Java is used around the world to create applications and is one of the most popular coding languages. The reason Java is so popular is because of it’s security and versatility provided by it’s Object Oriented nature.

1.1 Basics

public class Main {
  int x = 5;

  public static void main(String[] args) {
    Main myObj = new Main();
    System.out.println(myObj.x);
  }
}

1.2 Variables and Data Types

Variables

A Variable is a name given to a memory location that is holding the specified value. Here are some naming practices:

Use camel case. likeThis.
Don’t start with a number.
Spaces are not allowed.
No reserved characters, like $, @, and &

Java is a strongly typed language so you always need to declare the type of the variable. Variables can also be declared on their own or in the same line as when they are given a value:

int primitive5;
primitive4 = 1;

//Or...
int primitive6 = 1;

What are the greatest values integers and doubles can store?

Primitive Data

Primitive data determines the size and type of information. Primitive types are the most simple type of variable. They are simply store a short amount of raw data, and are not associated with another class.

Here are the different primitive types:

byte: An 8-bit signed two’s complement integer.
short: A 16-bit signed two’s complement integer.
int: A 32-bit signed two’s complement integer.
long: A 64-bit signed two’s complement integer.
float: A single-precision 32-bit IEEE 754 floating point.
double: A double-precision 64-bit IEEE 754 floating point.
boolean: Stores either true or false.
char: Stores a single character.

For this class you need to know:

boolean primitive3 = true; //Stores a true of false binary value
int primitive1 = 0; //Whole number
double primitive2 = 1.1; //Decimal values. Floating point numbers.
char primitive4 = 'a'; //Single character

Data Type	Size (bits)
boolean	8
int	32
double	64
char	16

public class GreatestValue {
    public static void main(String[] args) {
        System.out.println(Integer.MAX_VALUE);
        System.out.println(Integer.MIN_VALUE);
        System.out.println(Double.MAX_VALUE);
        System.out.println(Double.MIN_VALUE);
    }
}
GreatestValue.main(null);

2147483647
-2147483648
1.7976931348623157E308
4.9E-324

Reference Types

Some data types, like String, start with a capital letter. This is because they are not primiative, but are refrence types. They are called this because they refrence an object.

“A reference type is a code object that is not stored directly where it is created, but that acts as a kind of pointer to a value stored elsewhere.”

int integer = 7120; //"int" starts with a lowercase
String string = "abc"; //"String" starts with an uppercase, because it is an object and not a primitive type

All Reference Types Reference Objects: String Example

String is the most common reference type. Here is an example of how a String type is really just referencing an object.

public class WorseString {
    private char[] charArray;

    public WorseString(String inputString) {
        this.charArray = inputString.toCharArray();
    }

    public String getChars() {
        return new String(this.charArray);
    }

    @Override
    public String toString() {
        return getChars();
    }
}

WorseString string = new WorseString("Hello, World!");
System.out.println(string);

Hello, World!

Therefore, these two things are the same:

String string = "abc";
String string = new String("abc");

Literal vs String literal

Literal: Source code representation of a fixed value — 3
String Literal: In double quotes, a String — “3”

1.3 Expressions and Assignment Statements

Calculations and evaluating arithmetic statements is important when coding to create algorithms and other code. Make sure you are doing arithmetic statements with int or double values and not String literals

Operators

Operator	Example Equation	Output	Use
+	5 + 3	8	Add numbers together.
-	5 - 3	2	Subtract one number from another.
*	5 * 3	15	Multiply numbers together.
/	5 / 3.0	1.67	Divide one number by another.
/	5/3	1	Divide one number by another
%	5 % 3	2	Find the remainder of a division operation.

Tip: In the AP subset, you only have to worry about operations with int values. However, it’s good to know how to use arithmetic statements with doubles and other types.

If you do an operation with two ints or doubles, it returns the respective type. If you mix types, Java returns the one with more bits, a double in this case.

Modulus

Modulus gets the remainder if you were to divide two numbers. One common use is to find odd/even numbers.

5 % 2 = 1
100 % 10 = 0

You try:

8 % 3 = ?
4 % 1 = ?

Modulus joins multiplication and division in the order of operations

Assignment Operator

= is called the assignment operator because it is used to assign a value to a variable. It is the last in the order of operations.

Casting

Casting is converting one type of variable to another ex: double to int

public class Casting {
    public static void main(String[] args) {
        double castTest = 3.2;
        System.out.println((int) castTest);
        castTest = 3.7;
        System.out.println((int) castTest);
        System.out.println((int) (castTest+0.5));

        int castTest2 = 3;
        System.out.println(castTest2/2);
        System.out.println(castTest2/2.0);
    }
}
Casting.main(null);

What will this output? castTest2 = 7; System.out.println(castTest2/3); System.out.println((int) (castTest2+0.5));

Wrapper Classes

For many operations in Java, you need to have a class. Some examples are:

ArrayLists or HashMaps
If you require nullability (meaning the value could be null)
Generics
Methods that require objects as input

To accomplish this, we use a wrapper class. A wrapper class is essentially a class which ‘wraps’ the primitive type and makes it into an object rather than a primitive.

What is a downside of using wrapper classes? Why not always use them?

Increased memory usage

//This code fails
ArrayList ArrayList = new ArrayList<int>();

|   ArrayList ArrayList = new ArrayList<int>();

unexpected type

  required: reference

  found:    int

//This code works
ArrayList ArrayList = new ArrayList<Integer>();

wrapper

public class Wrappers {
    Integer ageWrapper = 17;
    int age = Integer.parseInt("17");
    String gpaString = "3.9";
    double gpaDouble = Double.parseDouble(gpaString);

    public static void main(String[] args) {
        Wrappers wrapper = new Wrappers();
        System.out.println(wrapper.ageWrapper);
        System.out.println(wrapper.age);
        System.out.println(wrapper.gpaDouble);
    }
}
Wrappers.main(null);

17
17
3.9

How do you complete this output so that it outputs an integer String grade = “95”; ?

How do you complete this output so that it outputs a double? String grade = “95.5”; ?

Enums

What are they?

Enums are a type of data, which allows a variable to be a predetermined set of values

Uses

Examples: days of the week

Things you can do with Enums

ordinal
switch
for loops

public class EnumTest { 
    enum Units {
    PRIMITVE_DATA_TYPES,
    CLASSES,
    BOOLEAN,
    ITERATION,
    WRITING_CLASSES,
    ARRAY,
    ARRAY_LIST,
    TWO_DIMENSIONAL_ARRAY,
    INHERITANCE,
    RECURSION;
}
public static void main(String[] args) { 
  System.out.println("What is the third unit in AP CSA? Answer: " + Units.BOOLEAN);
  Units classUnit = Units.CLASSES;
  System.out.println("What is the unit is Classes in AP CSA? Answer: " + (classUnit.ordinal() + 1));
  Units selectedUnit = Units.ARRAY_LIST;

  switch(selectedUnit) {
    case PRIMITVE_DATA_TYPES:
      System.out.println("The selected unit is: primitive data types");
      break;
    case BOOLEAN:
       System.out.println("The selected unit is: boolean");
      break;
    case CLASSES:
      System.out.println("The selected unit is: classes");
      break;
    case ITERATION:
      System.out.println("The selected unit is: iteration");
      break;
    case WRITING_CLASSES:
      System.out.println("The selected unit is: writing classes");
      break;
    case ARRAY:
      System.out.println("The selected unit is: array");
      break;
    case ARRAY_LIST:
      System.out.println("The selected unit is: array list");
      break;
    case TWO_DIMENSIONAL_ARRAY:
      System.out.println("The selected unit is: 2d array");
      break;
    case INHERITANCE:
      System.out.println("The selected unit is: inheritance");
      break;
    case RECURSION:
      System.out.println("The selected unit is: recursion");
      break;
  }
  for (Units allUnits: Units.values()) {
    System.out.println(allUnits);
  }
} 
}
EnumTest.main(null);

What is the third unit in AP CSA? Answer: BOOLEAN
What is the unit is Classes in AP CSA? Answer: 2
The selected unit is: array list
PRIMITVE_DATA_TYPES
CLASSES
BOOLEAN
ITERATION
WRITING_CLASSES
ARRAY
ARRAY_LIST
TWO_DIMENSIONAL_ARRAY
INHERITANCE
RECURSION

System.out.println(((10 + 20) - (3 * 4)) / (40 % 6));

Homework

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