LECTURE 4 CLASSES AND OBJECTS.pdf

Solutions 1
public static int getMinIndex(int[] values) {
int minValue = Integer.MAX_VALUE;
int minIndex = -1;
for(int i=0; i<values.length; i++)
if (values[i] < minValue) {
minValue = values[i];
minIndex = i;
}
return minIndex;
}

Solutions 2
public static int getSecondMinIndex(int[] values) {
int secondIdx = -1;
int minIdx= getMinIndex(values);
for(int i=0; i<values.length; i++) {
if (i == minIdx)
continue;
if (secondIdx == -1 ||
values[i] < values[secondIdx])
secondIdx = i;
}
return secondIdx;
}
• What happens if values = {0}? values = {0, 0}? values = {0,1}?

Popular Issues 1
• Array Index vs Array Value
int[] values = {99, 100, 101};
System.out.println(values[0] ); // 99
Values 99 100 101
Indexes 0 1 2

Popular Issues 2
• Curly braces { … } after if/else, for/while
for (int i = 0; i < 5; i++)
System.out.println(“Hi”);
System.out.println(“Bye”);
• What does this print?

Popular Issues 3
• Variable initialization
int getMinValue(int[] vals) {
int min = 0;
for (int i = 0; i < vals.length; i++) {
if (vals[i] < min) {
min = vals[i]
}
}
}
Problem?
• What if vals = {1,2,3}?
• Set min = Integer.MAX_VALUE or vals[0]

Popular Issues 4
• Variable Initialization – secondMinIndex
int minIdx = getMin(vals)
int secondIdx = 0;
if (i == minIdx) continue;
if (vals[i] < vals[secondIdx])
secondIdx = i;
}
• What if vals = {0, 1, 2}?
• See solutions

Popular Issues 5
Defining a method inside a method
public static void main(String[] arguments) {
public static void foobar () {
}
}

Debugging Notes 1
• Use System.out.println throughout your code
to see what it’s doing
for ( int i=0; i< vals.length; i++) {
if ( vals[i] < minVal) {
System.out.println(“cur min: ” + minVal);
System.out.println(“new min: ” + vals[i]);
minVal = vals[i];
}
}

Debugging Notes 2
• Formatting
• Ctrl-shift-f is your friend
if (vals[i] < vals[minIdx]) {
minIdx=i;}
return minIdx;}
• Is there a bug? Who knows! Hard to read

Today’s Topics
Object oriented programming
Defining Classes
Using Classes
References vs Values
Static types and methods

• Represent the real world
Baby

• Represent the real world
Baby
Name
Sex
Weight
Decibels
# poops so far

Object Oriented Programming
• Objects group together
– Primitives (int, double, char, etc..)
– Objects (String, etc…)
Baby
String name
boolean isMale
double weight
double decibels
int numPoops

Why use classes?
• Why not just primitives?
// little baby alex
String nameAlex;
double weightAlex;
// little baby david
String nameDavid;
double weightDavid;

Why use classes?
// little baby alex
String nameAlex;
double weightAlex;
String nameDavid;
double weightDavid;
String nameDavid2;
double weightDavid2;
David2?
Terrible •

Why use classes?
// little baby alex
String nameAlex;
double weightAlex;
String nameDavid;
double weightDavid;
String nameDavid2;
double weightDavid2;
David2?
Terrible •
500 Babies? That Sucks!

Why use classes?
Name
Weight
Sex
…
Baby1

Why use classes?
Name
Weight
Sex
…
496
more
Babies
Name
Weight
Sex
…
Name
Weight
Sex
…
Name
Weight
Sex
…
Baby1 Baby2 Baby3 Baby4 …

Why use classes?
Baby1 Baby2 Baby3 Baby4
496 more
Babies …
Nursery

Why use classes?
Baby1 Baby2 Baby3 Baby4
496 more
Babies …
Nurse1 Nurse2 Nurse3 Nurse4
More nurses…
Nursery

Why use classes?
Baby
Nurse
[]
[]
Nursery

Why use classes?
Nurse
[] ER
[]
Baby
Nursery
Hospital

Class - overview
public class Baby {
String name;
boolean isMale;
double weight;
double decibels;
int numPoops = 0; Class
Definition
void poop() {
numPoops += 1;
System.out.println(“Dear mother, ”+
“I have pooped. Ready the diaper.”);
}
}

Class - overview
Class
Baby myBaby = new Baby();
Instance

Let’s declare a baby!
public class Baby {
}

Let’s declare a baby!
public class Baby {
fields
methods
}

Note
• Class names are Capitalized
• 1 Class = 1 file
• Having a main method means the class can
be run

Baby fields
public class Baby {
TYPE var_name;
TYPE var_name = some_value;
}

Baby fields
public class Baby {
String name;
double weight = 5.0;
boolean isMale;
int numPoops = 0;
}

Baby Siblings?
public class Baby {
String name;
boolean isMale;
int numPoops = 0;
XXXXX YYYYY;
}

Baby Siblings?
public class Baby {
String name;
boolean isMale;
int numPoops = 0;
Baby[] siblings;
}

Ok, let’s make this baby!
Baby ourBaby = new Baby();
But what about it’s name? it’s sex?

Constructors
public class CLASSNAME{
CLASSNAME ( ) {
}
CLASSNAME ([ARGUMENTS]) {
}
}
CLASSNAME obj1 = new CLASSNAME();
CLASSNAME obj2 = new CLASSNAME([ARGUMENTS])

Constructors
• Constructor name == the class name
• No return type – never returns anything
• Usually initialize fields
• All classes need at least one constructor
– If you don’t write one, defaults to
CLASSNAME () {
}

Baby constructor
public class Baby {
String name;
boolean isMale;
Baby(String myname, boolean maleBaby){
name = myname;
isMale = maleBaby;
}
}

Baby methods
public class Baby {
String name = “Slim Shady”;
...
void sayHi() {
System.out.println(
“Hi, my name is.. “ + name);
}
}

Baby methods
public class Baby {
String weight = 5.0;
void eat(double foodWeight) {
if (foodWeight >= 0 &&
foodWeight < weight) {
weight = weight + foodWeight;
}
}
}

Baby class
public class Baby {
String name;
boolean isMale;
int numPoops = 0;
Baby[] siblings;
void sayHi() {…}
void eat(double foodWeight) {…}
}

Classes and Instances
// class Definition
public class Baby {…}
// class Instances
Baby shiloh = new Baby(“Shiloh Jolie-Pitt”, true);
Baby knox = new Baby(“Knox Jolie-Pitt”, true);

Accessing fields
• Object.FIELDNAME
Baby shiloh = new Baby(“Shiloh Jolie-Pitt”,
true)
System.out.println(shiloh.name);
System.out.println(shiloh.numPoops);

Calling Methods
• Object.METHODNAME([ARGUMENTS])
Baby shiloh = new Baby(“Shiloh Jolie-Pitt”,
true)
shiloh.sayHi() ; // “Hi, my name is ...”
shiloh.eat(1);

Primitives vs References
• Primitive types are basic java types
– int, long, double, boolean, char, short, byte, float
– The actual values are stored in the variable
• Reference types are arrays and objects
– String, int[], Baby, …

How java stores primitives
• Variables are like fixed size cups
• Primitives are small enough that they just fit
into the cup
int double char boolean

How java stores objects
• Objects are too big to fit in a variable
– Stored somewhere else
– Variable stores a number that locates the object
Object

How java stores objects
• Objects are too big to fit in a variable
– Stored somewhere else
– Variable stores a number that locates the object
Object Object Object
Object Object Object
Object’s
location

References
• The object’s location is called a reference
• == compares the references
Baby shiloh1 = new Baby(“shiloh”);
Does shiloh1 == shiloh2?

References
• The object’s location is called a reference
• == compares the references
Does shiloh1 == shiloh2?
no

References
Name=“shiloh”
Name=“shiloh”
reference reference
shiloh1 shiloh2

References
Baby mybaby = new Baby(“davy”, true)
mybaby.name = “david”
mybaby’s
location
name = ‘davy’
ismale = true
…

References
Baby mybaby = new Baby(‘davy’, true)
mybaby.name = ‘david’
mybaby’s
location
name = ‘david’
Ismale = true
…

References
• Using = updates the reference.
baby1 = baby2
baby2
location
baby1
object
baby2
object
baby2
location
baby1 baby2

References
• Using = updates the reference.
baby1 = baby2
baby2
location
baby1
object
baby2
object
baby1 baby2

References
• using [ ] or
– Follows the reference to the object
– May modify the object, but never the reference
• Imagine
– Following directions to a house
– Moving the furniture around
• Analogous to
– Following the reference to an object
– Changing fields in the object

Methods and references
void doSomething(int x, int[] ys, Baby b) {
x = 99;
ys[0] = 99;
b.name = “99”;
}
...
int i = 0;
int[] j = {0};
Baby k = new Baby(“50”, true);
doSomething(i, j, k);
i=? j=? k=?

static
• Applies to fields and methods
• Means the field/method
– Is defined for the class declaration,
– Is not unique for each instance

static
public class Baby {
static int numBabiesMade = 0;
}
Baby.numBabiesMade = 100;
Baby b1 = new Baby();
Baby b2 = new Baby();
Baby.numBabiesMade = 2;
What is
b1.numBabiesMade?
b2.numBabiesMade?

static example
• Keep track of the number of babies that have
been made.
public class Baby {
int numBabiesMade = 0;
Baby() {
numBabiesMade += 1;
}
}

static field
• Keep track of the number of babies that have
been made.
public class Baby {
static int numBabiesMade = 0;
Baby() {
numBabiesMade += 1;
}
}

static method
public class Baby {
static void cry(Baby thebaby) {
System.out.println(thebaby.name + “cries”);
}
}
Or
public class Baby {
void cry() {
System.out.println(name + “cries”);
}
}

static notes
• Non-static methods can reference static
methods, but not the other way around
– Why?
public class Baby {
String name = “DMX”;
static void whoami() {
System.out.println(name);
}
}

main
• Why is main static?
public static void main(String[] arguments) {
}

Assignment 4
• Modeling Book and Libraries
– class Book {}
– class Library{}
• Books can be
– Borrowed
– Returned
• Library
– Keeps track of books
– Hint: use Book[]

MIT OpenCourseWare
http://ocw.mit.edu
6.092 Introduction to Programming in Java
January (IAP) 2010
For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.

LECTURE 4 CLASSES AND OBJECTS.pdf

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