Chao216 / hsp-java

Now I will start learning Java 2023年 06月 18日 星期日 19:42:08 CST

NOTE ! when you assign variable as char, Always use '', NOT "", because "" is for string, while '' is seen as char!!! :)

difference between i++ and ++j

when you use them for while/for loop, they behave the same
but when you assign to new variables, they works differently!

int i = 1;
int j = 1;

int k = i++; // step1 create var_temp = i, step 2 i = i + 1, step 3 k = temp
int l = ++j; // step1 j = j + 1, step2 temp = j, step3 l = j

// Note! both i and j will be 2 after ++ operation !!!

evaluate String

when you only want to compare value, not the address. you shoudl use String1.equals(String2), not ==

return must be placed in the end of code block inside methods;

some shortcuts in IDEA
Ctrl + Y to delete line
Ctrl + D to duplicate line
Ctrl + Alt + L to reformat codes
Shift + F10 to execute codes!

modifier	self	same package	child class	different package
public	x	x	x	x
protected	x	x	x	o
default	x	x	o	o
private	x	o	o	o

polymorphic

if Animal is a Super class, and Cat and Dog are sub class

and we define a speak() seperately in 3 classes above

public  void speak(){
        System.out.println("Hi I am an animal!");
    }

public void speak(){
        System.out.println("Hi, I am a cat!");
    }

public void speak(){
        System.out.println("Hi, I am a Dog!");
    }

if we do the follow

	Animal animal = new Cat();//编译类 Animal, 运行类Cat
        animal.speak();
        animal = new Dog(); //编译类还是ANmial, 运行类为Dog
        animal.speak();
        //Animal 是 Dog, Cat 的父类。

following results will show on screen

Hi, I am a cat!
Hi, I am a Dog!

polymorphic in Java

if we create super class Animal, and then make sub class Tiger, and Sheep
same idea A super class Food, then sub class Chicken,Grass
we can design method with parameters feed(Animal animal, Food food). But we can pass in sub class as arguments
such as *.feed(sheep1,grass1) where sheep1 , and grass1 from instances of subclass Sheep, and Grass

public static void main(String[] args) {
        Master Chiang = new Master("Chiang Kai-SHik");
        Grass nzGrass = new Grass("fresh new zealand grass");
        Sheep sheep1 = new Sheep("Shon");
        Chiang.feed(sheep1,nzGrass);
    }

Static variable and Static Method

Static method can only access static varibale, mormal method can access both static and regular varibales.

sequence of running priority static extends

1. super class static code block, varibale initialization
2. child class static code block, varibale initializtaion
3. super class non-static code block varibale initialization
4. super class constructor
5. child class non-static code block varibale initialization
6. child class constructor