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Object-Oriented Programming
95-712
MISM/MSIT
Carnegie Mellon University
Lecture 2: Program Control
Today We Look At
Java operators
 Control structures
 More example programs

Java Operators
An operator takes one or more “things” and
produces a resultant “thing”.
 “Things” are usually primitive types, but
they are sometimes objects.
 The “things” operated upon are called
operands.
 An operator is just a function, but with a
different syntax.

A Familiar Example
int i = 3, j = 4, k;
k = i + j;
The assignment operator and the addition
operator are used (each exactly once!).
 This is a more familiar syntax than, e.g.,

k.equals(i.add(j));
More Operator Facts
All operators produce a value.
 Sometimes they produce side effects, i.e.,
they change the value of an operand.
 Evaluation of a statement with several
operators follows precedence rules. Use
parentheses for readability.
 (x + y) * z / 3 is different than x + y * z / 3

Assignment Is Tricky, Part I
public class Number {
int i;
}
public class Assignment1 {
public static void main(String[] args) {
Number n1 = new Number();
Number n1 = new Number();
n1.i = 2;
n2.i = 5;
n1.i = n2.i;
n2.i = 10;
// what is n1.i?
}
}
Assignment Is Tricky, Part II
public class Assignment2 {
public static void main(String[] args) {
Number n1 = new Number();
Number n1 = new Number();
n1.i = 2;
n2.i = 5;
n1 = n2;
n2.i = 10;
// what is n1.i?
n1.i = 20;
// what is n2.i?
}
}
A Picture Might Help
Before assignment n1 = n2
Number objects
reference variables
n1
n2
n2.i
5
n1.i
2
After assignment n1 = n2
Number objects
reference variables
n1
n2
n2.i
n1.i
5
2
“Aliasing” In Function Calls
public class PassObject {
static void f(Number m) {
m.i = 15;
}
public static void main(String[] args) {
Number n = new Number();
f(n); // what is n.i now?
}
}
Math Operators




+, -, *, /, %
Integer division truncates, i.e., 16/3 = 5
Modulus operator returns remainder on integer
division, i.e., 16%3 = 1
Shorthand:
x += 4;
is the same as
x = x + 4;

This works for the other math operators as well.
Auto Increment and Decrement
++ increases by one, and -- decreases by
one.
 Two flavors of each: pre and post:

int i = 1, j;
j = i++;
j = ++i;
j = i--;
j = --i;
// j = 1, i = 2
// j = 3, i = 3
// j = 3, i = 2
// j = 1, i = 1
Booleans and Relational Operators
The boolean type has two possible values,
true and false.
 The relational operators >, >=, <, <=, ==
and != produce a boolean result.
 >, >=, <, <= are legal for all built-in types
except booleans, == and != are legal for all.

Testing for (Non-)Equivalence

The == and != operators need to be used
with care with objects.
public class Equivalence {
public static void main(String[] args) {
Integer n1 = new Integer(47);
Integer n2 = new Integer(47);
System.out.println(n1 == n2); // prints false
System.put.println(n1 != n2); // prints true
}
}
The equals( ) Operator

This exists for all objects (don’t need it for
built-in types).
Integer n1 = new Integer(47);
Integer n2 = new Integer(47);
System.out.println(n1.equals(n2);
// prints true
The equals( ) Operator (cont.)

But exists doesn’t necessarily mean
properly defined!
class Number {
int i;
}
:
Number n1 = new Number();
Number n2 = new Number();
n1.i = 3;
n2.i = 3;
System.out.println(n1.equals(n2)); // prints false
The equals( ) Operator (cont.)
The equals( ) operator is properly defined
for most Java library classes.
 The default behavior is to compare
references, so…
 When you define a class, if you’re planning
to use equals( ), you need to define it (i.e.,
override the default behavior).

Logical Operators
These are AND (&&), OR (||), and NOT (!).
 These work on booleans only; if you have
old “C” habits, forget them!
 Use parentheses freely to group logical
expressions.
 Logical expressions short-circuit; as soon as
the result is known, evaluation stops.

Short-Circuiting Example
public class ShortCircuit {
static boolean test1(int val) {return val < 1;}
static boolean test2(int val) {return val < 2;}
static boolean test3(int val) {return val < 3;}
public static void main(String[] args) {
if (test1(0) && test2(2) && test3(2))
System.out.println(“Expression is true”);
else
System.out.println(“Expression is false”);
}
}
Bitwise, Shift, Ternary Operators
Bitwise & shift operators manipulate
individual bits in integral primitive types.
 I almost never use them, except for masks,
when memory is tight. YMMV
 The ternary if-else operator looks like this:

boolean-expression ? value0 : value1

The result is either value0 or value1,
depending on the truth of the boolean.
The String + Operator
The + operator is “overloaded” for String
objects; it means concatenation.
 It reminds me of good old C++…
 If an expression begins with a String, then
all the following operands of + will be
converted into Strings:

int x = 0, y = 1; z = 2;
String myString = “x, y, z ”;
System.out.println(myString + x + y + z);
Casting
A cast produces a temporary new value of a
designated type.
 Implicit and explicit casts:

int i = 2;
float f = i;
// OK, since f can hold all of i
float g = 3.14159;
//! int j = g;
// not OK, loses information
int k = (int) g;
// OK, compiler is reassured
Execution Control: if-else
if (boolean_expression)
statement
else if(boolean_expression)
statement
:
else if(boolean_expression)
statement
else
statement
if-else Example
public int test(int testVal, int target) {
int result = 0;
if (testVal > target)
result = +1;
else if (testVal < target)
result = -1;
else {
System.out.println(“They are equal”);
result = 0;
}
return result;
}
Execution Control: return

Exit a method, returning an actual value or
object, or not (if the return type is void).
public int test(int testVal, int target) {
if (testVal > target)
return = +1;
else if (testVal < target)
return = -1;
else {
System.out.println(“They are equal”);
result = 0;
}
}
Three Kinds of Iteration
while (boolean_expression) // evaluate first
statement_or_block
do
statement_or_block
while (boolean_expression)
// evaluate last
for (initialization ; boolean_expression ; step)
statement_or_block
Example:
for (int i = 0; i < myArray.size(); i++) {
myArray[i] = 0;
}
public class BreakAndContinue {
public static void main(String[] args) {
for (int i = 0; i < 100; i++) {
if (i == 74) break;
// out of for loop
if (i % 9 != 0) continue;
// next iteration
System.out.println(i);
}
int i = 0;
// this does work
while (true) {
// an infinite loop
i++;
int j = i * 27;
if (j == 1269) break;
// out of loop
if (i % 10 != 0) continue;
// top of loop
}
}
Selection Via switch
for (int i = 0; i < 100; i++) {
char c = (char) (Math.random() * 26 + ‘a’);
switch(c) {
case ‘a’:
case ‘e’:
case ‘i’:
case ‘o’:
case ‘u’:
System.out.println(“Vowel”); break;
case ‘y’:
case ‘w’:
System.out.println(“Sometimes a vowel”); break;
default:
System.out.println(“Not a vowel”);
}
Digression on Random Numbers

Despite theory, most random number generators
are more like “kids playing with matches”.
– See “Random Number Generators: Good Ones Are
Hard to Find” by S. Park and K. Miller, CACM Oct.
1988.

Most random number generators use
“multiplicative linear congruential” schemes:
– A modulus m, a large prime integer
– A multiplier a, an integer in the range 2,3,…m-1
– These produce a sequence z1, z2, z3… using the iterative
equation
– zi+1 = f(zi) = a*z % m
Random Numbers (cont.)





The sequence is initiated by choosing a seed.
Example: f(z) = 6z %13. This produces the
sequence ...1 , 6, 10, 8, 9, 2, 12, 7, 3, 5, 4, 11, 1,...
Example: f(z) = 7z % 13. This produces the
sequence ...1 , 7, 10, 5, 9, 11, 12, 6, 3, 8, 4, 2, 1,...
Is the latter somehow "less random"?
Example: f(z) = 5z %13. This produces the
sequence ...1 , 5, 12, 8, 1...
Examples of Rotten RNGs

IBM's RANDU for SYSTEM/360:
– f(z) = 65539z % 231. Not full period, dependent on
results of overflow operations, low-order bits cycle w/
period 2!







Prime’s Scheffield Pascal, an even worse version
SAS
Modula 2
Many LISPs and Prologs
Hundreds of CS textbooks
Turbo Pascal
One generator had a fixed point!
Using Java’s RNGs (cont.)

java.lang.Math
– static double random()
random in [0, 1.0)
The sequence doesn’t seem to be repeatable
 Bad for debugging
 Good for experimental work

What’s a Seed?
Here is a sequence I generated on Friday at 9:30, with the code:
Random randomGenerator = new Random();
for (int i = 0; i < 10; i++)
System.out.println(randomGenerator.nextInt());
-1321140701
i=0
1591074477
i=1
646222811
i=2
Random number i = 3
1633627825
sequence
-2039264851
i=4
i=5
-315534709
What’s a Seed?
I ran the same code again at 10:30, and got this:
-403585738
i=0
-2092042741
i=1
-333693675
i=2
Random number i = 3
-331823022
sequence
1651689643
i=4
i=5
133526058
What’s a Seed?
I ran this code at 10:35
Random randomGenerator = new Random(1234);
for (int i = 0; i < 10; i++)
System.out.println(randomGenerator.nextInt());
-1517918040
i=0
1115789266
i=1
-208917030
i=2
Random number i = 3
1019800440
sequence
-6116528751
i=4
i=5
1362132786
What’s a Seed?
I ran this code at 10:45 and got the same sequence.
Random randomGenerator = new Random(1234);
for (int i = 0; i < 10; i++)
System.out.println(randomGenerator.nextInt());
-1517918040
i=0
1115789266
i=1
-208917030
i=2
Random number i = 3
1019800440
sequence
-6116528751
i=4
i=5
1362132786
Using Java’s RNGs (cont.)

java.util.Random
– Constructors:
» Random()
» Random(long seed)
– Methods:
»
»
»
»

nextInt()
nextInt(int n)
nextFloat()
setSeed(long seed)
random in (-231, 231-1)
random in [0, n)
random in [0, 1)
java.lang.Math
– static double random() random in [0, 1.0)
Random Numbers In Action:
The Monte Hall Paradox
Monte Hall hosts a TV game
show, “Let’s Make a Deal”.
 One contestant is shown three
doors.
 Behind one of the doors is a
“Fabulous Grand Prize”.
 Behind the other two doors are “worthless
prizes”.

The Monte Hall Paradox
The contestant selects a door.
 Monte opens a door not selected by the
contestant, and this always shows a
worthless prize.
 The contestant can choose to change her
selection to the other door not opened by
Monte Hall.
 Should the contestant change doors?

The Monte Hall Paradox

Very intelligent people are confused!
– Any initial door choice has a 1/3 chance of
winning.
– Why would changing the door choice increase
or decrease the odds of winning the “Fabulous
Grand Prize”?
– Can’t we settle this with a computer
simulation?
The Door Class
public class Door {
boolean open;
boolean hasGrandPrize;
boolean chosenByContestant;
}
The Game Class
import java.util.*;
public class Game {
Door door1, door2, door3;
void setUpGame() {
door1 = new Door();door2 = new Door();door3 = new Door();
// initialize all the Door variables to false
Random r = new Random();
int grandPrizeDoor = r.nextInt(3);
switch(grandPrizeDoor) {
case 0:
door1.hasGrandPrize = true; break;
case 1: // etc.
}
}
Reality Check For Game Class
void printStateOfDoors() {
System.out.println("Door 1 is " +
(door1.open ? " open, " : "not open, ") +
(door1.hasGrandPrize ? "is the grand prize, and " : "is no
(door1.chosenByContestant ? "is chosen." : "is not chosen
System.out.println("Door 2 is " +
(door2.open ? " open, " : "not open, ") +
(door2.hasGrandPrize ? "is the grand prize, and " : "is no
(door2.chosenByContestant ? "is chosen." : "is not chosen
System.out.println("Door 3 is " +
(door3.open ? " open, " : "not open, ") +
(door3.hasGrandPrize ? "is the grand prize, and " : "is no
(door3.chosenByContestant ? "is chosen." : "is not chosen
}
The Game Class
public class Game {
Door door1, door2, door3;
void setUpGame() { // shown earlier}
void contestantChooseDoor() {
Random r = new Random();
int contestantDoor = r.nextInt(3);
switch(contestantDoor) {
case 0: door1.chosenByContestant = true; break;
case 1: door2.chosenByContestant = true; break;
case 2: door3.chosenByContestant = true; break;
}
}
}
The PlayManyGames Class
public class PlayManyGames {
public static void main(String[] args) {
Game theGame = new Game();
theGame.setUpGame();
theGame.printStateOfDoors();
theGame.contestantChooseDoor();
theGame.printStateOfDoors();
}
}
A Problem: First Run
Door 1 is not open, is not the grand prize, and is not chosen.
Door 2 is not open, is not the grand prize, and is not chosen.
Door 3 is not open, is
the grand prize, and is not chosen.
Door 1 is not open, is not the grand prize, and is not chosen.
Door 2 is not open, is not the grand prize, and is not chosen.
Door 3 is not open, is
the grand prize, and is
chosen.
A Problem: Second Run
Door 1 is not open, is not the grand prize, and is not chosen.
Door 2 is not open, is
the grand prize, and is not chosen.
Door 3 is not open, is not the grand prize, and is not chosen.
Door 1 is not open, is not the grand prize, and is not chosen.
Door 2 is not open, is
the grand prize, and is
chosen.
Door 3 is not open, is not the grand prize, and is not chosen.
A Problem: Third Run
Door 1 is not open, is
the grand prize, and is not chosen.
Door 2 is not open, is not the grand prize, and is not chosen.
Door 3 is not open, is not the grand prize, and is not chosen.
Door 1 is not open, is
the grand prize, and is
chosen.
Door 2 is not open, is not the grand prize, and is not chosen.
Door 3 is not open, is not the grand prize, and is not chosen.
The Solution
The RNGs in both methods of the Game
class are starting from the same seed.
 Solution: Take the RNG out of the methods,
and into the Game class itself.
 But each instance of the Game class will
still start at the same place.
 Solution: Make the RNG a static variable.

The New Game Class
public class Game {
Door door1, door2, door3;
static Random r = new Random();
void setUpGame() {
int grandPrizeDoor = r.nextInt(3);
switch(grandPrizeDoor) {
// etc.
}
void contestantChooseDoor() {
int contestantDoor = r.nextInt(3);
switch(contestantDoor) {
// etc.
}
}
Completing the Simulation

Add Game methods for
– Monte choosing a door to open
– The player switching or not
– Deciding if the player has won

Add PlayManyGames code to
–
–
–
–
Call the new Game methods
Put everything in a loop
Prompt the user for # of trials, switching policy
Display the results