1 Make a Class

约 280 个字 99 行代码 2 张图片预计阅读时间 2 分钟

print("hello world") - JAVA: error: class, interface enum or record expected - class(类) is expected in JAVA \(\Rightarrow\) JAVA is an object-oriented(面向对象) programming language

public class HelloWorld{ //define
//main method is written in class
    public static void main(String[] args){

    }
}

This equals to the blank program written in python \(\rightarrow\)

the whole program is \(\rightarrow\)

public class HelloWorld{ //define
//main method is written in class
    public static void main(String[] args){
        System.out.println("Hello world");//print hello world
    }
}

In Java, all code must be part of a class, which defines with public class
inside must be public static void main(String[] args){ and }

2 Hello Number

we need to help Java find x

In Java, you have to declare that it exists first (Actually make it easier to read the code ~~maybe a bit longer~~ )

public class HelloWorld{ 
    public static void main(String[] args){
        int x;
        x=0;
        while(x<10){
            System.out.println(x);
            x=x+1;
        }
    }
}

variable has types. it is NOT admitted to say x='horse' after you declare int x;
- Actually python has types as well. You can't add horse and 5:

x='horse'+5
print(x)

\(\Rightarrow\) Invalid Syntax

in Java, code can't even run...

3 function

public class HelloWorld{ 
    public static int larger(int x, int y){
        if (x>y){
            return x;
        }
        else{
            return y;
        }   
    }
    public static void main(String[] args){
            System.out.println(larger(-5,10));
        }
    }

to define a function we use public static
it can only return ONE value

4 how Java functions?

alt text

5 Features of Object-Oriented Programming

Modularity(模块化)
- interact with the module without knowing what's inside
- how?
  - we write a class to build a module
  - write codes inside to create instances(创建实例) of that class

in Python, you write:

Class car:
    def __init__(self,m):  
    # constructor, to create a new instance
        self.model = m
        self.wheels = 4
    def drive(self):
        if self.wheels<4:
            print(self.model + "break down")
            return;
        print(self.model + "booooooooooooooom")
    def getNumWheels(self):
        return self.wheels
    def driveintoDitch(self,wheelsLost):
    # wheelsLost represents the num lost by driving into ditch
        self.wheels=self.wheels-wheelsLost
    # That means: in a class, you can also change the object itself:

c1 = Car("c1")
c2 = Car("c2")
c1.drive()
c1.driveIntoDitch(2)
c1.drive()
print(c2.getNumWheels())

in Java, it's:

...pay attention to this

public class Car{
    public String model;
    public int wheels;
    public Car(String m){
        this.model=m;
        this.wheels=4;
        // "this" is a reference to the current object being created here
    }
    public void drive(){
        if (this.wheels<4){
            System.out.println(this.model+"break down");
            return;
        }
        System.out.println(this.model+"boooooooooooom")
    }
    public int getNumWheels(){
        return this.wheels;
    }
    public void driveIntoDitch(int wheelsLost){
        this.wheels = this.wheels - wheelsLost
    }
    public static void main(String[] args){
    // to execute, put it in function main

    // firstly, declare that c1&c2 exists
        Car c1;
        Car c2;
        c1 = new Car("c1");
        c2 = new Car("c2"); //new helps Java know that you're creating a new object
        c1.drive();
        c1.driveIntoDitch(2);
        c1.drive();
        System.out.println(c2.getNumWheels());
}

Tips:

    public int getNumWheels(){
        return this.wheels;
    }

in this function, actually this is optional -> there's only ONE wheel in this script

alt text

6 Some Syntax

Some boolean operators are as follows:

Python	Java
`and`	`&&`
`or`	`
`not`	`!`
`==`	`==`

Exponentiation

Python	Java
`x = 2**10`	`int x = Math.pow(2, 10);`

Note that ^ in Java is the "XOR" operator, not the exponentiation operation. That is, 2 ^ 10 is valid code, but it will return 8, not 1024.