Question

我的目标是要求用户输入名字和姓氏，然后将其转换为自定义的FullName类，然后搜索HashMap以查找具有相同名字和姓氏的密钥并返回该值。出于某种原因，每当我运行代码并将名称放入时，我都会得到null。

package hw4;

import java.io.*;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Set;

public class Test2 {

public static void main(String[] args) throws FileNotFoundException, IOException {
    HashMap<FullName, Integer> map = new HashMap<>();

    FullName n = new FullName("John", "Smith");
    map.put(n, 1);

    String f, l;
    BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    System.out.print("First: ");
    f = in.readLine();
    System.out.print("Last: ");
    l = in.readLine();
    FullName fl = new FullName(f, l);
    System.out.print(map.get(fl));
    }
}

这是FullName类。

public class FullName
{
private final String firstName;
private final String lastName;

public FullName(String first, String last) {
    firstName = first;
    lastName = last;
}

public String  getFirstName() { return firstName; }
public String getLastName() { return lastName; }

public boolean equals(FullName otherName)
{
    if (firstName == otherName.firstName && lastName == otherName.lastName)
    {return true;}

    return false;
    }
}

Answer 1

你还没有真正重写equals方法。因为equals将Object作为参数。另外，使用equals来比较不是==的对象。 ==比较参考变量。

import java.io.*;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Set;

public class Test2 {

    public static void main(String[] args) throws FileNotFoundException, IOException {
        HashMap<FullName, Integer> map = new HashMap<>();

        FullName n = new FullName("John", "Smith");
        map.put(n, 1);
        System.out.println(n.hashCode());

        String f, l;
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
        f = in.readLine();
        l = in.readLine();
        FullName fl = new FullName(f, l);
        System.out.println(fl.hashCode());

            System.out.print(map.containsKey(fl));
    }
}

class FullName {
    private final String firstName;
    private final String lastName;

    public FullName(String first, String last) {
        firstName = first;
        lastName = last;
    }

    public String getFirstName() {
        return firstName;
    }

    public String getLastName() {
        return lastName;
    }

    @Override
    public boolean equals(Object obj) {
        FullName otherName = (FullName) obj;

        if (getFirstName().equals(otherName.getFirstName()) && getLastName().equals(otherName.getLastName())) {
            return true;
        }       
        return false;
    }



    @Override
    public int hashCode() {

        return firstName.length()+lastName.length();
    }
}

使用HashMaps时遇到困难

1 个答案: