Java 第15章 泛型 课堂练习+本章作业

ArrayList中的Comparator

ArrayList要先加入对象再调用sort才能排序……

对birthday的比较，最好是放到MyDate类中，这样比较两个日期大小就可以直接调用MyDate类中重写的compareTo方法即可。具体做法是让MyDate类实现Comparable接口，然后重写其中的compareTo方法

public class Homework03 {
    public static void main(String[] args) {
        Employee a = new Employee("a", 1, new MyDate(2003, 1, 1));
        Employee b = new Employee("a", 2, new MyDate(2001, 2, 1));
        Employee c = new Employee("a", 3, new MyDate(2002, 3, 1));
        ArrayList<Employee> employees = new ArrayList<>();
        employees.add(a);
        employees.add(b);
        employees.add(c);
        employees.sort(new Comparator<Employee>() {
            @Override
            public int compare(Employee o1, Employee o2) {
                if (! (o1.getName()).equals(o2.getName()))
                    return (o1.getName()).compareTo(o2.getName());
                else 
                    return ((o1.getDate()).compareTo(o2.getDate()));
            }
        });
        System.out.println(employees);
    }
}
class MyDate implements Comparable<MyDate>{
    private int year;
    private int month;
    private int day;

    public MyDate(int year, int month, int day) {
        this.year = year;
        this.month = month;
        this.day = day;
    }

    @Override
    public String toString() {
        return "MyDate{" +
                "year=" + year +
                ", month=" + month +
                ", day=" + day +
                '}';
    }

    public int getYear() {
        return year;
    }

    public void setYear(int year) {
        this.year = year;
    }

    public int getMonth() {
        return month;
    }

    public void setMonth(int month) {
        this.month = month;
    }

    public int getDay() {
        return day;
    }

    public void setDay(int day) {
        this.day = day;
    }
    @Override
    public int compareTo(MyDate o) {
        MyDate myDate2 = o;
        if (year == myDate2.getYear()) {
            if (month == myDate2.getMonth()) {
                return day - myDate2.getDay();
            }
            return month - myDate2.getMonth();
        }
        return year - myDate2.getYear();
    }
}
class Employee {
    private String name;
    private double sal;
    private MyDate date;

    public Employee(String name, double sal, MyDate date) {
        this.name = name;
        this.sal = sal;
        this.date = date;
    }
    
    @Override
    public String toString() {
        return "\nEmployee{" +
                "name='" + name + '\'' +
                ", sal=" + sal +
                ", date=" + date.toString() +
                '}';
    }
    

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public double getSal() {
        return sal;
    }

    public void setSal(double sal) {
        this.sal = sal;
    }

    public MyDate getDate() {
        return date;
    }

    public void setDate(MyDate date) {
        this.date = date;
    }
}

判断main中是否出现错误

public class CustomGeneric_ {
    public static void main(String[] args) {
        //T=Double, R=String, M=Integer
        Tiger<Double,String,Integer> g = new Tiger<>("john");
        g.setT(10.9);
        g.setT("yy");
        System.out.println(g);
        Tiger g2 = new Tiger("john~~");
        g2.setT("yy");
        System.out.println("g2=" + g2);
    }
}
class Tiger<T, R, M> {
    String name;
    R r; //属性使用到泛型
    M m;
    T t;
    T[] ts;
    public Tiger(String name) {
        this.name = name;
    }
    public Tiger(R r, M m, T t) {//构造器使用泛型
        this.r = r;
        this.m = m;
        this.t = t;
    }
    public Tiger(String name, R r, M m, T t) {//构造器使用泛型
        this.name = name;
        this.r = r;
        this.m = m;
        this.t = t;
    }
    /*
     get and set ...
    */
    @Override
    public String toString() {
        return "Tiger{" +
                "name='" + name + '\'' +
                ", r=" + r +
                ", m=" + m +
                ", t=" + t +
                ", ts=" + Arrays.toString(ts) +
                '}';
    }
}

Tiger g2 = new Tiger(“john~~”); 没错，但要能分析出来，由于<T, R, M>没有被确定所以都是Object类型；

g2.setT(“yy”); 没错，因为 T=Object “yy”=String 是Object子类

public class CustomGeneric_ {
    public static void main(String[] args) {
        //T=Double, R=String, M=Integer
        Tiger<Double,String,Integer> g = new Tiger<>("john");
        g.setT(10.9); //OK
//        g.setT("yy"); //错误，类型不对
        System.out.println(g);
        Tiger g2 = new Tiger("john~~");//OK T=Object R=Object M=Object
        g2.setT("yy"); //OK ,因为 T=Object "yy"=String 是Object子类
        System.out.println("g2=" + g2);
    }
}

泛型编程

不难，注意JUnit中Test在测试中的使用。
JUnit是一个Java语言的单元测试框架，多数Java的开发环境都已经集成了JUnit作为单元测试的工具

import org.junit.jupiter.api.Test;

import java.util.*;

public class Homework01 {
    public static void main(String[] args) {

    }
    @Test
    public void testList() {
        DAO<User> dao = new DAO<>();
        // save
        dao.save("01", new User(1, 11, "a"));
        dao.save("02", new User(2, 22, "b"));
        dao.save("03", new User(3, 33, "c"));
        // list
        List<User> list = dao.list();
        System.out.println(list);
        // delete
        dao.delete("01");
        list = dao.list();
        System.out.println(list);
        // update
        dao.update("02", new User(12, 32, "bb"));
        list = dao.list();
        System.out.println(list);
        // get
        System.out.println(dao.get("03"));
    }
}
class User {
    private int id;
    private int age;
    private String name;

    public User(int id, int age, String name) {
        this.id = id;
        this.age = age;
        this.name = name;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", age=" + age +
                ", name='" + name + '\'' +
                '}';
    }
}
class DAO<T> {
    private Map<String, T> map = new HashMap<>();
    public void save(String id, T entity) {
        map.put(id, entity);
    }
    public T get(String id) {
        return (T) map.get(id);
    }
    public void update(String id, T entity) {
        map.put(id, entity);
    }
    public List<T> list() {
        List<T> list = new ArrayList<>();
        Set<Map.Entry<String, T>> entries = map.entrySet();
        Iterator<Map.Entry<String, T>> iterator = entries.iterator();
        while (iterator.hasNext()) {
            Map.Entry<String, T> entry = iterator.next();
            list.add(entry.getValue());
        }
        return list;
    }
    public void delete(String id) {
        map.remove(id);
    }
}