Java Utility Classes CS 21b

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

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)

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)); }

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)

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”));

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()

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()); }

Introduction to Databases and JDBC CS 21b

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

Database Example PRODUCT CodeItemPrice BBBeyblade ZDZoid SFStickfas SALE NumCodeQuantity 001BB2 002ZD1 003BB5 004SF1

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

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)

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 Relational DB Management System Java Program - load driver - establish connection - send SQL statements

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)

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

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);

Creating a Statement 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();

Methods of the Statement 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)

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

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);

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

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