Java --- 泛型

目录

一、泛型

二、集合中，较器中使用泛型 

三、自定义泛型类和接口

四、自定义泛型方法

四、通配符 ？ 

一、泛型

所谓泛型，就是允许在定义类、接口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或参数的类型。这个类型参数将在使用时（例如，继承或实现这个接口、创建对象或调用方法时）确定（即传入实际的类型参数，也称为类型实参）。

二、集合中，较器中使用泛型 

@Test
    public void test1(){
        ArrayList arrayList = new ArrayList<>();
        for (int i = 1; i <= 4 ; i++) {
            arrayList.add(i);
        }
        Iterator iterator = arrayList.iterator();
        while (iterator.hasNext()){
            Integer next = iterator.next();
            System.out.println(next);
        }
    }

@Test
    public void test2(){
        HashMap map = new HashMap<>();
        map.put("杰瑞",3);
        map.put("汤姆",3);
        var entries = map.entrySet();
        var iterator = entries.iterator();
        while (iterator.hasNext()){
            var next = iterator.next();
            String key = next.getKey();
            Integer value = next.getValue();
            System.out.println(key + "\t" + value);
        }
    }

在集合中使用泛型解决存在的问题

①、数据类型不安全，add()的参数是Object类型，意味着任何类型的对象都可以添加成功。

②、需要使用强制转换，繁琐。还可能导致ClassCastExcetion异常。

使用说明

集合框架在声明接口和其实现类时，使用了泛型(JDK5.0),在实例化集合对象时，如果没有使用泛型，默认操作是Object类型的数据，如果使用了泛型，则需要指明泛型的具体类型。

public class Employee implements Comparable{
    private String name;
    private Integer age;
    private MyDate birthday;

    public Employee() {
    }

    public Employee(String name, Integer age, MyDate birthday) {
        this.name = name;
        this.age = age;
        this.birthday = birthday;
    }

    public String getName() {
        return name;
    }

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

    public Integer getAge() {
        return age;
    }

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

    public MyDate getBirthday() {
        return birthday;
    }

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

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

    @Override
    public int compareTo(Employee employee) {
        //按照年龄从低到高
        return  this.age.compareTo(employee.age);

    }
}

public class MyDate {
    private int year;
    private int month;
    private int day;

    public MyDate() {
    }

    public MyDate(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 year + "年" + month + "月" + day + "日";
    }
}

@Test
    public void test1(){
        TreeSet set = new TreeSet<>();
        for (int i = 1; i <= 5 ; i++) {
            set.add(new Employee("杰瑞" + i,i,new MyDate(2022 + i,i,i)));
        }
        var iterator = set.iterator();
        while (iterator.hasNext()){
            Employee employee = iterator.next();
            System.out.println(employee);
        }
    }
    @Test
    public void test2(){
        Comparator comparator = new Comparator<>(){
            @Override
            public int compare(Employee o1, Employee o2) {
                int year = o1.getBirthday().getYear() - o2.getBirthday().getYear();
                if (year != 0){
                    return year;
                }
                int month = o1.getBirthday().getMonth() - o2.getBirthday().getMonth();
                if (month != 0){
                    return month;
                }
                return  o1.getBirthday().getDay() - o2.getBirthday().getDay();

            }
        };
        TreeSet set = new TreeSet<>();
        for (int i = 1; i <= 5 ; i++) {
            set.add(new Employee("杰瑞" + i,i,new MyDate(2022 + i,i,i)));
        }
        var iterator = set.iterator();
        while (iterator.hasNext()){
            Employee employee = iterator.next();
            System.out.println(employee);
        }
    }

三、自定义泛型类和接口

public class Order {
    T t;
    String name;

    public Order() {
    }

    public Order(T t, String name) {
        this.t = t;
        this.name = name;
    }

    public T getT() {
        return t;
    }

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

    public String getName() {
        return name;
    }

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

    @Override
    public String toString() {
        return "Order{" +
                "t=" + t +
                ", name='" + name + '\'' +
                '}';
    }
}

@Test
    public void test1(){
        //默认泛型是Object
        Order