javaLearning

personal learning note for JAVA

basic steps

1.create MyFirstJavaClass.java (the class name (CamelCase) is same as the file name.)

public class HelloWorld {
    public static void main(String[] args){
            System.out.println("Hello World");
    }
}

2.javac MyFirstJavaClass.java to created MyFirstJavaClass.class

3.java MyFirstJavaClass to run MyFirstJavaClass

extends

use super() to inheritate parent constructions and functions use @Override if child function needs to override parent function

JAVA_HOME (win version)

to tell the operation system the jdk path. For example: Path %JAVA_HOME%\bin

type

byte, short, int(default), long

float, double (default), char, boolean

String

auto type transfer (byte,short,char) == int -> long -> float -> double

type a = 12
int b = a
System.out.println(b) // 12

**two digit

System.out.printf("%.2f\n", d);

force type transfer

int a = 20;
byte b = (byte)a;

operations

"/" int divide int is still int. e.g. 5 / 2 = 2. To get 5 / 2 = 2.5, use 1.0 * 5 / 2 = 2.5

++,-- :plus 1, minus 1

int a = 10;
a++ // a=11;

int i = 10;
int rs = ++1;
// plus first, then set value
// rs=11; i=11;

int j = 10;
int rs2 = j++;
// set value first, then plus
// rs2=11; j=10;

=，+=，-=，*=, /=, %= // calculate first, then set value

, >=,<,<=, ==, !=

&, |, !, ^ , &&, ||

&& is Short-circuit evaluation, only run left logic if left is false. its result is the same as &
&& is prior to ||
*/ is prior to +-
^ return if two logic are different
2 >1 ^ 3>1 // =false,

flow

if, switch, for, while, do{}while(),

random

import java.util..Random;
Random r = new Random();
int data = r.nextInt(10); // 0-9

array

int[] arr = {20,10,80};

String[] arr2 = {"Adam","Ben", "Charlie"};

int arr3[] = {1,2,3};

int[] arr = new int[3] // create an array whose length is 3

print multidimensional Array: Arrays.deepToString()

input: Scanner

allow user to input a double

import java.util.Scanner
Scanner sc = new Scanner(System.in)
double score = sc.nextDouble()

method

              void
public static int sum(int a,int b){
    int c = a+b;
    return c; // type is the same as the return type above
}

constructor

init Class Object

public Student(){

}
public Student(String name, double score){

}

Encapsulation

private, public

import

import package.class

Java Base API

java.lang.String (No need to import java.lang.*)

String name = "test"; (string is unchanged object)

String str = public String(STRING/CHAR[]/BYTE[]);

length()

charAt(int index)

toCharArray()

equals(Object anObject)

String s1="abc";
String s2="ab";
String s3=s2+"c;
String s4="a"+"b"+"c"

// s1 == s3 returns false;
// s1.equals(s3) returns true;
// s1 == s4 returns true;

equalsIgnoreCase(String str)

substring(int startIndex, int endIndex?)

replace(CharSequence target, CharSquence replacement)

contains(CharSequence s)

startsWith(String prefix)

split(String regex)

java.util.ArrayList ArrayList()

add(E element) // to the last

add(int index, E element)

remove(int index);

remove(Object o);

get(int index)

size()

set(int index,E element)

input

import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in); // create Scanner
        System.out.print("Input your name: "); 
        String name = scanner.nextLine(); // input string
        System.out.print("Input your age: "); 
        int age = scanner.nextInt(); // input integer
        System.out.printf("Hi, %s, you are %d\n", name, age);
    }
}

check args

public class Main {
    public static void main(String[] args) {
        for (String arg : args) {
            if ("-version".equals(arg)) {
                System.out.println("v 1.0");
                break;
            }
        }
    }
}

abstract class

cannot be used with 'new', only for inheritage.

abstract class Person {
    public abstract void run();
}

interface is the abstract class with only functions class can implements more than 1 interface