【CS 61b study notes 1】Java Intro

Intro

Java is a statically typed language, which means that all variables, parameters and methods must have a declared type. After declaration , the type can never change. And before executing the code ,Java will check the type first , so it will never run into type erroe.

Basic syntax

• All statements must end with a semi-colon ‘;’

• Curly brace{} : which enclose section of code (functions classes and etc.)

• Declare varible ：variable must be declared before it used , and must be given a type.

• the difference between System.out.println() and System.out.print()

  • println will include a new line (or a return)
  • System.out.print() must have the parameter(s) in the brackets

• Commentary

  • Java comments can either start with ‘//’ or can extend over any number of lines using “/*” and “*/”
  • “/**” : documentation comments, which can be interpreted by some tools

• Class

  • Every function and variable in Java is contained in the class , which is similar as class in Python
  • All code lives with classes
  • All classes , in turn , belong to some package.

• Method(Function)

  • Function Headers in Java contain more information that those in Python.
    • NEED to specity the types of values returned by the function and taken as parameters to the function , like int, void etc.
    • type void has no possible values; the main function here returns nothing
    • type String is like the str in Python
    • the trailing ‘[]’ means array of . Arrays are like Python lists ,except that their size is fixed once created.
    • Functions named main are special : they are what get called when one runs a Java program.(In python ,the main function is essentially anonymous)
    • Any code we want to run must be inside of a function public static void main(String[] args)

• Selection

  • In python a.b means the thing names b that is in or that applies to the thing identified (or computed) by a
  • Thus System.out means the variable named out that is found in the class named System
  • Likewise System.out.println means the method named println that applies to the object referenced by the value of variable System.out

• Access

  • Every declared entity in Java has access permissions indicating what pieces of code may mention it.
  • In particular , public classes , methods and variables may be referred to anywhere else in the program.
  • We sometimes refer to them as exported from their class( for methods or variables) or package( for classes)
  • Static method / variable
    • Static methods and variables are “one-of” things
    • A static method is just like an ordinary python function (outside of any class) or a function in a python class that annotated @staticmethod
    • A static variable is like a Python variable defined outside of any class or a variable selected from a class ,as opposed to from a class instance.
    • Other variables are local variables (in functions) or instance variables(in classes), and these are as in python.

• *Using java the implement 'print(“hello world”)

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

Basic code compared with Python

# python
class Car:
    def __init__(self, m):
        self.model = m 
        self.wheels = 4
    
    def drive(self):
        if self.wheels < 4:
            print(self.model +" no go vroom")
            return 
        print(self.model + " goes vroom") # driving makes noise
    
    def getNumWheels(self):
        return self.wheels
        
    def driveIntoDitch(self, wheelsLost):
        self.wheels = self.wheels - wheelsLost

c1 = Car("Civil type R")
c2 = Car("Toyota Camry")

c1.drive()
c1.driveIntoDitch(2)
c1.drive()

print(c2.getNumWheels())

// java
public class Car{
   
	public String model;
	public int