1. Switch, Rounding Errors, Libraries
CMSC 131 Object-Oriented Programming I
Switch, Rounding Errors, Libraries
Dept of Computer Science
University of Maryland College Park
This material is based on material provided by Ben Bederson, Bonnie Dorr, Fawzi Emad, David Mount, Jan Plane

2. Accessing Private Instance Variables
IMPORTANT: Remember that methods of a class can access private elements of parameters that belong to the same class
Assume we have a Student class with a private instance variable called name and a method called checking
public boolean checking(Student p) {
// We can access p.name here even though is private
// We don’t need a get method. The method checking
// can access its own private instance variables and
// also those of objects that belong to the same class }

3. Ternary Operator
We can rewrite your maximum method by using the ternary operator:
Expr ? exprValueIfExprIsTrue : exprValueIfExprIsFalse;
Rewriting maximum
int maximum = x > y ? x : y;

4. Switch Statement
You can use a switch statement instead of a cascaded if statement if the expression you are testing is an integer, string or enum type (to be seen later on)
Example: SwitchExample.java

5. The Switch Statement Example:
Switch Statement: is a convenient (and often more efficient) way to perform a multi-way conditional based on a single control value.
Example:
switch ( option ) {
case 1:
System.out.println( "Read image" );
break;
case 2:
System.out.println( "Double" );
case 9:
System.out.println( "Quit" );
default:
System.out.println( "Sorry, invalid" ); }
if ( option == 1 )
System.out.println( “Read image” );
else if (option == 2 )
System.out.println( “Double” );
else if ( option == 9 )
System.out.println( “Quit” );
else
System.out.println( “Sorry, invalid” );
Original:
The case that is
chosen depends on
the value of “option”
The “default” case
is chosen if none of
the cases match

6. The Switch Statement General form: switch ( control-expression ) {
The control-expression is
one of the following types:
char, int, short, byte
General form:
switch ( control-expression ) {
case case-label-1 :
statement-sequence-1
break;
case case-label-2 :
statement-sequence-2 …
case case-label-n :
statement-sequence-n
default :
default-statement-sequence }
Each case label must be of
a type that is compatible
with the control expression.
You may have any number of statements,
including nesting of if-else and loops.
The “break” statement jumps
out of the switch statement.
The “default” case is optional, and
is executed if none of the other
cases match.

7. The Switch Statement not float or double, not boolean or long
The control expression can be of one of the following types: char, int, short, byte, String, enum
not float or double,
not boolean or long
The “break” statement jumps out of the switch statement. Otherwise control flow just “falls through” into the next case
int option = 2;
switch ( option ) {
case 1:
System.out.println( "Read image" );
case 2:
System.out.println( "Double" );
case 9:
System.out.println( "Quit" );
default:
System.out.println( "Sorry, invalid" ); }
This is not correct!
Double
Quit
Sorry, invalid
Output:
This is probably not
what you intended.

8. The Switch Statement
The falling though behavior is handy, because it allows you to combine cases.
Example: Allowing either upper-case or lower-case for characters:
char command = 'D';
switch ( command ) {
case 'i':
case 'I':
MyUtility.insert( );
numberOfItems++;
break;
case 'd':
case 'D':
MyUtility.delete( );
numberOfItems--; … }
Note: This is a char, not a String.
This is performed for
either ‘I’ or ‘i’

9. About Naming Constants
If a constant is static then use uppercase letters
final static int MAX = 10;
If a constant is not static then do not use uppercase letters
Use camel case
final int maxPressure = 50;
Example: ScienceExperiment.java

10. Floating Point Calculations
What is the output of the following code:
double difference = ;
Example: FloatCalculations.java
Floating point numbers in Java are stored in binary representation, and frequently numbers that are easily represented in base 10 cannot be represented precisely in base 2
What can we do?

11. Floating Point Calculations
Two important rules:
You can never use == to compare floating point values. Instead, check if two numbers are within a certain tolerance of each other:
Math.abs(( ) - 0.1) < EPSILON
Never use floating point values to represent money, e.g., 3.52 to represent $ Instead, use integer 352 to represent 352 pennies

12. Libraries in Java FileNewPackage Library
Implementation of useful routines shared by different programs
Java mechanism for creating libraries: packages
Package: group of related classes
Example: java.util (contains Scanner class)
To create a package in Eclipse use
FileNewPackage

13. Libraries in Java
To use a class from a package you can use a fully qualified name
Fully qualified name  package name + class name
java.util.Scanner s = new java.util.Scanner(System.in);
You can also import the class in the beginning of the file
import java.util.Scanner;
To import class in a package:
import java.util.*;
Imports Scanner as well as other classes in package

14. Package java.lang A special package containing widely used classes:
String
Math
etc.
java.lang.* is automatically imported by every Java program

15. package <name of package>;
Package Management
A class can be added to a package by including in the source file (usually very first line):
package <name of package>;
The variables/methods provided by a class/package are often called its API (= Application Programmers Interface)
APIs should be documented
java.lang documentation:
summary.html