# ~/

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

# CPSC 220 Lecture 4

## Practice Problem

1. Create a class called Car
• Create a private variable of int type called year
• Create a private variable of String type called make
2. Create accessor methods for all data members.
3. Create mutator methods for all data methods.
``````public class car { // begin car
private int year;
private String make;
public int getYear(void) {
return year;
}
public String getMake() {
return make;
}
public void setYear(int y) {
if (y > 1890) {
year = y;
} else {
System.out.println(y + " is not a valid year.");
}
}
public void setMake(String m) {
make = m;
}
}
``````

Local variables are only existent within the curly braces that it is defined in.

## If Statements and Boolean Expressions

Boolean expressions return a boolean

``````1 < 4; // 1 is less than 4: TRUE
3 > 5; // 3 is greater than 5: FALSE
5 == 5; // 5 is equal to 5: TRUE
5 != 5; // 5 is not equal to 5: FALSE
1 >= 1; // 1 is greater than or equal to 1: TRUE
5 <= 1; // 5 is less than or equal to 1: FALSE
``````

If statements only occur if the boolean expression is true, otherwise the `else` block is executed.

``````if (true) {
System.out.println("I am always printed");
} else {
System.out.println("I am never printed");
}
``````

You can only have one `else` per `if`. If you have an `if` you don’t necessarily need an `else`

## Local vs Class Variables

If you have a local variable and the class variable sharing the same name, then the local variable is always used first.

``````public class car { // begin car
private int year;
public void setYear(int year) {
year = year;
}
}
``````

This is a redundant statement, it makes the argument that is passed in equal to itself.

To avoid this situation, use the keyword `this` to access the class variable

``````public class car {
private int year;
public void setYear(int year) {
this.year = year;
}
}
``````

The code above runs as expected.

Rewriting our class with `this`

``````public class car { // begin car
private int year;
private String make;
public int getYear(void) {
return year;
}
public String getMake() {
return make;
}
public void setYear(int year) {
if (y > 1890) {
this.year = year;
} else {
System.out.println(y + " is not a valid year.");
}
}
public void setMake(String make) {
this.make = make;
}
}
``````

## Unreachable Code

When the code hits a `return` statement, it stops executing the rest of the code in the method. Also throws an Unreachable Code Error.

``````public int add(int x, int y) {
return x + y;
System.out.println("x + y = " + x + y);
}
System.out.println("Hello");
``````

Here the code above will not compile, though assuming the error doesn’t exist then it would only print out “Hello”

## Constructors

You cannot have a private or protected constructors.

Constructors are used to initialize your objects.

You want to have the class variables to the left of the assignment statement.

``````public class car {
private int year;
private String make;
car() {
year = 1890;
make = "Ford";
}
car(int year, String make) {
this.year = year;
this.make = make;
}
}
``````

## Testers

Testers are useful to check that the class is implemented correctly. Both the tester and the class have to be in the same folder/directory.

``````public class carTester {
public static void main(String[] args) {
Car myCar; // Declaration
myCar = new Car(); // Initilization
Car yourCar = new Car(2009, "Hyundai"); // Declaration + Initialization
}
}
``````

``````public class Car {
private String name;
private int odometer;
public void setOdometer(int od) {
odometer = od;
}
public void setName(String n) {
this.name = n;
}
public void changeOilRequest(String name, int od) {
if (name == this.name) {
int difference = od - this.odometer;
if (difference > = 3000) {
// You can call other methods in the class
setOdo(od);  // Equivalent to "this.setOdo(od);"
this.odometer = od;
} else {
System.out.println(name + " not ready for oil change.")
}
} // end if
} // end changeOil request
} // end class
``````

To call public methods outside the class use the variable name to do so.

``````public class CarTester {
public static void main(String[] args) {
Car myCar = new Car();
myCar.setName("Honda")
myCar.changeOilRequest("Honda", 3400);
}
}
``````

## Math library

The `ceil` method rounds up while the `floor` method runs down.

``````Math.ceil(3.2); // 4
Math.ceil(4.1); // 4
``````