如何使用 Java 的 map containsKey() 方法解决？

Question

在键入 ADD 时，我检查了代码并将数据保存到 HashMap 是正确的。然后在选择 FIND 选项后，我可以使用专用功能，但即使 100% 正确，该方法也无法显示找到的对象。 请检查此代码并告诉我为什么它在“public void showInfo(String name, String secondName)”中找不到正确的对象，该对象由在 CompanyApp 类中键入“FIND”引发的 Company 类

import java.util.InputMismatchException;
import java.util.Objects;
import java.util.Scanner;

public class CompanyApp {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        options[] values = options.values();
        int choose;
        int EXIT_NR = 2;
        Company ref = new Company();
        do {
            System.out.println("Available options: ");
            for (options one : values) {
                System.out.println(one.getDescription() + " - " + one.name());
            }
            System.out.println("Choose one: ");
            try {
                choose = options.valueOf(in.nextLine()).ordinal();
                if (Objects.equals(EXIT_NR, choose)) break;
                if (choose < 0 || choose >= options.values().length) {
                    throw new IllegalArgumentException("Choose 0, 1 or 2!");
                }
                options(choose);
            } catch (InputMismatchException e) {
                System.out.println("Choose a number ");
            }


        } while (1 == 1);
    }

    static void options(int choose){
        Company ref = new Company();
        Scanner info = new Scanner(System.in);

        switch (choose){
            case 0:
                System.out.println("Type in the name of the worker: ");
                String name = info.nextLine();
                System.out.println("Type in the second name of the worker: ");
                String secondName = info.nextLine();
                System.out.println("Type in the salary: ");
                double salary = info.nextDouble();
                info.nextLine();
                ref.add(new Employee(name, secondName, salary));
                break;
            case 1:
                System.out.println("Type in the name of the worker you want to find: ");
                String name2 = info.nextLine();
                System.out.println("Type in the second name of the worker you want to 
                find: ");
                String secondName2 = info.nextLine();
                ref.showInfo(name2, secondName2);
                break;
        }
    }
}

import java.util.HashMap;
import java.util.Map;
import java.util.TreeMap;

public class Company {
    private Map<String, Employee> map = new HashMap<>();

    public void add(Employee employee){
        String key = employee.getName() + " " + employee.getSecondName();
        if(!map.containsKey(key)){
        map.put(key, employee);
            System.out.println("Added object to map");}

    }

    public void showInfo(String name, String secondName){

        String key = name + " " + secondName;
        System.out.println("in showinfo method");
        if(map.containsKey(key)){
            System.out.println("found an object");
            Employee employee = map.get(key);
            System.out.println(employee.getName());
        }}}  

enum options {

     ADD("Add employee "), FIND("Find employee"), EXIT("Exit program");
    private String description;

     options(String description) {
         this.description = description;
     }

     public String getDescription() {
         return description;
     }

     public void setDescription(String description) {
         this.description = description;
     }

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

    String name;
    String secondName;
    double salary;

    public Employee(String name, String secondName, double salary) {
        this.name = name;
        this.secondName = secondName;
        this.salary = salary;
    }

    public String getName() {
        return name;
    }

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

    public String getSecondName() {
        return secondName;
    }

    public void setSecondName(String secondName) {
        this.secondName = secondName;
    }

    public double getSalary() {
        return salary;
    }

    public void setSalary(double salary) {
        this.salary = salary;
    }





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

Answer 1

问题在于方法static void options(int choose)。你需要传递Company-Object 并像这样在那里使用它：

从主方法调用（ref 是您在主方法中创建的Company-Object）

options(choose, ref);

options 方法以 Company 作为第二个参数：

  static void options(int choose, Company ref){
    Scanner info = new Scanner(System.in);

    switch (choose){
      case 0:
        System.out.println("Type in the name of the worker: ");
        String name = info.nextLine();
        System.out.println("Type in the second name of the worker: ");
        String secondName = info.nextLine();
        System.out.println("Type in the salary: ");
        double salary = info.nextDouble();
        info.nextLine();

        //use the passed Company here
        ref.add(new Employee(name, secondName, salary));
        break;
      case 1:
        System.out.println("Type in the name of the worker you want to find: ");
        String name2 = info.nextLine();
        System.out.println("Type in the second name of the worker you want to find: ");
        String secondName2 = info.nextLine();
 
        //and here
        ref.showInfo(name2, secondName2);
        break;
    }
  }

---

解释你的代码发生了什么

如前所述，问题出在方法static void options(int choose)。

在这里您创建一个新的Company-Object，它不会以任何方式传递给 main 方法。

当您使用 ADD 和之后的 FIND 时会发生这种情况：

1. 用ADD从主方法调用options
2. 新的Company-对象在options中创建
3. 新的Employee-Object 从上一点添加到Company
4. 方法结束 -> 创建的Company-Object 被“丢弃”（符合垃圾收集条件）
5. 用FIND从主方法调用options
6. 新的Company-对象在options中创建（因此其中没有员工）
7. 找不到Employee，因为新创建的Company的map中没有条目

Answer 2

当您尝试使用 FIND 选项从中获取数据时，地图为空。原因是您在 options 方法中重新创建 Company 对象：
公司参考 = 新公司（）；
同时，地图也被重新创建，所以里面没有记录。

另外，main方法中的Company对象也没有使用。