I1, I2 { } 

objects can have multiple types

In the following code, it can be seen that a C instance has the type of C as well as I1 and I2:

C c = new C();  boolean isC = (c instanceof C);   //true boolean isI1 = (c instanceof I1); //true boolean isI2 = (c instanceof I2); //true 

Now let's declare a class B which implements I1 as well:

class B implements I1 { } 

if a variable is declared to be the type of an interface, its value can reference any object that is instantiated from any class that implements the interface.

If we declare a variable of type I1, we can set it to an instance of C, and then reassign it to an instance of B:

I1 i1 = new C(); i1 = new B(); 

We can also reassign it to an instance of D, where D extends C:

i1 = new D();  ...  class D extends C { } 

Answer 2

Consider the following example:

Serializable s = new ArrayList(); 

In Java, this is valid code, even though Serializable is an interface, because ArrayList implements Serializable. So in this case, we're treating s as a variable of type Serializable.

Now suppose we follow up the above code with the following:

s = "String object"; 

This is also valid becauseString also implements Serializable. Since we declared s as type Serializable, it can point to any object that implements that interface.