6 核心类2

public class Weekday {
    public static final int SUN = 0;
    public static final int MON = 1;
    public static final int TUE = 2;
    public static final int WED = 3;
    public static final int THU = 4;
    public static final int FRI = 5;
    public static final int SAT = 6;
}

public class Color {
    public static final String RED = "r";
    public static final String GREEN = "g";
    public static final String BLUE = "b";
}

if (day == Weekday.SAT || day == Weekday.SUN) {
    // Work at home
}

if (Color.RED.equals(color)) {
    // Process red color
}

enum Weekday {
    SUN, MON, TUE, WED, THU, FRI, SAT;
}

Weekday day = Weekday.SUN;
if (day == Weekday.SAT || day == Weekday.SUN) {
    System.out.println("Work at home!");
    // Work at home!
}

int day = 1;
if (day == Weekday.SUN) { // 编译错误
}

Weekday x = Weekday.SUN; // ok
Weekday y = Color.RED; // 编译错误

public final class Color extends Enum { // 继承自Enum，标记为final class
    // 每个实例均为全局唯一:
    public static final Color RED = new Color();
    public static final Color GREEN = new Color();
    public static final Color BLUE = new Color();
    // private构造方法，确保外部无法调用new操作符:
    private Color() {}
}

public class demo30 {
    public static void main(String[] args) {
        Weekday day = Weekday.SUN;
        if(day == Weekday.SAT || day == Weekday.SUN){
            System.out.println("Work at home!");
        }else{
            System.out.println("Work at office!");
        }
        // Work at home!
    }
}

enum Weekday{
    SUN,MON,TUE,WED,THU,FRI,SAT;
}

package Exp03;

public class demo31 {
    public static void main(String[] args) {
        Weekday day = Weekday.FRI;

        //name():返回常量名,不可被覆写
        System.out.println(day.name());
        // FRI

        // ordinal():返回定义的常量的顺序，从0开始计数(不建议使用)
        System.out.println(day.ordinal());
        // 4

        // dayValue:建议使用自定义字段来做逻辑判断
        if (day.dayValue == 6 || day.dayValue == 0) {
            System.out.println("Today is " + day + ". Work at home!");
        }else{
            System.out.println("Today is " + day + ". Work at office!");
        }
        // Today is 星期五. Work at office!
        // 返回“星期五”而不是“5”或者“FRI”或者别的，是因为自动调用了day.toString()方法

        // 覆写的toString()
        System.out.println(day.toString());
        // 星期五
    }
}

enum Weekday{
    MON(1, "星期一"), TUE(2, "星期二"), WED(3, "星期三"),
    THU(4, "星期四"), FRI(5, "星期五"), SAT(6, "星期六"), SUN(7, "星期日");

    public final int dayValue;
    private final String chinese;

    private Weekday(int dayValue,String chinese){
        this.dayValue = dayValue;
        this.chinese = chinese;
    }

    @Override
    public String toString(){   //覆写toString()
        return this.chinese;
    }
}

public class Main {
    public static void main(String[] args) {
        Weekday day = Weekday.SUN;
        switch(day) {
        case MON:
        case TUE:
        case WED:
        case THU:
        case FRI:
            System.out.println("Today is " + day + ". Work at office!");
            break;
        case SAT:
        case SUN:
            System.out.println("Today is " + day + ". Work at home!");
            break;
        default:
            throw new RuntimeException("cannot process " + day);
        }
        // Today is 星期五. Work at office!
    }
}

enum Weekday {
    MON, TUE, WED, THU, FRI, SAT, SUN;
}

public final class Point {
    private final int x;
    private final int y;

    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }

    public int x() {
        return this.x;
    }

    public int y() {
        return this.y;
    }

    // 还需覆写 equals(), hashCode(), toString()
}

record Point(int x, int y) {}

final class Point extends Record {
    private final int x;
    private final int y;

    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }

    public int x() {
        return this.x;
    }

    public int y() {
        return this.y;
    }

    public String toString() {
        return String.format("Point[x=%s, y=%s]", x, y);
    }

    public boolean equals(Object o) {
        ...
    }
    public int hashCode() {
        ...
    }
}

public class demo32 {
    public static void main(String[] args) {
        Point p = new Point(10,20);
        System.out.println(p.x());
        // 10
        System.out.println(p);
        // Point[x=10, y=20]
    }
}

record Point(int x,int y){
}

record Point(int x,int y){
    public Point{
        if(x<0 || y<0){
            throw new IllegalArgumentException("坐标不能为负数");
        }
    }
}

record Point(int x, int y) {
    public static Point of() {
        return new Point(0, 0);
    }

    public static Point of(int x, int y) {
        return new Point(x, y);
    }
}

var p1 = Point.of();
var p2 = Point.of(100, 200);

public class Utils {
    public static int square(int x) {
        return x * x;
    }
}

// 调用方式
int result = Utils.square(5);
// 25

-2^63 ~ 2^63 - 1（约 ±9.2×10^18）

import java.math.BigInteger;

public class demo33 {
    public static void main(String[] args) {
        BigInteger bi = new BigInteger("1234567890");
        BigInteger bi2 = new BigInteger("9876543210");

        System.out.println(bi.add(bi2));
        // 11111111100

        System.out.println(bi2.subtract(bi));
        // 8641975320

        System.out.println(bi.multiply(bi2));
        // 12193263111263526900

        System.out.println(bi2.divide(bi));
        // 8

        System.out.println(bi.pow(5)); 
        // 2867971860299718107233761438093672048294900000
    }
}

BigDecimal d1 = new BigDecimal("123.45");
BigDecimal d2 = new BigDecimal("123.4500");
BigDecimal d3 = new BigDecimal("1234500");

System.out.println(d1.scale()); 
// 2
System.out.println(d2.scale()); 
// 4
System.out.println(d3.scale()); 
// 0

BigDecimal d1 = new BigDecimal("123.4500");
BigDecimal d2 = d1.stripTrailingZeros();

System.out.println(d1.scale()); 
// 4
System.out.println(d2.scale()); 
// 2

BigDecimal d3 = new BigDecimal("1234500");
BigDecimal d4 = d3.stripTrailingZeros();

System.out.println(d3.scale()); 
// 0
System.out.println(d4.scale()); 
// -2   （表示整数末尾有两个0）

import java.math.BigDecimal;
import java.math.RoundingMode;

public class Main {
    public static void main(String[] args) {
        BigDecimal d1 = new BigDecimal("123.456789");

        // 四舍五入
        BigDecimal d2 = d1.setScale(4, RoundingMode.HALF_UP);
        // 直接截断
        BigDecimal d3 = d1.setScale(4, RoundingMode.DOWN);

        System.out.println(d2);
        // 123.4568
        System.out.println(d3);
        // 123.4567
    }
}

BigDecimal d1 = new BigDecimal("123.456");
BigDecimal d2 = new BigDecimal("23.456789");
// 保留10位小数并四舍五入
BigDecimal d3 = d1.divide(d2, 10, RoundingMode.HALF_UP);
// 报错：ArithmeticException，因为除不尽
BigDecimal d4 = d1.divide(d2); 

import java.math.BigDecimal;

public class Main {
    public static void main(String[] args) {
        BigDecimal n = new BigDecimal("12.345");
        BigDecimal m = new BigDecimal("0.12");

        BigDecimal[] dr = n.divideAndRemainder(m);

        // 商:12.345/0.12 = 102.875
        System.out.println(dr[0]);
        // 102

        // 余数:12.345/0.12 ... 0.105
        System.out.println(dr[1]);
        // 0.105
    }
}

BigDecimal n = new BigDecimal("12.75");
BigDecimal m = new BigDecimal("0.15");
BigDecimal[] dr = n.divideAndRemainder(m);
if (dr[1].signum() == 0) {
    System.out.println("n是m的整数倍数");
    // n是m的整数倍数
}

BigDecimal d1 = new BigDecimal("123.456");
BigDecimal d2 = new BigDecimal("123.45600");
System.out.println(d1.equals(d2)); 
// false
System.out.println(d1.equals(d2.stripTrailingZeros())); 
// true
System.out.println(d1.compareTo(d2));
// true

public class BigDecimal extends Number implements Comparable<BigDecimal> {
    private final BigInteger intVal;
    private final int scale;
}

public class demo35 {
    public static void main(String[] args) {
        double min = 5;
        double max = 15;
        double x = Math.random(); // [0,1)
        double y = x * (max - min) + min; // [min,max)
        System.out.println(y);
        // 7.01306198489437
        long n = (long) y; // 取整后的随机整数
        System.out.println(n);
        // 7
    }
}

import java.util.HexFormat;

public class Main {
    public static void main(String[] args) throws InterruptedException {
        // 将字符串 "Hello" 转换为对应的 byte 数组
        byte[] data = "Hello".getBytes();
        // byte[] {72, 101, 108, 108, 111}

        // 创建一个默认配置的HexFormat实例
        HexFormat hf = HexFormat.of();

        // 自定义格式:分隔符为空格，添加前缀0x，大写字母:
        HexFormat hf2 = HexFormat.ofDelimiter(" ").withPrefix("0x").withUpperCase();

        // 将byte[]数组格式化为十六进制字符串
        String hexData = hf.formatHex(data); 
        String hexData2 = hf2.formatHex(data); 

        System.out.println(hexData);
        // 48656c6c6f

        System.out.println(hexData2);
        // 0x48 0x65 0x6C 0x6C 0x6F

         // 将十六进制字符串转化为byte[]
        byte[] bs = hf.parseHex(hexData);
        String str = new String(bs);

        System.out.println(str);
        // Hello

        System.out.println(Arrays.toString(bs));
        //[72, 101, 108, 108, 111]
    }
}

import java.util.Random;

public class demo37 {
    public static void main(String[] args) {
        //种子为：12345
        Random r = new Random(12345);
        for (int i = 0; i < 5; i++) {
            // 每次输出都获得相同的随机数
            System.out.println(r.nextInt(100));
            // 51
            // 80
            // 41
            // 28
            // 55
        }
    }
}

import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;
import java.util.Arrays;

public class demo37 {
    public static void main(String[] args) {
        SecureRandom sr = null;
        try {
            // 获取高强度安全随机数生成器
            sr = SecureRandom.getInstanceStrong(); 
        } catch (NoSuchAlgorithmException e) {
            // 获取普通的安全随机数生成器
            sr = new SecureRandom(); 
        }

        byte[] buffer = new byte[5];
        // 用安全随机数填充buffer
        sr.nextBytes(buffer); 
        System.out.println(Arrays.toString(buffer));
    }
}

// 实例方法
public void greet() {}

// 静态方法
public static int sum(int a, int b) {
    return a + b;
}

// 工厂方法（静态构造）
public static Dog of(String name) {
    return new Dog(name);
}

// 构造方法（无返回值）
public Dog(String name) {
    this.name = name;
}

// 默认方法（接口中）
default void log() {
    System.out.println("Logging...");
}

// 抽象方法（必须被实现）
abstract void draw();

public class Dog {
    private String name;

    // 构造方法
    public Dog(String name) {
        this.name = name;
    }

    // 实例方法
    public void bark() {
        System.out.println(name + " says woof!");
    }

    // 静态方法
    public static void info() {
        System.out.println("Dogs are great pets.");
    }

    // 工厂方法
    public static Dog of(String name) {
        return new Dog(name);
    }
}

// 调用方式
Dog.info();                 // 静态方法
Dog d = Dog.of("Buddy");    // 工厂方法
d.bark();                   // 实例方法