PreparedStatement.executeBatch returns unexpected values

@Test

public void test() throws SQLException {

    String url = "jdbc:mariadb://localhost:3366/test?user=user&password=password";

    Connection con = DriverManager.getConnection(url);

    Statement stmt = con.createStatement();

    stmt.execute("DROP TABLE IF EXISTS t0");

    stmt.execute("CREATE TABLE t0(c0 REAL SIGNED  PRIMARY KEY NOT NULL) engine=InnoDB");

    stmt.close();

    PreparedStatement pstmt = con.prepareStatement("INSERT INTO t0 VALUES(?)");

    for (int i = 0; i < 2; i++) {

        pstmt.setDouble(1, i);

        pstmt.addBatch();

    }

    int[] res = pstmt.executeBatch();

    for (int t : res) {

        System.out.println(t);

    }

}