Interface can never instantiate in java,But What happened,when Interface is used return type

Question

We know,interface can never instantiate in java. We can, however, refer to an object that implements an interface by the type of the interface

public interface A
{
}
public class B implements A
{
}

public static void main(String[] args)
{
    A test = new B();  //upcating
    //A test = new A(); // wont compile
}

But I have become confused when interface is used return type,such as

Method of DriverManager class which return Connection object
public static Connection getConnection(String url);

Connection con=DriverManager.getConnection(String url);

Same problem

Method of Connection interface which return Statement object
public Statement createStatement();

Statement stat=con.createStatement();

I cannot understand,what happened when interface is used return type. Please help me by explain.

Thanks

You seem to understand that you can upcast, so what do you have trouble about in the example `public A getA() { return new B(); }` — clcto, Jan 20 '15 at 18:10
@clctoThanks a lot,I am beginner in java,I can understand clearly.before i think how is it possible. — Sakib Hasan, Jan 20 '15 at 18:15
" interface is used return type." it mean a reference to an object which implements that interface is returned (or null) — Peter Lawrey, Jan 20 '15 at 18:21

score 2 · Accepted Answer · answered Jan 20 '15 at 18:10

2

When an interface is used as the return type of a method, what is actually being returned is an instance of a class that implements that interface.

answered Jan 20 '15 at 18:10

Eran

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Niels Billen · Answer 2 · 2015-01-20T18:18:00.760

The returned object is than a subclass which implements said interface.

Test classes

public interface Printable {
    public String print();
}
public class A implements Printable {
    public String print() { return "I am A"; }
}
public class B implements Printable {
    public String print() { return "I am B"; }
}

Example code

private static final A a = new A();
private static final B b = new B();

public static void main(String[] args) {
    Random random = new Random(0);
    System.out.println(getA().print());
    System.out.println(getB().print());
    for (int i = 0; i < 5; ++i)
        System.out.println(getAorB(random).print());
}

public static Printable getA() {
    return a;
}

public static Printable getB() {
    return b;
}

public static Printable getAorB(Random random) {
    return random.nextBoolean() ? a : b;
}

Output

I am A
I am B
I am A
I am A
I am B
I am A
I am A

Interface can never instantiate in java,But What happened,when Interface is used return type

2 Answers2