Question

I understand how interfaces work in general but I am confused about a thing. I started learning about JDBC and I can’t understand this statement:

Connection conn = null; // set a reference pointing to nothing
Statement stmt = null;
PreparedStatement preparedStmt = null;
conn = DriverManager.getConnection('mysql/derby/other db', 'user', 'password');  

//This is what I don't understand
stmt = conn.createStatement(); // Interface reference calling another interface's method
// Or this
preparedStmt = conn.preparedStatement("INSERT INTO db.table values (?, ?, ?, ?)";

I have a Statement interface reference pointing to another interface’s method Connection interface. Where is this method defined?

Answer 1

While you can have variables that are of interfaces types (e.g. Connection, Statement, …), the actual objects at runtime need to be of some specific implementation type.

For example DriverMananger.getConnection() takes over the job of finding the Connection implementation that can handle the JDBC URL you provided.

Let’s assume that it’s the fictional MyDBConnection class (that implements Connection of course).

Now your conn variable holds a reference of to a MyDBConnection.

When you call createStatement() on that object, it’s actually MyDBConnection.createStatement() that gets executed. That method could look like this:

public Statement createStatement() {
    return new MyDBStatement(this.someConnectionState);
}

After this, your stmt variable would hold a reference to a MyDBStatement object.

Remember: if any method promises to return something of type A, it can equally well return something of a sub-type of A (i.e. either a sub-class of A or a class implementing A, if A is an interface). In fact, when A is an interface it has to return a class implementing A. Since a pure A object can’t exist.