Download import java.util.InputMismatchException

Introduction to Java
CSC212 Lecture 7
D. Thiebaut, Fall 2014
More on Exceptions
A Simple Class
import java.util.Scanner;!
!
public class Bomb {!
!
public static void main(String[] args) {!
Scanner input = new Scanner( System.in );!
!
while ( true ) {!
System.out.println( "Please enter 2 integers"!
+" (-1 to stop): ");!
int n1 = input.nextInt();!
if ( n1==-1 ) !
break;!
int n2 = input.nextInt();!
System.out.println( n1 + " / " + n2 + " is " + (n1/n2) );!
}!
}!
}
What could go wrong?
import java.util.Scanner;!
!
public class Bomb {!
!
public static void main(String[] args) {!
Scanner input = new Scanner( System.in );!
!
while ( true ) {!
System.out.println( "Please enter 2 integers"!
+" (-1 to stop): ");!
int n1 = input.nextInt();!
if ( n1==-1 ) !
break;!
int n2 = input.nextInt();!
System.out.println( n1 + " / " + n2 + " is " + (n1/n2) );!
}!
}!
}
First Place to Crash:
import java.util.Scanner;!
!
public class Bomb {!
!
public static void main(String[] args) {!
Scanner input = new Scanner( System.in );!
!
while ( true ) {!
System.out.println( "Please enter 2 integers"!
+" (-1 to stop): ");!
int n1 = input.nextInt();!
if ( n1==-1 ) !
break;!
int n2 = input.nextInt();!
System.out.println( n1 + " / " + n2 + " is " + (n1/n2) );!
}!
}!
}
Fix #1
while ( true ) {!
System.out.println( "Please enter 2 integers (-1 to stop): ");!
int n1 = input.nextInt();!
int n2 = input.nextInt();!
if ( n1==-1 ) !
break;!
try {!
System.out.println( n1 + " / " + n2 + " is " + (n1/n2) );!
}!
catch ( ArithmeticException e ) {!
System.out.println("Exception: " + e.getMessage() + !
"\nReenter numbers." );!
}!
}!
What Else Could Go Wrong?
import java.util.Scanner;!
!
public class Bomb {!
!
public static void main(String[] args) {!
Scanner input = new Scanner( System.in );!
!
while ( true ) {!
System.out.println( "Please enter 2 integers"!
+" (-1 to stop): ");!
int n1 = input.nextInt();!
if ( n1==-1 ) !
break;!
int n2 = input.nextInt();!
System.out.println( n1 + " / " + n2 + " is " + (n1/n2) );!
}!
}!
}
Badly Formatted Ints:
import java.util.Scanner;!
!
public class Bomb {!
!
public static void main(String[] args) {!
Scanner input = new Scanner( System.in );!
!
while ( true ) {!
System.out.println( "Please enter 2 integers"!
+" (-1 to stop): ");!
int n1 = input.nextInt();!
if ( n1==-1 ) !
break;!
int n2 = input.nextInt();!
System.out.println( n1 + " / " + n2 + " is " + (n1/n2) );!
}!
}!
}
Fix #2
import java.util.Scanner;!
import java.util.InputMismatchException;!
!
public class Bomb {!
!
public static void main(String[] args) {!
Scanner input = new Scanner( System.in );!
!
while ( true ) {!
System.out.println( "Please enter 2 integers (-1 to stop): ");!
int n1, n2;!
try {!
n1 = input.nextInt();!
if ( n1==-1 ) break;!
n2 = input.nextInt();!
} catch ( InputMismatchException e ) {!
// add your code here...!
}!
!
//... some code missing...
Fix #2
import java.util.Scanner;!
import java.util.InputMismatchException;!
!
public class Bomb {!
!
public static void main(String[] args) {!
Scanner input = new Scanner( System.in );!
!
while ( true ) {!
Both Input!
System.out.println( "Please enter 2 integers (-1 to stop): ");!
Statements!
int n1, n2;!
Protected!
try {!
n1 = input.nextInt();!
if ( n1==-1 ) break;!
n2 = input.nextInt();!
} catch ( InputMismatchException e ) {!
// add your code here…!
! ! ! ! ! !
continue;!
}!
!
//... some code missing...
But There's a Problem…
!
Infinite Loop…
!
It Has to Do With The
Input Buffer…
Input Buffer
1 2 3 _ 3 7 \n
input.nextInt() —-> 123
input.nextInt() —-> 37
Input Buffer
1 2 A _ 3 7 \n
input.nextInt() —->
input.nextInt() —->
Solution: input.nextLine();
Input Buffer
1 2 A _ 3 7 \n
Fix #2 Fixed
import java.util.Scanner;!
import java.util.InputMismatchException;!
!
public class Bomb {!
!
public static void main(String[] args) {!
Scanner input = new Scanner( System.in );!
!
while ( true ) {!
System.out.println( "Please enter 2 integers (-1 to stop): ");!
int n1, n2;!
try {!
n1 = input.nextInt();!
if ( n1==-1 ) break;!
Get rid!
n2 = input.nextInt();!
of the rest of!
} catch ( InputMismatchException e ) {!
the unread!
// add your code here…!
information!
! ! ! ! ! !
input.nextLine();!
! ! ! ! ! !
continue;!
}!
!
//... some code missing...
Reading Files…
(and dealing with
more exceptions)
import java.io.File;!
import java.util.Scanner;!
!
Example
public class Lab5ReadFile {!
public static void main( String[] args ) {!
!
String fileName;!
int n1, n2;!
Scanner inFile = null;!
!
// if user forgot the enter file name on command line, remind her!
if ( args.length == 0 ) {!
System.out.println( "Syntax: java progName textFileName" );!
System.out.println( "
where textFileName is a name of a data file.");!
return;!
}!
!
// set file name to first argument on command line!
fileName = args[0];
!
inFile = new Scanner( new File( fileName ) );!
!
// read the file!
while ( inFile.hasNext() ) {!
n1 = inFile.nextInt();!
n2 = inFile.nextInt();!
System.out.println( n1 + " / " + n2 + " is " + n1/n2 );!
}!
}!
}
When we compile the
program, the compiler stops
and requires the use of a
try/catch statement!
$: javac Lab5ReadFile.java
Lab5ReadFile.java:23: error: unreported exception FileNotFoundException; must be caught or
declared to be thrown
inFile = new Scanner( new File( fileName ) );
^
1 error
Search the Error on the Web
import java.io.File;!
import java.util.Scanner;!
import java.io.FileNotFoundException;!
!
public class Lab5ReadFile {!
public static void main( String[] args ) {!
!
String fileName;!
int n1, n2;!
Scanner inFile = null;!
…!
!
// set file name to first argument on command line!
fileName = args[0]; !
!
try {
!
inFile = new Scanner( new File( fileName ) );!
}!
catch ( FileNotFoundException e ) {!
System.out.println( "File not found." );!
return;!
}!
!
…!
}!
}
(Lab5ReadFile.java)
Throwing Exceptions
Why?
•
Sometimes the method in which the exception
occurs is too low level to be able to fix the
problem.
private static int[] get2Ints( Scanner inFile ) {!
int n1 = inFile.nextInt(); !
int n2 = inFile.nextInt();!
return new int[] { n1, n2 };!
}
private static int[] get2Ints( Scanner inFile ) !
! ! ! ! ! ! ! ! ! ! throws InputMismatchException{!
int n1 = inFile.nextInt(); !
int n2 = inFile.nextInt();!
return new int[] { n1, n2 };!
}
while (inFile.hasNext()) {!
int[] a;!
try {!
a = get2Ints( inFile );!
} catch ( InputMismatchException e ) {!
System.err.println( "Wrong number format. Please reenter!" );!
inFile.nextLine();!
continue;
}!
System.out.println( a[0] + " / " + a[1] + " is " + a[0] / a[1] );!
}
private static int[] get2Ints( Scanner inFile ) !
! ! ! ! ! ! ! ! ! ! throws InputMismatchException{!
int n1 = inFile.nextInt(); !
int n2 = inFile.nextInt();!
return new int[] { n1, n2 };!
}
while (inFile.hasNext()) {!
int[] a;!
try {!
a = get2Ints( inFile );!
} catch ( InputMismatchException e ) {!
System.err.println( "Wrong number format. Please reenter!" );!
inFile.nextLine();!
continue;
}!
System.out.println( a[0] + " / " + a[1] + " is " + a[0] / a[1] );!
}
We Create Our
Own Exception Object
class MyException extends Exception {!
MyException( String message ) {!
super( message );!
}!
}
private static int[] get2Ints( Scanner inFile ) !
! ! !
throws InputMismatchException, MyException {!
int n1 = inFile.nextInt(); !
int n2 = inFile.nextInt();!
if ( n2 == 0 ) {!
throw new MyException( "Invalid 0\n" );!
}!
return new int[] { n1, n2 };!
}!
(Lab5ReadFileThrow3.java)
DATA
STRUCTURES!
“ In computer science, a data structure
is a particular way of organizing data in
a computer so that it can be used
efficiently. Different kinds of data
structures are suited to different kinds of
applications, and some are highly
specialized to specific tasks.”
–en.wikipedia.org/wiki/Data_structureWikipedia
Efficiency
•
SPEED!
•
MEMORY
Java’s Vector
Data Structure
(Chapter 3)
Where Do I Find Information?
http://docs.oracle.com/javase/7/docs/api/java/util/Vector.html
Some Useful Vector
Methods
•
Vector(), Vector(initialCapacity)
•
add( )
•
contains( Object )
•
get( index )
•
indexOf( Object )
•
size()
Build a Vector…
import java.util.Iterator;
import java.util.Vector;
!
!
public class Vector1 {
!
}
static public void main( String[] args ) {
Vector list = new Vector( );
for ( int i=0; i<10; i++ ) list.add( (Integer) (i * 3) );
… }
Build a Vector…
import java.util.Iterator;
import java.util.Vector;
!
!
public class Vector1 {
!
}
static public void main( String[] args ) {
Vector list = new Vector( );
for ( int i=0; i<10; i++ ) list.add( (Integer) (i * 3) );
… }
Type Cast!
Access Element At Index
try {
System.out.println( "Element at 4 = " + list.get( 4 ) );
} catch (ArrayIndexOutOfBoundsException e ) {
// do nothing
}
!
try {
System.out.println( "Element at 20 = " + list.get( 20 ) );
} catch (ArrayIndexOutOfBoundsException e ) {
// do nothing
}
Iterating Through Vector
for ( int i=0; i<list.size(); i++ ) {
int x = list.get( i );
System.out.println( x );
}
Iterator<Integer> it = list.iterator();
while ( it.hasNext() ) {
int x = it.next();
System.out.println( x );
}
import java.util.Iterator;
import java.util.Vector;
!
public class Vector1 {
!
static public void main( String[] args ) {
Vector list = new Vector( );
for ( int i=0; i<10; i++ ) list.add( (Integer) (i * 3) );
try {
System.out.println( "Element at 4 = " + list.get( 4 ) );
} catch (ArrayIndexOutOfBoundsException e ) {
// do nothing
}
!
}
}
try {
System.out.println( "Element at 20 = " + list.get( 20 ) );
} catch (ArrayIndexOutOfBoundsException e ) {
// do nothing
}
Iterator<Integer> it = list.iterator();
while ( it.hasNext() ) {
int x = it.next();
System.out.println( x );
} import java.util.Iterator;
import java.util.Vector;
!
public class Vector2 {
static public void main( String[] args ) {
Vector<Integer> list = new Vector<Integer>( );
for ( int i=0; i<10; i++ ) list.add( i * 3 );
try {
System.out.println( "Element at 4 = " + list.get( 4 ) );
} catch (ArrayIndexOutOfBoundsException e ) {
// do nothing
}
try {
System.out.println( "Element at 20 = " + list.get( 20 ) );
} catch (ArrayIndexOutOfBoundsException e ) {
// do nothing
}
Iterator<Integer> it = list.iterator();
while ( it.hasNext() ) {
int x = it.next();
System.out.println( x );
}
}
}