Type of object while instantiating a class that implements interface

Question

Let's say I have a class named TestClass that implements interface named TestInterface. What is the difference in creating following objects:

TestInterface test1 = new TestClass();
TestClass test2 = new TestClass();

If there is no differences, which one is the better convention?

using test1 object, you can only access to the methods declared in the TestInterface whereas using test2, you can access to all the methods and proprrties defined in TestClass. — Md Shahbaz Ahmad, Jul 13 '20 at 15:29
[This](https://stackoverflow.com/questions/383947/what-does-it-mean-to-program-to-an-interface) might help. — Andrew S, Jul 13 '20 at 15:31
I would recommend reading about "coding To Interfaces".. A simple example : https://medium.com/javarevisited/oop-good-practices-coding-to-the-interface-baea84fd60d3 — Rahul Singh, Jul 13 '20 at 15:34

score 0 · Answer 1 · answered Jul 13 '20 at 15:46

interface TestInterface {
    public void function2();
}

public class TestClass implements TestInterface {
    public static void main(String... args) {
        TestInterface testInterface = new TestClass();
        TestClass testClass = new TestClass();

        /* function1 belongs to TestClass class only */
        testInterface.function1(); // This gives you compile time error as function1 doesn't belong to TestInterface
        testClass.function1(); // Whereas this is ok.

        /* function2 belongs to TestInterface and TestClass, so function2 can be called from both object (testClass and testInterface) */
        testInterface.function2(); 
        testClass.function2();
    }
    public void function1() {
        System.out.println("This is function 1");
    }
    public void function2() {
        System.out.println("This is function 2");
    }
}

Type of object while instantiating a class that implements interface

1 Answers1