这个课程的参考视频和图片来自youtube。

主要学到的知识点有：

• Class: Blueprint for an object. (e.g. dog is a class)
• Object: custom variables contain state an behavior. (e.g. a two-year old husky is an object of a class dog, it's age is 2, and behavior is bark. then it will have methods like "bark()", "eat(food)")

Define an object:

• If we want to use the classes defined in Java, like Scanner, Random . Then need to use import method to import the class libraries. ("Ctrl + Shifl + i "will automatic import/unimport the packages can be refered) Might also need to import the packages of others or packages in other projects.

import java.util.Random;

• Show javadoc will help you to understand how to use the class or method. This is an example of the Random class we just import above.

// Generator a random number between 0 - 9

Random generator = new Random();

int i = generator.nextInt(10);

• Reverse a string use StringBuilder.

  String forward = "This is a test";
  
  StringBuilder sb = new StringBuilder();
  
  sb.append(forward);
  // StringBuilder sb = new StringBuilder(forward);  is the same like last two lines.
  
  String reverseString = sb.reverse().toString();
  
  System.out.println(reverseString);
  
  // System output result will be:
  
  // !tset a si sihT

• Math.PI will present pi which is 3.14.... in math. ("Tab" is also our friend to see methods that we can use, so press "Tab" after Math.)

double circ = 2 * Math.PI * radius;