codecademy 的 Learn Java 笔记
Java 基础语法
基础
数据类型
- int（integer 数字）
- boolean（对错）
- char（character 字母）
变量
需要明确数据类型
int myNumber = 42;
boolean isFun = true;
char movieRating = 'A';
代码格式
用空格优化代码格式，看起来更具可读性
注释
单行注释
// I'm a single line comment!
多行注释
/*
Hello,
Java!
*/
算术运算符
加减乘除 +, -, *, and /
取余数 ％
int sum = 34 + 113;
int difference = 91 - 205;
int product = 2 * 8;
int quotient = 45 / 3;
int myRemainder = 7 % 5;
关系运算符
<, <=, >, >=
比较运算符
==, !=
条件与流程控制
布尔运算符
与，或，非 &&, ||, !
优先次序：!, &&, ||
括号内优先
条件表达式
if
if/else
if/else if/else
三元条件运算符(Ternary Conditional)
char gameResult = (pointsScored > 20) ? 'W' : 'L';
Switch
int restaurantRating = 2;
switch (restaurantRating) {
case 1: System.out.println("This restaurant is not my favorite.");
break;
case 2: System.out.println("This restaurant is good.");
break;
default: System.out.println("I've never dined at this restaurant.");
break;
}
面向对象 (OPP)
class, constructor, object
class Dog {
// instance variables
int age;
// class constructor
public Dog(int dogsAge){
age = dogsAge;
}
// main method
public static void main(String[] args) {
// create a Dog object
Dog spike = new Dog(2);
}
}
method
关键词 void 表示执行方法后，不返回任何值
class Dog {
int age;
public Dog(int dogsAge) {
age = dogsAge;
}
// 添加 method
public void bark() {
System.out.println("Woof!");
}
// 添加带参数的 method
public void run(int feet) {
System.out.println("Your dog ran " + feet + " feet!");
}
// return value
public int getAge() {
return age;
}
public static void main(String[] args) {
Dog spike = new Dog(5);
// 使用 method
spike.bark();
spike.run(40);
// 使用返回值
int spikeAge = spike.getAge();
System.out.println(spikeAge);
}
}
继承
关键字 extends
class Car extends Vehicle {
int modelYear;
public Car(int year) {
modelYear = year;
}
//Other methods omitted for brevity...
public static void main(String[] args){
Car myFastCar = new Car(2007)
myFastCar.checkBatteryStatus();
}
}
class Vehicle {
public void checkBatteryStatus() {
System.out.println("The battery is fully charged and ready to go!");
}
}
数据结构
for 循环
- 初始化计数器
- 计数器与某个有限值进行比较
- 每次执行循环，计数器的值都要递增
for (int waterLevel = 0; waterLevel < 7; waterLevel++) {
System.out.println("The pool's water level is at " + waterLevel + " feet.");
}
数组
import java.util.ArrayList;
public class Temperatures {
public static void main(String[] args) {
// create an ArrayList object
// 声明一个数组
ArrayList<Integer> weeklyTemperatures = new ArrayList<Integer>();
// 在尾部添加数据
weeklyTemperatures.add(78);
weeklyTemperatures.add(67);
// 在指定位置添加数据
weeklyTemperatures.add(2, 111);
// 获取数据
weeklyTemperatures.get(0)
// 获取数组的每一个数据
for (int j = 0; j < weeklyTemperatures.size(); j++) {
System.out.println( weeklyTemperatures.get(j) );
}
// for each 循环，相当于for循环的简写
// : 可读为 'in'
for (Integer temperature : weeklyTemperatures) {
System.out.println(temperature);
}
}
}
hashMap
像字典一样，存储键值对
import java.util.HashMap;
public class Restaurant {
public static void main(String[] args) {
// 声明一个HashMap
HashMap<String, Integer> restaurantMenu = new HashMap<String, Integer>();
// 添加数据
restaurantMenu.put("Turkey Burger",13);
restaurantMenu.put("Naan Pizza",11);
// 获取数据
System.out.println( restaurantMenu.get("Naan Pizza") );
// 获取数据的条数
System.out.println(restaurantMenu.size());
// 遍历
// The keySet method of HashMap returns a list of keys.
for (String item : restaurantMenu.keySet()) {
System.out.println("A " + item + " costs " + restaurantMenu.get(item) + " dollars.");
}
}
}
觉得文章不错就支持一下呗～
