Java Vocab

Type Casting

Type casting in Java is when you assign one primitive data type to another data type. The two types of casting are as follows:

• Widening Casting (automatically) - converting a smaller type to a larger type size
  byte -> short -> char -> int -> long -> float -> double

• Narrowing Casting (manually) - converting a larger type to a smaller size type
  double -> float -> long -> int -> char -> short -> byte

public class wideningCasting {

    public static void main(String args[]) {

        int myInt = 9;
        double myDouble = myInt;

        System.out.println(myInt);
        System.out.println(myDouble);

    }
}

// widening is done automatically when you assign a smaller type to a larger one
wideningCasting.main(null);

9
9.0

public class narrowingCasting {

    public static void main(String args[]) {

        double myDouble = 9.5;
        int myInt = (int) myDouble;

        System.out.println(myDouble);
        System.out.println(myInt);
    }
}

/* 
in order to convert a larger data type to a smaller one, you must use (dataType) when defining the variable, or 
you will recieve an error about lossy conversion 
*/

narrowingCasting.main(null);

9.5
9

Wrapper Classes

A wrapper class provdies a way to use primitive data types as objects. Examples of primitive types and wrapper classes below.

Primitive Type	byte	short	int	long	float	double	boolean	char
Wrapper Class	Byte	Short	Integer	Long	Float	Double	Boolean	Character

Java example -

ArrayList<int> myNums = new ArrayList<int>(); // invalid

ArrayList<Integer> myNums = new ArrayList<Integer>(); // valid

Concatenation

In java, concatenation is the combination of multiple strings to make one string. Below are the two ways of doing this -

// just add the strings and java does the work for you

public class additionConcat {

    public static void main(String args[]) {

        String string1 = "Hello ";
        String string2 = "World!";
        String string3 = string1 + string2;

        System.out.println(string3);
    }
}

additionConcat.main(null);

Hello World!

// the manual concat() method

public class manualConcat {

    public static void main(String args[]) {

        String string1 = "Hello ";
        String string2 = "World!";

        System.out.println(string1.concat(string1));
        System.out.println(string1.concat(string2));
        System.out.println(string2.concat(string2));
        System.out.println(string2.concat(string1));
    }
}

// using the concat() method is a more dynamic and customizable way to concatenate strings
manualConcat.main(null);

Hello Hello 
Hello World!
World!World!
World!Hello 

Math Class

The Math class in java has many useful methods that run math functions, such as: absolute value, round up, round down, generate a random number, etc...

import java.lang.Math;

public class mathRandom {

    public static void main(String args[]) {

        /*
         * This generates a random number between 0-1, multiplies this large decimal by 100, and then rounds it,
         * so it's basically generating a random number between 0-100
         */
        for(int i = 0; i < 10; i++) {
            System.out.println(Math.round(Math.random()*100));
        }
    }
}

// the random class is much better in my opinion than Math.random, but this method will generate a random double between 0-1
mathRandom.main(null);

3
96
57
24
48
66
82
22
8
43

Compound Boolean Expressions / De Morgan's Law / Truth Tables

AND	OR	NOT
&&	||	!

De Morgan's Law:

When the NOT expression is applied to an AND, it will switch it to an OR. The same happens from OR to AND.

Truth Tables:

A truth table is just a way of organizing true/false conditions.

A = true
B = false
C = true
D = true

	AND	OR	XOR
A, B	false	true	false
C, D	true	true	true

public class compBools {

    public static void main(String args[]) {

        if(true && false) {
            System.out.println("True and false.");
        }

        if(true || false) {
            System.out.println("True or false.");
        }

        
// "NOT(true AND false)" is equivalent to "false OR true"
        if(!(true && false)) {
            System.out.println("False or true.");
        }
    }
}

compBools.main(null);

True or false.
False or true.