覆盖GetHashCode变体-Java 学习之路

我有一个理论类Name_Order，它有一个字符串 Name 和一个int Order .

我需要指出两个 Name_Order 是不同的，如果NameOrder对不同，那么，或者名称或顺序是不同的 .

现在，重写Equals没问题，但我有一些"issues"与GetHashCode：

Public Class Name_Order
  Public Property Name As String
  Public Property Order As Integer

  Public Overrides Function Equals(ByVal obj As Object) As Boolean
    If TypeOf obj Is Name_Order Then
      Dim no = DirectCast(obj, Name_Order)
      Return Me.Name = no.Name AndAlso Me.Order = no.Order
    Else
      Return MyBase.Equals(obj)
    End If
  End Function

  Public Overrides Function GetHashCode() As Integer
    Dim hcName = 0
    If Me.Name IsNot Nothing Then
      hcName = Me.Name.GetHashCode
    End If

    Dim hcOrder = Me.Order.GetHashCode

    Return hcName + hcOrder
  End Function
End Class

在这种情况下，对哈希码求和，留下一个（小但是真实的）可能性，即具有不同名称或顺序的两个不同的Name_Orders是“相同的” .

说，添加 7 154 给出与添加 154 7 相同的结果...

该方法的替代覆盖？

2 回答

0
首先，虽然避免碰撞是好的，但如果发生碰撞则不是问题 . 但常见的方法是：
```
return 7 * x.GetHashCode() + y.GetHashCode();
```
这在例如点/位置处更明显，其中避免明显的对角线碰撞（例如（2,3）vs（3,2））会很好 .

代码中一个更大的问题是hash / equals中的属性是可变的;如果它们被改变，字典等中的任何用法都将停止工作 . 您应该更喜欢只读键值 .
回复于 2024-04-28T16:45:49+08:00

MSDN在这里给出了该案例答案的变体（在Example的部分中）：

http://msdn.microsoft.com/en-us/library/bb338049.aspx

Public Class Product
    Public Property Name As String
    Public Property Code As Integer
End Class

' Custom comparer for the Product class
Public Class ProductComparer
    Implements IEqualityComparer(Of Product)

    Public Function Equals1(
        ByVal x As Product, 
        ByVal y As Product
        ) As Boolean Implements IEqualityComparer(Of Product).Equals

        ' Check whether the compared objects reference the same data.
        If x Is y Then Return True

        'Check whether any of the compared objects is null.
        If x Is Nothing OrElse y Is Nothing Then Return False

        ' Check whether the products' properties are equal.
        Return (x.Code = y.Code) AndAlso (x.Name = y.Name)
    End Function

    Public Function GetHashCode1(
        ByVal product As Product
        ) As Integer Implements IEqualityComparer(Of Product).GetHashCode

        ' Check whether the object is null.
        If product Is Nothing Then Return 0

        ' Get hash code for the Name field if it is not null.
        Dim hashProductName = 
            If(product.Name Is Nothing, 0, product.Name.GetHashCode())

        ' Get hash code for the Code field.
        Dim hashProductCode = product.Code.GetHashCode()

        ' Calculate the hash code for the product.
        Return hashProductName Xor hashProductCode ' here.....................
    End Function
End Class

回复于 2024-04-28T16:45:49+08:00

覆盖GetHashCode变体

2 回答

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