Declaring and Initializing Variables:

Variables defined in a method are called “local varibales” since they are local to the method that was created.

public class MyFirstClass {
    public static void main(System[] args){
        System.out.println("Hello World");
        int x;
        x = 10;
        System.out.println(x);
        x = 5;
        System.out.println(x);
        x = x + 1;
        System.out.println(x);

        int MyFinalScore = 10;
        System.out.println(MyFinalScore);

    }
}

MyFirstClass.main(null);

Hello World
10
5
6
10

8 Primitive Data Types:

These 4 all hold integer values but having different ranges:

Byte: -128 to 127 (inclusive)

Int: -32768 to 327687 (inclusive)

Short: -2^31 to 2^31-1

Long: -2^63 to 2^63-1

The next two hold decimals, but double is more precise:

Float

Decimal

Rest are self-explantory:

Boolean: True or False

Char: Holds a character

For the AP Test, you need to know: int, double, and boolean

public class PrimitiveDataTypes {
    public static void main(String[] args){
        int a = 3;
        double b = 3.0;
        b = 3;
        //a = 3.0; doesn't convert because double can't convert to int.
        a = (int)3.999;
        System.out.println(a);

        boolean c = true;
        c = false;
        System.out.println(c);
    }
}

PrimitiveDataTypes.main(null);

3
false

Arithmetic and Uninary Operators:

public class Operators{
    public static void main(String[] args){
        int x = 3;
        System.out.println(x);
        System.out.println(3 + 3);

        x = 3 * 3;
        x = 3 * x;
        System.out.println(x);
        System.out.println(4 % 3); //output is remainder

        //x++; is x = x + 1;
        //x--; is x = x - 1;
        //x =* 3;  is x = x * 3;
        
        boolean z = true;
        System.out.println(!z);

    }
}

Operators.main(null);

3
6
27
1
false

public class IntDoubleDivision {
    public static void main(String[] args){
        System.out.println(5/3); //int division, will return 1
        
        System.out.println(5.0/3.0); //double division, will return 1.6667

        //System.out.println(5/0) will return error

        System.out.println(5/0.0); //Infinity

        System.out.println(-5/0.0); //Negative Infinity

        System.out.println(0.0/0.0); //Returns Nan (Not a Number)
    
    }
}

IntDoubleDivision.main(null);

1
1.6666666666666667
Infinity
-Infinity
NaN

Operation Precdence:

Some operators will be processed first, depending on their precedence (PEMDAS)
Precedence is same? Code goes left to right
Best used with parenthesis to clarify the order of execution

public class PEMDAS {
    public static void main(String[] args){
        int x = 9;
        int y = 6;
        System.out.println((double)x++/(3-1*2) > ((5-2) * 3));
        System.out.println(x);
        System.out.println((double)x);
        // ((double)x++/1 > 9)
        // ((double)9.0 > 9)
        // returns false
        // if we returned x after, it would return 10

    }
}

PEMDAS.main(null);

false
10
10.0

Casting a Primitive and Overflow:

Casting primitive data type allows us to convert a peice of data from one type to another type
Widening means converting a data type to a data type with more precise/more space
Narrowing means converting a data type to a data type with less precision/less space
For AP Exam, you only need to know about int, double, boolean

public class Casting{
    public static void Example1() {
        double a = 3.0;
        System.out.println(a);
 
        double b = 4; //in this case, the system automatically **widens** 4 to 4.0 
        System.out.println(b);
 
        double c = (double)5; //manually widens 5 to 5.0
        System.out.println(c);
 
        // int d = c; will cause error, c needs to narrow down from a double to int
 
        //narrowing must be done manually
        int d = (int)c;
        System.out.println(d);
 
        int e = (int)3.95;
        System.out.println(e); //truncate 3.95 --> 3
    }

    public static void Example2() {
        int x = 10;
        byte y; //value from -128 to 127

        // y = x; will cause error because of "lossy conversion", meaning system doesn't want to loose data
        y = (byte)x;

        System.out.println(y);

    }

    public static void main(String[] args){
        System.out.println("Example 1:");
        Example1();

        System.out.println(" ");
        System.out.println("Example 2:");
        Example2();
    }
}

Casting.main(null);

Example 1:
3.0
4.0
5.0
5
3
 
Example 2:
10

Swapping Variable Values:

Switch values of two variables

public class SwitchVariables {
    public static void WrongWay() {
        int x = 3;
        int y = 6;

        //wrong way, you are essentially declaring all variables equal to 6
        x = y;
        System.out.println(x); //x = 6 (y is 6);
        y = x;
        System.out.println(y); //y = 6;
    }

    public static void RightWay() {
        int x = 3;
        int y = 6;

        //correct way, declare a temporary variable that holds a value
        int temp = y;
        y = x;
        x = temp;
        System.out.println("x-value (or temp): " + temp);
        System.out.println("y-value: " + y);
    }

    public static void main(String[] args){
        System.out.println("Wrong Way:");
        WrongWay();

        System.out.println(" ");
        System.out.println("Right Way:");
        RightWay();
    }
}

SwitchVariables.main(null);

Wrong Way:
6
6
 
Right Way:
x-value (or temp): 6
y-value: 3

Unit 1 Study • 11 min read

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