关于Equal和HashCode的实现

重点提示：

Equals示例：

Public Overrides Function Equals(ByVal obj As Object) As Boolean

'Check for null and compare run-time types.

If obj Is Nothing OrElse Not [GetType]().Equals(obj.GetType()) Then

Return False

End If

Dim p As Point = CType(obj, Point)

Return x = p.x AndAlso y = p.y

End Function 'Equals

public override bool Equals(Object obj) {

//Check for null and compare run-time types.

if (obj == null || GetType() != obj.GetType()) return false;

Point p = (Point)obj;

return (x == p.x) && (y == p.y);

}

GetHashCode示例：

Public Overrides Function GetHashCode() As Integer

Return a.GetHashCode() Xor b.GetHashCode() Xor c.GetHashCode()

End Function

public override int GetHashCode () {

return a.GetHashCode() ^ b.GetHashCode() ^ c.GetHashCode();

}

equals()反映的是对象或变量具体的值。

而hashCode()是对象或变量通过哈希算法计算出的哈希值。

之所以有hashCode方法，是因为在批量的对象比较中，hashCode要比equals来得快，很多集合都用到了hashCode，比如HashTable。

两个obj，如果equals()相等，hashCode()一定相等。

两个obj，如果hashCode()相等，equals()不一定相等（Hash散列值有冲突的情况，虽然概率很低）。

所以：

可以考虑在集合中，判断两个对象是否相等的规则是：

第一步，如果hashCode()相等，则查看第二步，否则不相等;

第二步，查看equals()是否相等，如果相等，则两obj相等，否则还是不相等。

1、首先equals()和hashcode()这两个方法都是从object类中继承过来的。

equals()是对两个对象的地址值进行的比较（即比较引用是否相同）。

hashCode()是一个本地方法，它的实现是根据本地机器相关的。

2、Java语言对equals()的要求如下，这些要求是必须遵循的：

? 对称性：如果x.equals(y)返回是“true”，那么y.equals(x)也应该返回是“true”。

? 反射性：x.equals(x)必须返回是“true”。

? 类推性：如果x.equals(y)返回是“true”，而且y.equals(z)返回是“true”，那么z.equals(x)也应该返回是“true”。

? 一致性：如果x.equals(y)返回是“true”，只要x和y内容一直不变，不管你重复x.equals(y)多少次，返回都是“true”。

? 任何情况下，x.equals(null)，永远返回是“false”；x.equals(和x不同类型的对象)永远返回是“false”。

3、equals()相等的两个对象，hashcode()一定相等；

反过来：hashcode()不等，一定能推出equals()也不等；

hashcode()相等，equals()可能相等，也可能不等。

引用：http://honda418.javaeye.com/blog/342731

Effective Java

讲得比较详细

equals（）方法

1． 自反

2． 对称

3． 传递

4． 一致

5． X非空,x.equals(null)返回false

为了实现高质量的equals()改写，effective java提醒我们要注意一下几点：

1． 使用==操作符检查是否指向同一个对象，这是一个性能优化的手段。

2． 使用instanceof来检查是否为某个类，同样也是性能优化的手段。

3． 把实参转换到正确的类型，上面一步保证了这一步不会有错误。

4． 检查类里的每一个关键域是否都相等。

hashCode()则是一种计算 对象的散列值

任何 classes 如果复写了 equals(), 便应该同时也复写 hashCode()

Visual Basic :

Imports System

Class Point

Inherits Object

Protected x, y As Integer

Public Sub New()

Me.x = 0

Me.y = 0

End Sub 'New

Public Sub New(ByVal X As Integer, ByVal Y As Integer)

Me.x = X

Me.y = Y

End Sub 'New

Public Overrides Function Equals(ByVal obj As Object) As Boolean

'Check for null and compare run-time types.

If obj Is Nothing OrElse Not [GetType]().Equals(obj.GetType()) Then

Return False

End If

Dim p As Point = CType(obj, Point)

Return x = p.x AndAlso y = p.y

End Function 'Equals

Public Overrides Function GetHashCode() As Integer

Return x ^ y

End Function 'GetHashCode

End Class 'Point

Class Point3D

Inherits Point

Private z As Integer

Public Sub New(ByVal X As Integer, ByVal Y As Integer, ByVal Z As Integer)

Me.x = X

Me.y = Y

Me.z = Z

End Sub 'New

Public Overrides Function Equals(ByVal obj As Object) As Boolean

Return MyBase.Equals(obj) AndAlso z = CType(obj, Point3D).z

End Function 'Equals

Public Overrides Function GetHashCode() As Integer

Return MyBase.GetHashCode() ^ z

End Function 'GetHashCode

End Class 'Point3D

Class [MyClass]

Public Shared Sub Main()

Dim point2D As New Point(5, 5)

Dim point3Da As New Point3D(5, 5, 2)

Dim point3Db As New Point3D(5, 5, 2)

If Not point2D.Equals(point3Da) Then

Console.WriteLine("point2D does not equal point3Da.")

End If

If Not point3Db.Equals(point2D) Then

Console.WriteLine("Likewise, point3Db does not equal point2D.")

End If

If point3Da.Equals(point3Db) Then

Console.WriteLine("However, point3Da equals point3Db.")

End If

End Sub 'Main

End Class '[MyClass]

' ----------------------------------

' Output should be:

'

' point2D does not equal point3Da.

' Likewise, point3Db does not equal point2D.

' However, point3Da equals point3Db.

C# :

using System;

class Point: Object {

protected int x, y;

public Point() {

this.x = 0;

this.y = 0;

}

public Point(int X, int Y) {

this.x = X;

this.y = Y;

}

public override bool Equals(Object obj) {

//Check for null and compare run-time types.

if (obj == null || GetType() != obj.GetType()) return false;

Point p = (Point)obj;

return (x == p.x) && (y == p.y);

}

public override int GetHashCode() {

return x ^ y;

}

}

class Point3D: Point {

int z;

public Point3D(int X, int Y, int Z) {

this.x = X;

this.y = Y;

this.z = Z;

}

public override bool Equals(Object obj) {

return base.Equals(obj) && z == ((Point3D)obj).z;

}

public override int GetHashCode() {

return base.GetHashCode() ^ z;

}

}

class MyClass {

public static void Main() {

Point point2D = new Point(5, 5);

Point3D point3Da = new Point3D(5, 5, 2);

Point3D point3Db = new Point3D(5, 5, 2);

if (!point2D.Equals(point3Da)) {

Console.WriteLine("point2D does not equal point3Da.");

}

if (!point3Db.Equals(point2D)) {

Console.WriteLine("Likewise, point3Db does not equal point2D.");

}

if (point3Da.Equals(point3Db)) {

Console.WriteLine("However, point3Da equals point3Db.");

}

}

}

// ----------------------------------

// Output should be:

//

// point2D does not equal point3Da.

// Likewise, point3Db does not equal point2D.

// However, point3Da equals point3Db.

Visual Basic :

Imports System

Public Class SomeType

Public Overrides Function GetHashCode() As Integer

Return 0

End Function

End Class

Public Class AnotherType

Public Overrides Function GetHashCode() As Integer

Return 1

End Function

End Class

Public Class LastType

Public Overrides Function GetHashCode() As Integer

Return 2

End Function

End Class

Public Class Demo

Private a As New SomeType()

Private b As New AnotherType()

Private c As New LastType()

Public Overrides Function GetHashCode() As Integer

Return a.GetHashCode() Xor b.GetHashCode() Xor c.GetHashCode()

End Function

End Class

C# :

using System;

public class SomeType {

public override int GetHashCode() {

return 0;

}

}

public class AnotherType {

public override int GetHashCode() {

return 1;

}

}

public class LastType {

public override int GetHashCode() {

return 2;

}

}

public class MyClass {

SomeType a = new SomeType();

AnotherType b = new AnotherType();

LastType c = new LastType();

public override int GetHashCode () {

return a.GetHashCode() ^ b.GetHashCode() ^ c.GetHashCode();

}

}