# ~/

## Brandon Rozek

PhD Student @ RPI studying Automated Reasoning in AI and Linux Enthusiast.

# CPSC 220 Lecture 3

## Variables

Variable – Storage of information

The type cannot change in a variable.

Examples of types include

• int
• float
• double
• String
• char
• boolean

Declaration: int num;

Initialization: num = 5;

Declaration + Initialization: int num = 5;

### Possible Errors

You cannot declare a variable multiple times.

Undefined variables are when you do not declare a variable before attempting to use it.

### Casting

You need to cast if you are attempting to lose data or store a larger memory type into a smaller one.

double -> float -> int (casting required)

double gpa = 3.2;
int num1 = 10 * (int)gpa // 30

# Operations

The basic number operations are

• /
• % (the remainder)

Examples

0 % 2 // 0
1 % 2 // 1
2 % 2 // 0

3 % 2 // 1
4 % 2 // 0
5 % 2 // 1

3 % 5 // 3
7 % 5 // 2

You can test if something is even using modulus %

// Assuming i was initiliazed to a value earlier
if (i % 2 == 0) {
System.out.println("i is even");
} else {
System.out.println("i is odd");
}

# System input

Here is sample code using a Scanner as input

import java.util.Scanner;
public class ScannerExample {
public static void main(String[] args) {
Scanner in;
in = new Scanner(System.in);

// Grab numerical values
int num = in.nextInt();
float gpa = in.nextFloat();
double weight = in.nextDouble();

// Grab a single character
in.nextLine()
char initial = in.next().charAt(0);

// To get the entire line of a string
in.nextLine();
String name = in.nextLine();
}
}

You need to use in.nextLine() to grab the carriage return that is left after grabbing a numeric value.

# Classes and Objects

Classes are a new type that you can have multiple things of.

These classes are blueprints that are made up of primitives or more basic types.

First create a Pet.java file (Name of the class must match the name of the file)

public class Pet {
private String name;
private int years;
}

You can then use the Pet class in your main program. The terminology here is that you can create instances or objects of the class.

In PetTester.java

public class PetTester {
public static void main(String[] args) {
Pet myPet;
myPet = new Pet();
}
}

Both Pet.java and PetTester.java must be in the same directory/folder

### Mutators/Accessors

Since the variables are private we cannot access them in the main program. To work around this, we can write what is called a mutator method.

public class Pet {
private String name;
private int years;

// Mutators
public void setName(String n) {
name = n;
}
public void setYears(int y) {
if (y >= 0) {
years = y;
} else {
System.out.println("No one is less than 0 years old.")
}
}
}

Now let’s use these new methods

public class PetTester {
public static void main(String[] args) {
Pet mypet;
myPet = new Pet();
myPet.setName("Fred");
myPet.setAge(20);
}
}

We need a method that will allow us to access the data type. Let’s add accessors to our pet class.

public class Pet {
private String name;
private int years;

// Mutators
public void setName(String n) {
name = n;
}
public void setYears(int y) {
if (y >= 0) {
years = y;
} else {
System.out.println("No one is less than 0 years old.")
}
}

// Accessors
public String getName() {
return name;
}
public int getYears() {
return years;
}
}

Now let’s get some information from the pet object, such as the age.

public class PetTester {
public static void main(String[] args) {
Pet mypet;
myPet = new Pet();
myPet.setName("Fred");
myPet.setYears(20);

int year = myPet.getYears();
}
}

### Constructors

Constructors lets us initialize variables in the class without having to use mutator methods.

public class Pet {
private String name;
private int years;

// Default Constructor
public Pet() {
name = "";
years = 0;
}
// Non-Default Constructor
public Pet(int y, String n) {
name = n;
years = y;
}

// Mutator Methods
public void setName(String n) {
name = n;
}
public void setYears(int y) {
if (y >= 0) {
years = y;
} else {
System.out.println("No one is less than 0 years old.")
}
}

// Accessor Methods
public String getName() {
return name;
}
public int getYears() {
return years;
}
}

Now let us see this in action.

public class PetTester {
public static void main(String[] args) {
Pet yourPet = new Pet(10, "Fluffy");
}
}

You can have as many constructors as you want, but they must be different.

Example:

public class Pet {
...
pet() {
name = "";
years = 0;
}
pet(int y, String n) {
name = n;
years = y;
}
pet(String n) {
years = 1;
name = n;
}
...
}