Java Classes and Objects:

In OOP (Object Oriented Programming), objects have…

States (fields in Java)
Behaviors (methods in Java)

Example: Lightbulb Object

States:
- color
- wattage
Behaviors:
- turnOn
- turnOff
- flash

What is a class?

A class is a blueprint (or template) for a object

Lightbulb Example (Again)

    Lightbulb x = new Lightbulb();
    Lightbulb y;
    y = new Lightbulb();
    Lightbulb z = new Lightbulb("green");
    y.turnOff(); //example, doesn't run because some methods aren't defined

Methods that don’t need an object are called static methods
Methods where you need to create an object first are called nonstatic methods

Different Ways to Create Objects

    Lightbulb w = new Lightbulb(); //typical
    String x = "hello"; //string
    int[] y = new int[5]; //array
    int[] z = {3, 4, 1} // array

Methods:

Methods are blocks of code that can be called from another program
May be returned to the part of the prgoram that called them
They pass parameters that allows us to pass them data
Void return type means return statement is optional
- Method with any other return type means data of the correct type MUST be returned back
Methods end as soon as there is a return type, regardless even if there is more code

public class Method {
    public static int addTwoNums(int num1, int num2){ //must return a type int, given 2 parameters
        int total = num1 + num2;
        return total;
    }

    public static int averageOfTwo(int numA, int numB){
        return addTwoNums(numA, numB) / 2;
    }

    public static void printAverage(int num3){
        System.out.println("Average is " + num3);
    }

    public static void main(String[] args){
       int sum = addTwoNums(5,3); //parameters have to match
       int avg = averageOfTwo(7,5);
       printAverage(avg);
    }
}

Method.main(null);

Average is 6

Extra Notes:

A method can have multiple return statements, but only one is activated every time the method is run
You must have the same number, type, and order of the argument(s) as parameter(s)

Escape Sequence

Definition: A way to represent certain special characters inside a String

Example:

System.out.println("This is a test message.");

This prints out “This is a test message.”

Incorrect Way:

System.out.println("This is a "test" message.");

Will lead to error, test is not printed out as a string.

Correct Way:

System.out.println("This is a \"test\" message.");

This will print out “This is a “test” message.”

Chart with all Escape Sequences:

For the AP Test, you only need to know first 3.

public class IntDoubleDivision {
    public static void main(String[] args){
        System.out.println("abc \\ \' def \n \" ghi");
    }
}

IntDoubleDivision.main(null);

abc \ ' def 
 " ghi

String Class:

Strings are a data type that hold text
Internally Strings are an array of the pirimitve type char
String class has a variety of methods to compare and modify Strings

public class StringClass {
    public static void main(String[] args){
        String a = "hello";
        String b = "hello"; //in a heap, both object `b` and `a` are directed to the string "hello" to save memory

        String c = new String("hello"); //will create a second object in heap
        String d = new String("hello");

        System.out.println(a == b); //both 'a' and 'b' are pointed to the same memory location
        System.out.println(c == d); //two different memory locations where the data is stored

        System.out.println(a.equals(b)); //true
        System.out.println(c.equals(d)); //false, two different memoring locations

        System.out.println(c.equals("hello")); //true

        a = "hola"; //strings are immutable, we can't change the data on the heap
        // will create a as a brand new object under "hola" instead of "hello"

        String e = "hi";
        String f = e;
        String g = "howdy";
        String h = "HI";

        System.out.println(e.equals(f)); //true
        System.out.println(e.equals(h)); //false, capitalizatoin is accounted for

        System.out.println(e.equalsIgnoreCase(h)); //true, ignoring capital case
        System.out.println(g.length()); //5
        System.out.println(g.substring(2)); //returns "wdy", starting at index 2
        System.out.println(g.substring(1,4)); //(inclusive, exclusive), includes index 1 but not 4!! Returns "owd"
        System.out.println(h.compareTo(e)); //-32

        int i = h.length();

        if (e.equals("hi")){
            System.out.println("e equals hi");
        }
    }
}

StringClass.main(null);

true
false
true
true
true
true
false
true
5
wdy
owd
-32
e equals hi

Math Class:

Math class contains a variety of methods
All methods are static
Can’t extend or create an instance of the methods
Can pass a int, double, float, or long
Will return the same data type (usually)

Examples:

Math.abs(-3.2) //taking absolute value, returns 3.2

Math.pow(3,2.0) //this method will always return double. (base, exponent), in this cause it returns 9.0

Math.sqrt(9) //returns a double like Math.pow, this one returns 3.0

Math.PI //is a public variable, a double type approximately equal to pi
double circumference = 2 * Math.PI * radius //ex

Other Useful Methods:

Random Numbers with Math.random()

Several ways to genearte random numbers in Java, one of which is Math.random()
Includes many mathematical methods including random()
It’s a static method, you can call it without an instance of Math class

For example:

0.0 <= Math.random() < 1.0 //number between 0.0 and 0.99999...

int randNum = (int)(Math.random()*11); //number between 0 and 10
System.out.println("Random Number between 0 and 10: " + randNum);

int randNum2 = (int)(Math.random()*10 + 1); //number between 1 and 10
System.out.println("Random Number between 1 and 10: " + randNum2);

System.out.println(" ");

String[] name = {"Dorothy", "Glenda", "Toto"};
int randNum3 = (int)(Math.random() * 3);
String chosenName = name[randNum3];
System.out.println("Random Name chosen from array \"name\": " + chosenName);

int coinFlip = (int)(Math.random() * 2);
if (coinFlip == 0) {
    System.out.println("Heads");
}
else {
    System.out.println("Tails");
}

Random Number between 0 and 10: 8
Random Number between 1 and 10: 4
 
Random Name chosen from array "name": Dorothy
Tails

Scanner:

public class UserInput {
    public static void main(String[] args){
        Scanner keyboard = new Scanner(System.in);

        System.out.println("Type in your height: ");
        double x = keyboard.nextDouble();
        System.out.print(x);

        System.out.println(" \nWhat is your age? ");
        int y = keyboard.nextInt();
        System.out.println(y);

        System.out.println("Are you female? "); //can only respond with true or false
        boolean isFemale = keyboard.nextBoolean();
        System.out.println(isFemale);
    }
}

UserInput.main(null);

Type in your height: 
12.3 
What is your age? 
2
Are you female? 
true

Special case with keyboard.next() and keyboard.nextLine()

public class NextvsNextline {
    public static void main(String[] args) {
        Scanner keyboard = new Scanner(System.in);

        // Using next()
        System.out.print("Enter your first name: ");
        String firstName = keyboard.next(); // Reads the first word, ex. Derrick Huang will only read Derrick
        System.out.println("First Name: " + firstName);

        // Using nextLine()
        System.out.print("Enter your full name: ");
        keyboard.nextLine(); // Consume the newline character left by next()
        String fullName = keyboard.nextLine(); // Reads the entire line, ex. Derrick Huang will read Derrick Huang
        System.out.println("Full Name: " + fullName);

        keyboard.close();
    }
}

NextvsNextline.main(null);

Enter your first name: First Name: Derrick
Enter your full name: Full Name: Derrick Huang

Unit 2 Study • 17 min read

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