java 通配符的使用

简介

java 通配符 是java写库的人必备的技能树

感觉有点模糊

jdk 版本要求较高 否则可能出现变异不通过的情况 最好用最新版本（jdk14） 虽然java 核心编程要求的是jdk1.8 但是我的jdk8 是编译不过去的

code

package com.company;

import com.company.Manager;

public class PairTest3
{
    public static void main(String[] args)
    {
        Manager ceo = new Manager("Gus Greedy", 800000, 2003, 12, 15);
        Manager cfo = new Manager("Sid Sneaky", 600000, 2003, 12, 15);
        Pair<Manager> buddies = new Pair<Manager>(ceo, cfo);
        printBuddies(buddies);

        ceo.setBonus(1000000);
        cfo.setBonus(500000);
        Manager[] managers = { ceo, cfo };

        Pair<Employee> result = new Pair<Employee>();
        minmaxBonus(managers, result);
        System.out.println("first: " + result.getFirst().getName()
                + ", second: " + result.getSecond().getName());
        maxminBonus(managers, result);
        System.out.println("first: " + result.getFirst().getName()
                + ", second: " + result.getSecond().getName());
    }

    public static void printBuddies(Pair<? extends Employee> p)
    {
        Employee first = p.getFirst();
        Employee second = p.getSecond();
        System.out.println(first.getName() + " and " + second.getName() + " are buddies.");
    }

    public static void minmaxBonus(Manager[] a, Pair<? super Manager> result)
    {
        if (a == null || a.length == 0) return;
        Manager min = a[0];
        Manager max = a[0];
        for (int i = 1; i < a.length; i++)
        {
            if (min.getBonus() > a[i].getBonus()) min = a[i];
            if (max.getBonus() < a[i].getBonus()) max = a[i];
        }
        result.setFirst(min);
        result.setSecond(max);
    }

    public static void maxminBonus(Manager[] a, Pair<? super Manager> result)
    {
        minmaxBonus(a, result);
        PairAlg.swapHelper(result); // OK--swapHelper captures wildcard type
    }
}

class PairAlg
{
    public static boolean hasNulls(Pair<?> p)
    {
        return p.getFirst() == null || p.getSecond() == null;
    }

    public static void swap(Pair<?> p) { swapHelper(p); }

    public static <T> void swapHelper(Pair<T> p)
    {
        T t = p.getFirst();
        p.setFirst(p.getSecond());
        p.setSecond(t);
    }
}

package com.company;

public class Manager extends Employee{
    private double bonus;
    public Manager(String name, double salary, int year, int month, int day) {
        super(name, salary, year, month, day);
        bonus = 0;
    }

    public double getSalary() {
        double baseSalary = super.getSalary();
        return baseSalary + bonus;
    }

    public void setBonus(double bonus) {
        this.bonus = bonus;
    }

    public boolean equals(Object otherObject){
        if (!super.equals(otherObject)) return false;
        Manager other = (Manager) otherObject;
        return bonus == other.bonus;
    }

    public int hashCode(){
        return super.hashCode() + 17 * new Double(bonus).hashCode();
    }

    public String toString() {
        return super.toString() + "[bonus=" + bonus + "]";
    }

    public double getBonus() {
        // TODO Auto-generated method stub
        return this.bonus;
    }
}

package com.company;

public class Pair<T>
{
    public Pair() { first = null; second = null; }
    public Pair(T first, T second) { this.first = first;  this.second = second; }

    public T getFirst() { return first; }
    public T getSecond() { return second; }

    public void setFirst(T newValue) { first = newValue; }
    public void setSecond(T newValue) { second = newValue; }

    private T first;
    private T second;
}

package com.company;

import java.time.*;
import java.util.Objects;
public class Employee{
    private String name;
    private double salary;
    private LocalDate hireDay;

    public Employee(String name, double salary, int year, int month, int day){
        this.name = name;
        this.salary = salary;
        hireDay = LocalDate.of(year, month, day);
    }

    public String getName() {
        return name;
    }

    public double getSalary() {
        return salary;
    }

    public LocalDate getHireDay() {
        return hireDay;
    }

    public String getDescription() {
        return String.format("an emplyee with a salary of $%.2f", salary);
    }

    public void raiseSalary(double byPercent) {
        double raise = salary * byPercent / 100;
        salary += raise;
    }

    public boolean equals(Object otherObject) {
        if (this == otherObject) return true;
        if (otherObject == null) return false;
        if (getClass() != otherObject.getClass()) return false;

        Employee other = (Employee) otherObject;
        return Objects.equals(name, other.name) && salary == other.salary
                && Objects.equals(hireDay,  other.hireDay);
    }

    public int hashCode(){
        return Objects.hash(name, salary, hireDay);
    }

    public String toString() {
        return getClass().getName() + "[name=" + name + ",salary=" + salary + ",hireDay=" + hireDay + "]";
    }
}

Q&A

? super Manager 的作用 <? extends Employee>

带有超类型限定的通配符可以向范型对象写入，带有子类型限定的通配符可以从泛型对象读取。

Tips

初看有点不懂，还是等以后工作的时候一边写代码一边熟悉。