Java-equals（）和hashCode（）实现

Question

I have a class PointDensity that is implemented as:

import java.sql.Date;

public class PointDensity {

    private int id_place;

    private String algorithm;

    private Date mission_date;

    private int mission_hour;

    private int x;

    private int y;

    public PointDensity(int id_place, String algorithm, Date mission_date, int mission_hour, int x, int y) {

        this.id_place = id_place;
        this.algorithm = algorithm;
        this.mission_date = mission_date;
        this.mission_hour = mission_hour;
        this.x = x;
        this.y = y;
    }


    @Override
    public boolean equals(Object obj){
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        PointDensity other = (PointDensity) obj;
        return id_place == other.id_place
            && algorithm.equals(other.algorithm) 
            && mission_date.equals(other.mission_date)

            && mission_hour == other.mission_hour
            && x == other.x
            && y== other.y;
    }

    @Override
    public int hashCode(){
        final int prime = 31;
        int result = 1;
        result = prime * result + y;
        result = prime * result + ((algorithm == null) ? 0 : algorithm.hashCode());
        result = prime * result + x;
        result = prime * result + ((mission_date == null) ? 0 : mission_date.toString().hashCode());
        result = prime * result + mission_hour;
        result = prime * result + id_place;
        return result;
    }
}

I read some stuff and get values from a database. And then I want to use a HashMap() in order to store a PointDensity object as key and the times it has been encountered as a value. However, it never finds that the object is the same.

    Map<PointDensity, Integer> pointDensities = new HashMap<>();
    while (resultSet.next()){
        PointDensity pointDensity = 
            new PointDensity(
                resultSet.getInt(1), 
                resultSet.getString(2),
                new Date(timestampValues.getLong(i)), 
                new java.util.Date(timestampValues.getLong(i)).getHours(),
                xValues.getInt(i), 
                yValues.getInt(i)
            );  
        if (pointDensities.containsKey(pointDensity)){
            //IT NEVER ENTERS HERE!!!
            System.out.println("exists");
            int times = pointDensities.get(pointDensity);
            pointDensities.replace(pointDensity, times++);
        }else{
            pointDensities.put(pointDensity, 1);
        }
    }        

Thanks in advance :)

Answer 1

The following code works for me, your hashcode and equals methods seem OK. The error must be in code you aren't showing us, or in the database data itself.

    public static void main( String[] args ) {
       Map<PointDensity, Integer> pointDensities = new HashMap<>();
       PointDensity pointDensity = new PointDensity( 10, "TEST",
               new Date( 3, 1, 2019 ), new java.util.Date().getHours(),
               42, 24 );
       pointDensities.put( pointDensity, 1 );
       System.out.println( "pointDensity equal to self:"
               + pointDensity.equals( pointDensity ) );
       if( pointDensities.containsKey( pointDensity ) ) {

          //IT NEVER ENTERS HERE!!!
          System.out.println( "exists" );
          int times = pointDensities.get( pointDensity );
          pointDensities.replace( pointDensity, times++ );
       } else {
          pointDensities.put( pointDensity, 1 );
       }
    }
}

Output:

run:
pointDensity equal to self:true
exists
BUILD SUCCESSFUL (total time: 0 seconds)