API的使用 compare接口的用法

Math 类

math.直接调用

public static void main(String[] args) {
        System.out.println(Math.E);
        System.out.println(Math.ceil(2.1)); // 上天花板数
        System.out.println(Math.floor(2.9)); // 下天花板数
        System.out.println(Math.abs(3.5)); //绝对值
        System.out.println(Math.round(3.5)); //四舍五入
        System.out.println(Math.random()); //0.0~1.0的随机数、
        System.out.println(Math.pow(2, 3)); //返回2的3次方
        System.out.println(Math.sqrt(81)); //开平方
    }

calendar类：

public static void main(String[] args) {
        
        //受保护的构造方法的获得实例的方式 
        
        Calendar ca = Calendar.getInstance();
        
        //日历返日期
        Date date = ca.getTime();
        System.out.println(date);
        //获取时间
        System.out.println(ca.get(ca.YEAR));
        System.out.println(ca.get(ca.MONTH)+1);
    }

simpledateformat：日期格式化

public static void main(String[] args) {
            
            //日期格式化  a 代表的是上午
            Date date = new Date();
            SimpleDateFormat sf = new SimpleDateFormat("yyyy-MM-dd-HH:mm:ss a" );
            String format = sf.format(date);
            System.out.println(format);
            
        }

comparable接口

实现这个接口

class Dog implements Comparable<Dog>{
    int age;
    String name;
    public Dog(int age, String name) {
        super();
        this.age = age;
        this.name = name;
    }
    @Override
    public int compareTo(Dog o) {
        //比较信息
        if(this.age > o.age){
            return 1;
        }else{
            return -1;
        }
    }
}
public class CompareTest {
    
    public static void main(String[] args) {
        Dog dog1 = new Dog(2, "小胡");
        Dog dog2 = new Dog(3, "胖胖");
        System.out.println(dog1.compareTo(dog2));
    }
    
}

package com.qc.java1801.api;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
import java.util.TreeSet;

class Student{
    String name;
    int score;
    public Student(String name, int score) {
        super();
        this.name = name;
        this.score = score;
    
    }
    @Override
    public String toString() {
        return "Student [name=" + name + ", score=" + score + "]";
    }
}

/*class compar implements Comparator<Student>{

    @Override
    public int compare(Student o1, Student o2) {
        return o1.score - o2.score;
    }
}*/

public class ComparetoTest {

    public static void main(String[] args) {
        List<Student> students = new ArrayList<Student>();
        students.add(new Student("张三", 90));
        students.add(new Student("李四", 80));
        students.add(new Student("王二麻", 100));
        //用了匿名内部类
        Collections.sort(students, new Comparator<Student>() {

            @Override
            public int compare(Student o1, Student o2) {
                return o1.score - o2.score;
            }
        });
        System.out.println(students);
    }
}

comparator 和 comparable区别；

comparable自己实现

comparator需要第三方实现

抓取异常和抛异常

    public static void main(String[] args) {
        Park.ticket = 10000;
        Scanner sc = new Scanner(System.in);
        System.out.println("请输入年龄");
            
            try {
                int age = sc.nextInt();
                System.out.println(Park.sellticket(age));
            } catch (InputMismatchException e) {
                
                System.out.println("输入的格式不正确");
            } catch (Exception e) {
                System.out.println(e.getMessage());
            }
}


static class Park{
    
    static int ticket;
    public static int sellticket(int age) throws Exception{
        if(age > 100 || age < 0){
            throw new Exception("年龄不合适");
            }else if(age > 60){
                ticket = 0;
            }else if(age < 10){
                ticket = ticket/2;
            }else{
                return ticket;
            }
        
        return ticket;
    }
    }