如何为特定类编写hashCode方法？ [英] How to write hashCode method for a particular class?

问题描述

我正在尝试为我的简单类生成一个hashCode（）方法，但我没有得到它的任何地方。我将不胜感激任何帮助。我已经实现了equals（）方法，如下所示，并且还想知道我是否需要实现compareTo（）方法。我已经导入了java.lang.Character来使用character.hashCode（）但它似乎不起作用。

I'm trying to generate a hashCode() method for my simple class but i'm not getting anywhere with it. I would appreciate any help. I've implemented the equals() method, which looks as follows, and would also like to know if I need to implement compareTo() method. I've imported java.lang.Character to use character.hashCode() but it doesn't seem to work.

private class Coord{
    private char row;
    private char col;
    public Coord(char x, char y){
        row = x;
        col = y;
    }
    public Coord(){};

    public char getX(){
        return row;
    }

    public char getY(){
        return col;
    }

    public boolean equals(Object copy){
        if(copy == null){
            throw new NullPointerException("Object entered is empty");
        }
        else if(copy.getClass()!=this.getClass()){
            throw new IllegalArgumentException("Object entered is not Coord");
        }
        else{
            Coord copy2 = (Coord)copy;
            if(copy2.row==this.row && copy2.col==this.col)
                return true;
            else
                return false;
        }
    }

}

谢谢提前...

给我java.lang.Comparable转换错误的comparTo（）方法..

The comparTo() method that is giving me java.lang.Comparable casting error..

public int compareTo(Object copy){
        if(copy==null){
            throw new NullPointerException("Object entered is empty");
        }
        else if(copy.getClass()!=this.getClass()){
            throw new IllegalArgumentException("Object entered is not Coord");
        }
        else{
            Coord copy2 = (Coord)copy;
            if(copy2.row==this.row && copy2.col==this.col){
                return 0;
            }
            else if(copy2.col < this.col){
                return -1;
            }
            else{
                return 1;
            }
        }
    }

谢谢......

推荐答案

要实现hashCode，请覆盖Object的默认实现：

To implement hashCode, you override the default implementation from Object:

@Override
public int hashCode()
{
    return row ^ col;
}

这不是一个理想的哈希，因为它的结果是非常可预测的两个不同的 Coord 对象很容易返回相同的值。更好的哈希将使用 java.util 中的内置 Arrays 类（ http://docs.oracle.com/javase/7/docs/api/java/util/Arrays。 html ）：

This isn't really an ideal hash, since its results are very predictable and it is easy for two different Coord objects to return the same value. A better hash would make use of the built-in Arrays class from java.util (http://docs.oracle.com/javase/7/docs/api/java/util/Arrays.html):

@Override
public int hashCode()
{
    return Arrays.hashCode(new Object[]{new Character(row), new Character(col)});
}

您可以使用此方法生成包含任意数量字段的非常好的哈希。

You can use this method to generate a pretty good hash with any number of fields.

要实现compareTo，你需要你的类实现 Comparable ：

To implement compareTo, you'll want your class to implement Comparable:

public class Coord implements Comparable<Coord>

完成此操作后，您可以使compareTo采用 Coord 而不是输入 Object ，这将为您省去检查其类型的麻烦。

Once you've done this, you can make compareTo take an argument of type Coord rather than type Object, which will save you the trouble of checking its type.

这篇关于如何为特定类编写hashCode方法？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！