《Head First Java》| 1 进入Java 的世界

设定Java环境

我此前安装过IntelliJ IDEA，因此已经有JDK-17

JDK与SDK的关系：

JDK	SDK	关系
Java Development Kit（Java开发工具包）	soft development kit （软件开发工具包）	JDK是SDK的子集

使用cmd键入javac，如果返回javac相关信息，说明jdk环境设置正确。

Java的工作流程

编译源代码——编译器(javac)编译——输出字节码.class——Java虚拟机(JVM)运行
main()：程序的起点，不论程序有多大（包含多少个类），一定都会有一个main()来作为程序的起点。

程序基本语法

• 语句以分号结束；
• 以两条斜线开始的行是注释 //
• 类型+名称 来声明变量 int weight;
• 类和方法都必须定义在花括号中
• 一个等号：赋值运算符；两个等号：等号运算符
• 可以把boolean测试放在括号中 while(x==4)
• print 输出后在同一行；println 换行输出

3种循环

Java有三种循环结构

while循环、do-while循环、for循环

注意：while循环括号中的条件判断，只能是True和False，不能用0，1代替。

// 这是错误的！
int x = 1;
while(x) { }

while循环示例：

public class whileloop {
    public static void main (String[] args) {
        int x = 1;
        System.out.println("Before the Loop");
        while(x<4) {
            System.out.println("In the Loop");
            System.out.println("Value of x is "+x);
            x = x+1;
        }
        System.out.println("The loop is done");
    }
}

Before the Loop
In the Loop
Value of x is 1
In the Loop
Value of x is 2
In the Loop
Value of x is 3
The loop is done

条件分支

if - else条件语句

public class IFTest {
    public static void main (String[] args){
        int x=2;
        if (x==3) {
            System.out.println("x must be 3");
        }
        else {
            System.out.println("x is not 3");
        }
    }
}

x is not 3

专家术语学习机

作用：3组单字随机挑出来排列组合输出。

public class PhraseOMatic {
    public static void main (String[] args) {
        // 专家术语
        String[] wordListOne = {"rebellion","996-work","sociological","swelter","html","adulation"};
        String[] wordListTwo = {"clown","conceive","surge","embodiment","embody"};
        String[] wordListThree = {"antecedent","dwinding","lyrics","piety","rambler","里世界郊游"};

        // 计算每一组有多少个术语
        int oneLength = wordListOne.length;
        int twoLength = wordListTwo.length;
        int threeLength = wordListThree.length;

        // 产生随机数字
        int rand1 = (int) (Math.random() * oneLength);
        int rand2 = (int) (Math.random() * twoLength);
        int rand3 = (int) (Math.random() * threeLength);

        // 组合出专家术语
        String phrase = wordListOne[rand1]+" " +wordListTwo[rand2]+" "+wordListThree [rand3];

        // 输出
        System.out.println("What we need is a "+phrase);
    }
}

What we need is a 996-work surge piety

生成随机数时，我们先用random()函数返回0-1之间的随机数，乘以数组长度后取整。
取整：浮点数转换为整型

> int x = (int) 24.6;

练习

排排看：要求输出 a-b c-d

public class Shuffle1 {
    public static void main(String[] args) {
        int x = 3;
        if (x > 2) {
            System.out.print("a");
        }
        x = x - 1;
        while (x > 0) {
            System.out.print("-");
            if (x==2) {
                System.out.print("b c");
            }
            if (x==1) {
                System.out.println("d");
            }
            x = x-1;
        }
    }
}

泳池迷宫
要求输出：

a noise
annoys
an oyster

public class PoolPuzzleOne {
    public static void main(String [] args){
        int x=0;
        while(x<1) {
            System.out.print("a");
            if (x<1) {
                System.out.print(" ");
            }
            x = x+1;
            if (x>0) {
                System.out.println("noise");
                x = x - 1;
            }
            x = x+2;
        }
        if(x>1) {
            System.out.println("annoys");
            x = x+1;
        }
        if(x<4){
            System.out.print("an");
            x = x-2;
        }
        System.out.println(" oyster");
    }
}