object 的类型，同时实例化实现接口的 class

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?

Answer 1

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");
    }
}