Types

Note

There are two kinds of types in Java: the primitive types for storing simple values and the reference types for storing complex objects.

Primitive types

Type	Bytes	Range
byte	1	[-128, 127]
short	2	[-32k, 32k]
int	4	[-2B, 2B]
long	8
float	4
double	8
char	2	A,B,…
boolean	1	true/false

public class Main {
    
    public static void main(String[] args) {
        byte age = 30;
        short numCountries = 200;
        int income = 12_000;
        // L for long
        long viewsCount = 3_123_456_789L;

        // F for float
        float price = 10.99F;
        double newPrice = 10.88;

        // single quotation mark for char
        char letter = 'A';
        
        boolean isEligible = false;
    }
}

Reference types

All the other types except the 8 primitive types, for example the Date type:

import java.util.Date;

public class Main {
    
    public static void main(String[] args) {
        // need to allocate memory
        Date now = new Date();
        System.out.println(now);
    }
}

How to init reference types:

TypeName name = new TypeName(Arguments);

Primitive types vs Reference types

import java.awt.*;

public class Main {
    
    public static void main(String[] args) {
        // x and y: different address in memory
        byte x = 1;
        byte y = x;
        x = 2;
        System.out.println(y);  // print 1

        // point1 and point2: reference the same address in memory
        Point point1 = new Point(1, 1);
        Point point2 = point1;
        point1.x = 2;
        System.out.println(point2);  // print java.awt.Point[x=2,y=1]
    }
}

Strings

Initialize:

String message = new String("Hello World");
// we can initialize String variable like Primitive types.
String newMessage = "Hello World";

Usefull methods (String is immutable, methods that modifies a String will return a new String object):

String message = "Hello World" + "!!";
System.out.println(message.length());
// strip
System.out.println(message.trim());
System.out.println(message.toLowerCase());
System.out.println(message.endsWith("!!"));
// first index appears, if does not exists, return -1
System.out.println(message.indexOf("W"));
System.out.println(message.replace("!", "*"));

Escape sequences:

String path = "\\User\\facer";
String quotation = "\"A quotated sentence\""
String basicEscape = "\n\t"

Arrays

import java.util.Arrays;

public class Main {
    
    public static void main(String[] args) {
        // arrays are reference types, need specify the size of the array
        int[] numbers = new int[5];
        numbers[0] = 1;
        numbers[1] = 2;
        
        // will print a wired string based on the address of the array
        System.out.println(numbers);
        // print numbers
        System.out.println(Arrays.toString(numbers));
    }
}

Simpler way to initialize an array:

int[] numbers = {2, 3, 5, 1, 4};

Sort array:

int[] numbers = {2, 3, 5, 1, 4};
Arrays.sort(numbers);
System.out.println(Arrays.toString(numbers));

Multi-dimensional arrays

// 2 rows, 3 columns
int[][] numbers = new int[2][3];
numbers[0][0] = 1;

// use deepToString to print numbers
System.out.println(Arrays.deepToString(numbers));

// simpler way to initialize
int[][] newNumbers = {{1, 2, 3}, {4, 5, 6}};

Constants

// use upper case name
final float PI = 3.14F;

Casting

Implicit casting (chain: byte > short > int > long > float > double):

byte x = 1;
// byte > int
int y = x;

Explicit casting:

double x = 1;
// double > int
int y = (int)x;

When explicit casting, types needs to be compatible. We cannot cast String to int directly, the right way to do this:

// int y = (int)x will raise a Exception
String x = "1";
// similarly, we have Short.parseShort(), Float.parseFloat() ...
int y = Integer.parseInt(x) + 2;