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
System.out.printIn ++ i;
| System.out.printIn ++ i;
';' expected
| System.out.printIn ++ i;
not a statement
| System.out.printIn ++ i;
cannot find symbol
symbol: variable printIn
| 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);
| int i ++;
';' expected
| System.out.printLine + i;
not a statement
int i = Integer.MAX_VALUE;
System.out.println(i);
i++;
System.out.printLn(1);
2147483647
| System.out.printLn(1);
cannot find symbol
symbol: method printLn(int)
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.”
System.out.println(2 + 7 % 5 * 2);
System.out.println(2 * 2 + 7 % 5);
System.out.println("1" + "2");
System.out.println((10 + 2) / 0);
6
6
12
---------------------------------------------------------------------------
java.lang.ArithmeticException: / by zero
at .(#63:1)
int a = 10;
int b = 20;
int c = 30;
int d = 40;
int e = 50;
boolean result = ((a + b - c * d / e) % 2 == 0) && ((a & b | c ^ d) != e) && !(c < a || d > e);
System.out.println(result);
true
System.out.println(((10 + 20) - (3 * 4)) / (40 % 6));
4
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");
int a = 4;
System.out.println(a + 2 / 1.2);
5.666666666666667
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 | 1 | Divide one number by another. |
| 5/3.0 | 1.67 | Divide number by a double | |
| % | 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);
3
3
4
1
1.5
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?
//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>();
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?
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
double roundMe = 4.7;
double roundedValue = Math.round(roundMe);
System.out.println(roundedValue);
5.0
Homework
All of your homework is on this form. (Link is https://forms.gle/M6FgxZwX1AnWdZmL9)
Create a program using enums and a switch statement. The values in the enum are the days of the week. If it is Tuesday, then print, “Today is Tuesday!”.
public class DayOfWeekChecker {
enum DayOfWeek {
SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY
}
public static void main(String[] args) {
DayOfWeek today = DayOfWeek.TUESDAY;
switch (today) {
case TUESDAY:
System.out.println("Today is Tuesday!");
break;
default:
System.out.println("It's not Tuesday today.");
break;
}
}
}
DayOfWeekChecker.main(null)
Today is Tuesday!
Create a calculator program that uses both int and double types depending on user input. Include at least 3 operations.
import java.util.Scanner;
public class Calculator {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
System.out.println("Welcome to the Calculator!");
System.out.print("Choose data type (1 for int, 2 for double): ");
int dataTypeChoice = scanner.nextInt();
System.out.print("Enter the first operand: ");
double operand1;
if (dataTypeChoice == 1) {
operand1 = scanner.nextInt();
} else if (dataTypeChoice == 2) {
operand1 = scanner.nextDouble();
} else {
System.out.println("Invalid choice. Please choose 1 for int or 2 for double.");
scanner.close();
return;
}
System.out.print("Choose operation (1 for addition, 2 for subtraction, 3 for multiplication): ");
int operationChoice = scanner.nextInt();
System.out.print("Enter the second operand: ");
double operand2;
if (dataTypeChoice == 1) {
operand2 = scanner.nextInt();
} else {
operand2 = scanner.nextDouble();
}
double result = 0;
switch (operationChoice) {
case 1:
result = operand1 + operand2;
break;
case 2:
result = operand1 - operand2;
break;
case 3:
result = operand1 * operand2;
break;
default:
System.out.println("Invalid operation choice.");
scanner.close();
return;
}
System.out.println("Result: " + result);
scanner.close();
}
}
Calculator.main(null)
Welcome to the Calculator!
Choose data type (1 for int, 2 for double): Enter the first operand: Choose operation (1 for addition, 2 for subtraction, 3 for multiplication): Enter the second operand: Result: 0.0