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CMSC 433,Michael Hicks, U. Maryland
CMSC433, Fall 2002
Programming Language
Technology and Paradigms
Basic Java
Michael Hicks
Sep 12, 2002
CMCS 433, Fall 2002 - Michael Hicks 59
Exceptions
• On an error condition, we throw an
exception
• At some point up the call chain, the
exception is caught and the error is handled
• Separates normal from error-handling code
• A form of non-local control-flow
– Like goto, but structured
CMSC 433,Michael Hicks, U. Maryland
CMCS 433, Fall 2002 - Michael Hicks 60
Throwing an Exception
• Create a new object of the class Exception, and
throw it
if (i > 0 && i < a.length ) {
return (a[I])
else throw new ArrayIndexOutOfBounds();
• Exceptions thrown are part of return type
– when overriding a method in a superclass
– can’t throw anything that would surprise a superclass
object
CMCS 433, Fall 2002 - Michael Hicks 61
Method throws declarations
• A method declares the exceptions it might throw
– public void openNext() throws
UnknownHostException,
EmptyStackException
• Must declare any exception the method might
throw
– unless it is caught in the method
– includes exceptions thrown by called methods
– certain built-in exceptions excluded
CMSC 433,Michael Hicks, U. Maryland
CMCS 433, Fall 2002 - Michael Hicks 64
Example Application
public class BufferedReader {
public String readLine () throws IOException { … } …
public class Echo {
public static void main(String args[]) {
BufferedReader in = …
try {
while((s = in.readLine()) != null)
System.out.println(s);
} catch(IOException e) {
System.out.println(e.stackTrace());
CMCS 433, Fall 2002 - Michael Hicks 65
Creating New Exceptions
• User-defined exception is just a class that is
a subclass of Exception
class MyOwnException extends Exception {}
class MyClass {
void oops() throws MyOwnException {
if (some_error_occurred) {
throw new MyOwnException();
CMSC 433,Michael Hicks, U. Maryland
Java Libraries
CMCS 433, Fall 2002 - Michael Hicks 67
You should familiarize yourself
• Packages
– java.lang
– java.util
– java.net
– java.io
• Read the documentation on line
CMSC 433,Michael Hicks, U. Maryland
CMCS 433, Fall 2002 - Michael Hicks 70
I/O Classes
• OutputStream – byte stream going out
• Writer – character stream going out
• InputStream – byte stream coming in
• Reader – character stream coming in
CMCS 433, Fall 2002 - Michael Hicks 71
OutputStream - bytes
• Example classes
– ByteArrayOutputStream – goes to byte []
– FileOutputStream – goes to file
• Wrappers – wrapped around OutputStream
– BufferedOutputStream
– ObjectOutputStream – serialization of object
graph
CMSC 433,Michael Hicks, U. Maryland
CMCS 433, Fall 2002 - Michael Hicks 72
Writer - characters
• OutputStreamWriter
– wraps around OutputStream to get a Writer
– takes characters, converts to bytes
– can specify encoding used to convert
• Other wrappers
– PrintWriter – supports print, println
– BufferedWriter
• Other Writers
– CharArrayWriter
– StringWriter
CMCS 433, Fall 2002 - Michael Hicks 73
InputStream - bytes
• Example classes
– ByteArrayInputStream
– FileInputStream
• Wrappers – wrapped around InputStream
– BufferedInputStream
– PushedBackInputStream
CMSC 433,Michael Hicks, U. Maryland
CMCS 433, Fall 2002 - Michael Hicks 76
Input (JDK 1.1 and higher)
• Wrap System.in in an InputStreamReader
– converts from bytes to characters
• Wrap the result in a BufferedReader
– makes input operations efficient
– supports readline() interface
• readline() returns a string
– returns null if at EOF
CMCS 433, Fall 2002 - Michael Hicks 77
Example Echo Application
import java.io.*;
public class Echo {
public static void main(String [] args) {
String s;
BufferedReader in = new BufferedReader(
new InputStreamReader(System.in));
int i = 1;
try {
while((s = in.readLine()) != null)
System.out.println((i++) + “: “ + s); }
catch(IOException e) {
System.out.println(e); }
CMSC 433,Michael Hicks, U. Maryland
CMCS 433, Fall 2002 - Michael Hicks 78
Java Networking
• class Socket
– Client-side connections to servers
• class ServerSocket
– Server-side “listen” socket
– Awaits and responds to connection requests
CMCS 433, Fall 2002 - Michael Hicks 79
Example Client/Server
ServerSocket s = new ServerSocket(5001);
Socket conn = s.accept();
InputStream in = conn.getInputStream();
OutputStream out = conn.getOutputStream();
Socket conn = new Socket (5001);
InputStream in = conn.getInputStream();
OutputStream out = conn.getOutputStream();
Client code
Server code
server client
CMSC 433,Michael Hicks, U. Maryland
CMCS 433, Fall 2002 - Michael Hicks 82
Example Client/Server
ServerSocket s = new ServerSocket(5001);
Socket conn = s.accept();
InputStream in = conn.getInputStream();
OutputStream out = conn.getOutputStream();
Socket conn = new Socket (5001);
InputStream in = conn.getInputStream();
OutputStream out = conn.getOutputStream();
Client code
Server code
server client
Note: The server is still
listening other
connection requests
CMCS 433, Fall 2002 - Michael Hicks 83
Example Client/Server
ServerSocket s = new ServerSocket(5001);
Socket conn = s.accept();
InputStream in = conn.getInputStream();
OutputStream out = conn.getOutputStream();
Socket conn = new Socket (5001);
InputStream in = conn.getInputStream();
OutputStream out = conn.getOutputStream();
Client code
Server code
server client
in
out
CMSC 433,Michael Hicks, U. Maryland
CMCS 433, Fall 2002 - Michael Hicks 84
Example Client/Server
ServerSocket s = new ServerSocket(5001);
Socket conn = s.accept();
InputStream in = conn.getInputStream();
OutputStream out = conn.getOutputStream();
Socket conn = new Socket (5001);
InputStream in = conn.getInputStream();
OutputStream out = conn.getOutputStream();
Client code
Server code
server client
in
out
in
out
CMCS 433, Fall 2002 - Michael Hicks 85
Possible Failures
• Server-side
– ServerSocket port already in use
– Client dies on accept
• Client-side
– Server dead
– No one listening on port
• In all cases IOException thrown
– Must use appropriate throw/try/catch constructs
CMSC 433,Michael Hicks, U. Maryland
CMCS 433, Fall 2002 - Michael Hicks 88
Other libraries
• java.lang.Math
– abstract final class – only static members
– includes constants e and π
– includes static methods for trig, exponentiation,
min, max, …
• java.text
– text formatting tools
• class MessageFormat provides printf/scanf
functionality
– lots of facilities for internationalization
CMCS 433, Fall 2002 - Michael Hicks 89
Java Container Classes
• A unified architecture for representing and
manipulating collections of objects
• Container classes contain three things:
– Interfaces: abstract data types representing collections
of objects
– Implementations: concrete implementations of the
collection interfaces
– Algorithms: methods that perform computations on
objects that implement collection interfaces
CMSC 433,Michael Hicks, U. Maryland
CMCS 433, Fall 2002 - Michael Hicks 90
Container class hierarchy
CMCS 433, Fall 2002 - Michael Hicks 91
Collection Classes
• Collections contain groups of objects (elements)
• Collection interface is not implemented in Java.
Subinterfaces implemented
– Set: unordered, can’t contain duplicate elements
- Hashset – unordered, no duplicates
- TreeSet - ordered, no duplicates
– List: ordered, can contain duplicate elements
- LinkedList – unordered, dynamic size, add/delete quick
- ArrayList – unordered, dynamic size, random access
CMSC 433,Michael Hicks, U. Maryland
CMCS 433, Fall 2002 - Michael Hicks 94
compareTo Interface
• public int compareTo(Object o)
• The natural comparison method (i.e., default)
- Returns a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than o
- sgn(x.compareTo(y)) == -sgn(y.compareTo(x))
- implies that x.compareTo(y) must throw an exception iff
y.compareTo(x) throws an exception.
- (x.compareTo(y)>0 && y.compareTo(z)>0) => x.compareTo(z)>0.
- x.compareTo(y)==0 => sgn(x.compareTo(z)) == sgn(y.compareTo(z))
- Recommended that (x.compareTo(y)==0) == (x.equals(y))
CMCS 433, Fall 2002 - Michael Hicks 95
Comparator Interface
• When natural order isn’t acceptable
• public int compare(Object o1, Object o2)
- Returns a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second
- sgn(compare(x, y)) == -sgn(compare(y, x))
- (This implies that compare(x,y) must throw an exception if and only
if compare(y, x) throws an exception.)
- ((compare(x, y)>0) && (compare(y, z)>0)) => compare(x, z)>0.
- compare(x, y)==0 => sgn(compare(x,z))==sgn(compare(y, z))
- recommended (compare(x, y)==0) == (x.equals(y))
• public boolean equals(Object obj)
- Indicates whether some other object is "equal to" this Comparator.
CMSC 433,Michael Hicks, U. Maryland
CMCS 433, Fall 2002 - Michael Hicks 96
Example
import java.util.; import java.awt.; class MyPoint extends java.awt.Point implements Comparable { MyPoint(int x, int y) {super(x,y);} public int compareTo(Object o) { MyPoint p = (MyPoint)o; double d1 = Math.sqrt(xx + yy); double d2 = Math.sqrt(p.xp.x + p.yp.y); if (d1 < d2) {return -1;} else if (d2 < d1) {return 1;} return 0; } }
class Sort3 { public static void main(String[] args) { Random rnd = new Random(); MyPoint[] points = new MyPoint[10]; for (int i=0; i<points.length; i++) { points[i] = new MyPoint (rnd.nextInt(100),rnd.nextInt(100)); System.out.println(points[i]); } System.out.println("-----------"); Arrays.sort(points); //Print the points for (int i=0; i<points.length; i++){ System.out.println(points[i]); } } }
CMCS 433, Fall 2002 - Michael Hicks 97
Example
import java.util.*;
import java.awt.*;
class MyPoint extends
java.awt.Point implements
Comparable {
MyPoint(int x, int y) {
super(x, y);
public int
compareTo(Object o) {
MyPoint p = (MyPoint)o;
return x - p.x;
class Sort2 { public static void main(String[] args) { Random rnd = new Random(); MyPoint[] points = new MyPoint[10]; for (int i=0; i<points.length; i++) { points[i] = new MyPoint (rnd.nextInt(100), rnd.nextInt(100)); System.out.println(points[i]); } System.out.println("-----------"); Arrays.sort(points); //Print the points for (int i=0; i<points.length; i++){ System.out.println(points[i]); } } }
