Java Utility Classes

1. Java Utility Classes CS 21b

2. Some Java Utility Classes • Vector • Hashtable • StringTokenizer * import java.util.*;

3. Vector • Indexed data structure that automatically resizes • Selected Methods • void addElement(Object o) • int size() • Object elementAt(int index) • void setElementAt(Object elem, int index)

4. Vector Example Vector names = new Vector(); names.addElement(“Bart”); names.addElement(“Lisa”); names.addElement(“Maggie”); for (int i = 0; i < names.size(); i++) { System.out.println(names.elementAt(i)); } names.setElementAt(“Homer”,1); names.addElement(“Marge”); for (int i = 0; i < names.size(); i++) { System.out.println(names.elementAt(i)); }

5. Hashtable • Data structure that associates values with a key for efficient look up • Selected Methods • void put(Object key, Object value) • Object get(Object key) • Object remove(Object key)

6. Hashtable Example Hashtable grades = new Hashtable(); grades.put(“Martin”, “A”); grades.put(“Nelson”, “F”); grades.put(“Millhouse”, “C”); System.out.println(grades.get(“Nelson”)); System.out.println(grades.get(“Martin”)); grades.put(“Nelson”, “W”); grades.remove(“Martin”); System.out.println(grades.get(“Nelson”)); System.out.println(grades.get(“Martin”));

7. StringTokenizer • Breaks up a string into substrings (tokens) separated by a specified delimiter • Selected Methods • StringTokenizer(String str, String delim) • int countTokens() • String nextToken() • boolean hasMoreTokens()

8. StringTokenizer Example StringTokenizer st; String longstr = “This is the last slide”; st = new StringTokenizer(longstr, “ ”); int numwords = st.countTokens(); System.out.println(numwords); String firstword = st.nextToken(); System.out.println(firstword); while (st.hasMoreTokens()) { System.out.println(st.nextToken()); }

9. Introduction to Databasesand JDBC CS 21b

10. Crash Course inDatabases • (Relational) Database: set of tables • represents information • Table: columns (fields) and rows (values) • SQL - Structured Query Language: facilitates manipulation of data in a database

11. Database Example SALENum Code Quantity001 BB 2002 ZD 1003 BB 5004 SF 1 PRODUCT Code Item PriceBB Beyblade 150.00 ZD Zoid 799.95 SF Stickfas 500.00

12. SQL Statements • To add a row: INSERT INTO SALE VALUES(005,‘ZD’,2) • To retrieve a row: (e.g., product name given a code) SELECT ITEM FROM PRODUCT WHERE CODE = ‘BB’ • To retrieve several rows: (e.g., all details of all orders) SELECT * FROM SALE

13. JDBC • Java Database Connectivity • Database Access Interface • provides access to a relational database (by allowing SQL statements to be sent and executed through a Java program) • JDBC package: set of Java classes that facilitate this access (java.sql.*) • Comes with JDK (since 1.1)

14. JDBC Driver Need a driver, specific to the DB product, to mediate between JDBC and the database • the driver is a Java class that needs to be loaded first Java Program - load driver - establish connection - send SQL statements Relational DBManagement System

15. JDBC-ODBC Bridge • Driver that interfaces with ODBC (Object Database Connectivity--also an access interface) • Easiest way to access databases created by Microsoft products • register database as an ODBC data source • use JDBC-ODBC bridge as the JDBC driver (included in JDK 1.2 distribution)

16. Key Classes in JDBC • Connection • need to create an instance of this class when establishing a connection to the database • Statement • for issuing SQL statements • ResultSet (interface) • a ResultSet object represents the table returned by an SQL select statement

17. Establishing a Connection Use the getConnection() method • under the DriverManager class • String argument: "jdbc:driver:name” • returns a Connection object Class.forName(“sun.jdbc.odbc.JdbcOdbcDriver”); // above line loads the jdbc-odbc driver String dbname = “jdbc:odbc:MyDB”; Connection c = Driver.getConnection(dbname);

18. Creating aStatement Object Execute the createStatement() method on the Connection object • returns a Statement object • afterwards, run methods on the Statement object to execute an SQL statement Statement s = c.createStatement();

19. Methods of theStatement Class • executeQuery() • requires a String argument (a select statement) • returns a ResultSet object • executeUpdate() • requires a String argument (an insert, update, or delete statement) • returns an int (row count, in most cases)

20. The ResultSet Interface • A ResultSet object represents the table returned by the select statement sent • Navigation/retrieval methods • next(): moves to the next row (first row if called for the first time), returns false if no rows remain • getXXX methods return the value of a field for the current row

21. get Method Example: getInt() ResultSet rs; rs = s.executeQuery(“SELECT * FROM ORDER”); rs.next(); // gets the first row // suppose the Orders table has an integer field // called quantity int myvar = rs.getInt(“Quantity”); // if you knew that quantity is the 3rd field in the table myvar = rs.getInt(3);

22. Exercise • Create a Microsoft Access Database • insert sample rows • Add an ODBC data source • use the Microsoft Access driver • associate with the created database • Create a Java program • use JDBC-ODBC bridge • create a loop that lists all rows of the table

23. Summary • JDBC allows you to write Java programs that manipulate a database • A driver (often a separate product) is required that facilitates access • Key classes: Connection, Statement, and ResultSet • Other features: metadata, parameterized statements, and stored-proc invocation