泛型

泛型的引入

package com.generic;

import java.util.ArrayList;

public class Generic01 {
    public static void main(String[] args) {

        //传统方法
        ArrayList arrayList = new ArrayList();

        arrayList.add(new Dog("旺财", 9));
        arrayList.add(new Dog("小黑", 3));
        arrayList.add(new Dog("圆圆", 5));

        //意外添加了一只猫
        arrayList.add(new Cat("咪咪", 4));//会抛出异常


        for (Object obj : arrayList) {
            Dog dog = (Dog) obj;
            System.out.println(dog.getName() + "-" + dog.getAge());
        }
    }
}


class Dog {
    private String name;
    private int age;

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

    public String getName() {
        return name;
    }

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

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "Dog{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

 class Cat {
    private String name;
    private int age;

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

    public String getName() {
        return name;
    }

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

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "Dog{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

引入泛型之后

package com.generic.improve;

import java.util.ArrayList;

public class Generic02 {
    public static void main(String[] args) {

        //使用泛型
        //1、ArrayList<Dog>表示放到arrayList集合中的元素是Dog类型
        //2、当编译器发现添加的类型不满足要求，就会报错
        //3、遍历的时候可以直接定义Dog类型,不需要先定义成Object，再转成Dog类型
        ArrayList<Dog> arrayList = new ArrayList<Dog>();

        arrayList.add(new Dog("旺财", 9));
        arrayList.add(new Dog("小黑", 3));
        arrayList.add(new Dog("圆圆", 5));

        //意外添加了一只猫
        //arrayList.add(new Cat("咪咪", 4));//会报错


        for (Dog obj : arrayList) {
            System.out.println(obj.getName() + "-" + obj.getAge());
        }
    }
}


class Dog {
    private String name;
    private int age;

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

    public String getName() {
        return name;
    }

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

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "Dog{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

class Cat {
    private String name;
    private int age;

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

    public String getName() {
        return name;
    }

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

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "Dog{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

泛型说明

package com.generic;

public class Generic03 {
    public static void main(String[] args) {
        Person<String> person01 = new Person<>("小明");

        /**
         * 等价于
         *  class Person<String> {
         *     String s;//表示这个属性的类型是在创建的时候定义的,E就表示s的数据类型
         *
         *     public Person(String s) {//E也可以是参数类型
         *         this.s = s;
         *     }
         *
         *     public String f() {//返回类型使用E
         *         return s;
         *     }
         * }
         */
    }
}


//泛型可以在类声明时通过一个标识表示类中某个属性的类型
//或者是某个方法的返回值的类型，或者参数类型

class Person<E> {
    E s;//表示这个属性的类型是在创建的时候定义的,E就表示s的数据类型

    public Person(E s) {//E也可以是参数类型
        this.s = s;
    }

    public E f() {//返回类型使用E
        return s;
    }
}

泛型的语法

泛型的声明

interface 接口{} 和 class 类<K,V>{}

T,K,V可以用任意字母表示

泛型实例化

要在类名后面指定参数类型的值（类型）

应用实例

package com.generic;

import java.util.*;

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

        HashSet<Student> students = new HashSet<>();

        students.add(new Student("小明", 18));
        students.add(new Student("小红", 19));
        students.add(new Student("小白", 21));

        //遍历
        for (Student s : students) {
            System.out.println(s);
        }

        System.out.println("==================");

        //使用泛型方式给HashMap 放入3个学生
        HashMap<String, Student> stringStudentHashMap = new HashMap<String, Student>();
        stringStudentHashMap.put("tom", new Student("tom", 18));
        stringStudentHashMap.put("smith", new Student("smith", 19));
        stringStudentHashMap.put("hary", new Student("hary", 21));

        Set<Map.Entry<String, Student>> entrySet = stringStudentHashMap.entrySet();
        Iterator<Map.Entry<String, Student>> iterator = entrySet.iterator();
        while (iterator.hasNext()) {
            System.out.println(iterator.next());
        }


    }
}


class Student {
    public String name;
    public int age;

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

    public String getName() {
        return name;
    }

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

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

泛型语法细节

泛型指向数据类型要求是引用数据类型，不能是基本数据类型
在给泛型制定具体类型之后，可以传入该类型或者其子类型
简写
不指定具体的泛型类型，默认的时Object

package com.generic;

import java.util.ArrayList;
import java.util.List;

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


        //在给泛型制定具体类型之后，可以传入该类型或者其子类型
        Pig<A> aPig1 = new Pig<>(new A());
        aPig1.f();
        Pig<A> aPig2 = new Pig<A>(new B());
        aPig2.f();

        //泛型的使用形式
        ArrayList<Integer> list01 = new ArrayList<Integer>();
        List<Integer> list02 = new ArrayList<Integer>();
        //简写形式，推荐写法
        ArrayList<Integer> list03 = new ArrayList<>();
        List<Integer> list04 = new ArrayList<>();

        //不指定具体的泛型类型，默认的时Object
        ArrayList arrayList = new ArrayList();
        //等价于ArrayList<Object> arrayList = new ArrayList<>();

    }
}


class A {
}

class B extends A {
}

class Pig<E> {
    E e;

    public Pig(E e) {
        this.e = e;
    }

    public void f() {
        System.out.println(e.getClass());
    }
}

泛型练习

package com.generic;

import java.util.ArrayList;
import java.util.Comparator;

public class GenericExercise02 {
    public static void main(String[] args) {

        ArrayList<Employee> employees = new ArrayList<>();

        employees.add(new Employee("tom", 18000, new MyData(1995, 10, 1)));
        employees.add(new Employee("jack", 21000, new MyData(1997, 7, 1)));
        employees.add(new Employee("smith", 29000, new MyData(1990, 12, 1)));
        employees.add(new Employee("smith", 29000, new MyData(1990, 11, 1)));

        System.out.println(employees);

        System.out.println("=====对员工进行排序=====");
        employees.sort(new Comparator<Employee>() {
            @Override
            public int compare(Employee emp01, Employee emp02) {
                //先按照name排序，如果name相同，则按生日日期的先后排序
                if (!(emp01 instanceof Employee && emp02 instanceof Employee)) {
                    System.out.println("类型不正确...");
                    return 0;
                }

                //比较name
                int i = emp01.getName().compareTo(emp02.getName());
                if (i != 0) {
                    return i;
                }

                //如果name相同，则比较birthday
                int yearMinus = emp01.getBirthday().getYear() - emp02.getBirthday().getYear();
                if (yearMinus != 0) {
                    return yearMinus;
                }

                //如果年份相同，接下来比较月份
                int monthMinus = emp01.getBirthday().getMonth() - emp02.getBirthday().getMonth();
                if (monthMinus != 0) {
                    return monthMinus;
                }

                //如果年份和月份都相同，接下来比较具体哪天
                return emp01.getBirthday().getDay() - emp02.getBirthday().getDay();

            }
        });

        System.out.println(employees);

    }
}


class Employee {
    private String name;
    private double sal;
    private MyData birthday;

    public Employee(String name, double sal, MyData birthday) {
        this.name = name;
        this.sal = sal;
        this.birthday = birthday;
    }

    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 MyData getBirthday() {
        return birthday;
    }

    public void setBirthday(MyData birthday) {
        this.birthday = birthday;
    }

    @Override
    public String toString() {
        return "\nEmployee{" +
                "name='" + name + '\'' +
                ", sal=" + sal +
                ", birthday=" + birthday +
                '}';
    }
}


class MyData {
    private int year;
    private int month;
    private int day;

    public MyData(int year, int month, int day) {
        this.year = year;
        this.month = month;
        this.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 String toString() {
        return "MyData{" +
                "year=" + year +
                ", month=" + month +
                ", day=" + day +
                '}';
    }
}

优化后：

package com.generic.improve;

import java.util.ArrayList;
import java.util.Comparator;

public class GenericExercise03 {
    public static void main(String[] args) {

        ArrayList<Employee> employees = new ArrayList<>();

        employees.add(new Employee("tom", 18000, new MyData(1995, 10, 1)));
        employees.add(new Employee("jack", 21000, new MyData(1997, 7, 1)));
        employees.add(new Employee("smith", 29000, new MyData(1990, 12, 1)));
        employees.add(new Employee("smith", 29000, new MyData(1990, 11, 1)));

        System.out.println(employees);

        System.out.println("=====对员工进行排序=====");
        employees.sort(new Comparator<Employee>() {
            @Override
            public int compare(Employee emp01, Employee emp02) {
                //先按照name排序，如果name相同，则按生日日期的先后排序
                if (!(emp01 instanceof Employee && emp02 instanceof Employee)) {
                    System.out.println("类型不正确...");
                    return 0;
                }

                //比较name
                int i = emp01.getName().compareTo(emp02.getName());
                if (i != 0) {
                    return i;
                }

                //封装操作
                //比较birthday
                return emp01.getBirthday().compareTo(emp02.getBirthday());

            }
        });

        System.out.println(employees);

    }
}


class Employee {
    private String name;
    private double sal;
    private MyData birthday;

    public Employee(String name, double sal, MyData birthday) {
        this.name = name;
        this.sal = sal;
        this.birthday = birthday;
    }

    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 MyData getBirthday() {
        return birthday;
    }

    public void setBirthday(MyData birthday) {
        this.birthday = birthday;
    }

    @Override
    public String toString() {
        return "\nEmployee{" +
                "name='" + name + '\'' +
                ", sal=" + sal +
                ", birthday=" + birthday +
                '}';
    }
}


class MyData implements Comparable<MyData> {
    private int year;
    private int month;
    private int day;

    public MyData(int year, int month, int day) {
        this.year = year;
        this.month = month;
        this.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 String toString() {
        return "MyData{" +
                "year=" + year +
                ", month=" + month +
                ", day=" + day +
                '}';
    }


    @Override
    public int compareTo(MyData o) {

        //先比较year
        int yearMinus = year - o.getYear();
        if (yearMinus != 0) {
            return yearMinus;
        }

        //如果年份相同，接下来比较月份
        int monthMinus = month - o.getMonth();
        if (monthMinus != 0) {
            return monthMinus;
        }

        //如果年份和月份都相同，接下来比较具体哪天
        return day - o.getDay();
    }
}

自定义泛型

package com.generic;

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

    }
}


//自定义泛型
//1、T,R,M 泛型的标识符，一般单个字母大写
//2、泛型标识符可以有多个
//3、普通成员可以使用泛型（属性、方法）
//4、使用泛型的数组，不能初始化
//5、静态方法不能使用类的泛型
class Tiger<T, R, M> {
    String name;
    T t;
    R r;
    M m;
    T[] ts;//不能 new 数组，因为不能确定T的类型，无法开辟内存空间

    public Tiger(String name, T t, R r, M m) {//构造器使用泛型
        this.name = name;
        this.t = t;
        this.r = r;
        this.m = m;
    }

    //因为静态是和类相关的，在类加载时，对象还没有创建
    //所以如果静态方法和静态属性使用了泛型，JVM就无法完成初始化
    //static R r2;//有报错
    //public static void m1(M m) {}//有报错

    public String getName() {
        return name;
    }

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

    public T getT() {
        return t;
    }

    public void setT(T t) {
        this.t = t;
    }

    public R getR() {
        return r;
    }

    public void setR(R r) {
        this.r = r;
    }

    public M getM() {
        return m;
    }

    public void setM(M m) {
        this.m = m;
    }
}

自定义泛型接口

package com.generic;

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

    }
}


interface IUsb<U, R> {
    //普通方法中，可以使用接口泛型
    R get(U u);

    void hi(R r);

    void run(R r1, R r2, U u1, U u2);

    default R method(U u) {
        return null;
    }
}

//在继承接口时指定泛型接口的类型

interface IA extends IUsb<String, Double> {

}


//当我们去实现IA接口时，因为IA接口在继承IUsb接口时，指定了U为String R为Double
//在实现IUsb接口的方法时，使用String替换U，使用Double替换R
class AA implements IA {
    @Override
    public Double get(String s) {
        return null;
    }

    @Override
    public void hi(Double aDouble) {

    }

    @Override
    public void run(Double r1, Double r2, String u1, String u2) {

    }
}


//实现接口时，直接指定泛型接口的类型
//给U指定Integer，给R指定Float
//所以，当我们实现IUsb方法时，会使用Integer替换U，使用Float替换R
class BB implements IUsb<Integer, Float> {

    @Override
    public Float get(Integer integer) {
        return null;
    }

    @Override
    public void hi(Float aFloat) {

    }

    @Override
    public void run(Float r1, Float r2, Integer u1, Integer u2) {

    }
}


//没有指定类型，默认Object
class CC implements IUsb {//等价于class CC implements IUsb<Object, Object>

    @Override
    public Object get(Object o) {
        return null;
    }

    @Override
    public void hi(Object o) {

    }

    @Override
    public void run(Object r1, Object r2, Object u1, Object u2) {

    }
}

自定义泛型方法

package com.generic;

import java.util.ArrayList;

public class CustomMethodGeneric {
    public static void main(String[] args) {
        Car car = new Car();
        car.fly("宝马", 999);//当调用泛型方法时，传入参数，编译器会确认参数类型


        Fish<String, ArrayList> fish = new Fish<>();
        fish.hello(new ArrayList(), 11.3f);
    }
}


//泛型方法，可以定义在普通类中，也可以定义在泛型类中
class Car {
    //普通方法
    public void run() {
    }


    //<T, R>就是泛型
    //提供给fly方法用的
    public <T, R> void fly(T t, R r) {//泛型方法

    }
}

class Fish<T, R> {//泛型类

    public void run() {//普通方法
    }

    public <U, M> void eat(U u, M m) {//泛型方法

    }

    //下面这个方法不是泛型方法
    //是方法使用了类声明的泛型
    public void hi(T t) {
    }

    //泛型方法，可以使用类声明的泛型，也可以使用自己声明的泛型
    public <K> void hello(R r, K k) {
        System.out.println(r.getClass());
        System.out.println(k.getClass());
    }

}

泛型的继承和通配

泛型不具备继承性
：支持任意泛型类型
：支持A类以及A类的子类，规定了泛型的上限
：支持A类以及A类的父类，不限于直接父类，规定了泛型的下限

Junit

package com.junit_;

import org.junit.jupiter.api.Test;

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

    }



    @Test
    public void m1() {
        System.out.println("m1方法被调用");
    }

    @Test
    public void m2() {
        System.out.println("m2方法被调用");
    }

}

@Test ==> Alt + Enter ==>导入包

可以不经main函数直接调用方法测试

泛型练习

要求：定义一个泛型类DAO，在其中定义一个Map成员变量，Map的键为String类型，值为T类型。

分别创建以下方法：

public void save(String id,T entity):保存T类型的对象到Map成员变量中
public T get(String id)：从map中获取id对应的对象
public void update(String id,T entity)：替换map中key为id的内容，改为entity对象
public List list()：返回map中存放的所有T对象
public void delete(String id)：删除指定id对象

定义一个User类：

包含：private 成员变量(int类型) id,age;(String) name.

创建DAO类的对象，分别调用其save,get,update,list,delete方法来操作User对象，使用Junit单元测试类进行测试

package com.generic;

import org.junit.jupiter.api.Test;

import java.util.*;

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

    }

    @Test
    public void testList() {

        DAO<User> dao = new DAO<>();

        dao.save("001", new User("jack", 1, 18));
        dao.save("002", new User("xiaomi", 2, 22));
        dao.save("003", new User("keeper", 3, 27));

        System.out.println("===测试save===");
        System.out.println(dao);

        System.out.println("===测试get===");
        System.out.println(dao.get("001"));

        System.out.println("===测试update===");
        dao.update("002", new User("dami", 2, 31));
        System.out.println(dao.get("002"));

        System.out.println("===测试list===");
        System.out.println(dao.list());

        System.out.println("===测试delete===");
        dao.delete("002");
        System.out.println(dao.list());
    }
}

//定义一个泛型类DAO<T>

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 map.get(id);
    }


    public void update(String id, T entity) {
        map.replace(id, entity);
    }


    public List<T> list() {
        ArrayList<T> list = new ArrayList<>();

        Set<String> set = map.keySet();
        for (String s : set) {
            list.add(map.get(s));
        }
        return list;
    }


    public void delete(String id) {
        map.remove(id);

    }

    @Override
    public String toString() {
        return "\nDAO{" +
                "map=" + map +
                '}';
    }
}

//定义一个User类
class User {
    private String name;
    private int id;
    private int age;


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

    public String getName() {
        return name;
    }

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

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "\nUser{" +
                "name='" + name + '\'' +
                ", id=" + id +
                ", age=" + age +
                '}';
    }
}

posted @ 2022-08-03 19:37 每年桃花开的时候阅读(43) 评论(0) 收藏举报

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