重温算法与数据结构:(1)排序算法的预备知识

谈起排序,势必会想到比较。在.NET框架中什么和比较有关系呢?自然是IComparable、IComparable<T>、IComparer<T>、IEqualityComparer<T>这几个接口,其中IComparable<T>为IComparable的泛型版本。泛型的优点自然不必多说。在一切有可能的情况,我们应该优先使用泛型。老赵的这篇文章可以回答你有关泛型性能的问题。

.NET框架中的IComparable、IComparable<T>、IComparer<T>、IEqualityComparer<T>

它们的的定义如下:


public interface IComparer<T> 
{ 
	int Compare(T x, T y); 
} 
public interface IComparable 
{ 
	int CompareTo(object obj); 
} 
public interface IComparable<T> 
{
	int CompareTo(T other); 
} 
public interface IEqualityComparer<T> 
{ 
	bool Equals(T x, T y); 
	int GetHashCode(T obj); 
}

对于特定类型实现IComparer<T>接口,这样当你在实现排序算法时,即可调用IComparer<T>.Compare(T x, T y)进行对实例x,y的比较,方法返回值小于0,表明x小于y,若返回值等于0,表明x等于y,若返回值大于0,表明x大于y。对于IComparable<T>接口,当某类型实现该接口时,就可以再该类型实例上调用CompareTo(T other)方法,方法返回值小于0,表明实例小于other参数,若返回值等于0,表明实例等于other参数,若返回值大于0,表明实例大于other参数。对于IEqualityComparer<T>接口,可以根据对该接口的实现类,调用Equals(T x, T y)方法,返回true表明x和y相等。

通过实现接口IComparable<Employee>:

现在准备一个雇员类,内含三个成员(年龄、姓名、所在部门),根据年龄大小来判断大小,年龄大的雇员大于年龄小的雇员。代码如下:

public class Employee : IComparable<Employee>
{
        public int Age
        {
            get;
            set;
        }
        public string Name
        {
            get;
            set;
        }
        public string Department
        {
            get;
            set;
        }
        public Employee(int age,string name,string department)
        {
            this.Age = age;
            this.Name = name;
            this.Department = department;
        }

        #region IComparable<Employee> Members

        public int CompareTo(Employee other)
        {
            return this.Age - other.Age;
        }

        #endregion
}

测试代码:

static void Main(string[] args)
{
            Employee anders = new Employee(35, "Anders Wang", "Developer Division");
            Employee jeffrey = new Employee(28, "Jeffrey Chen", "Developer Division");
            Employee steve = new Employee(35, "Anders Wang", "Developer Division");
            Console.WriteLine(anders.CompareTo(jeffrey)); //output > 0
}

从Employee的CompareTo看出,由于anders的Age>jeffrey的Age,所以anders.CompareTo(jeffrey)返回1,表明实例anders大于实例jeffrey。

通过实现接口IComparer<Employee>:

代码如下:

class EmployeeComparer:IComparer<Employee>
{
        #region IComparer<Employee> Members

        public int Compare(Employee x, Employee y)
        {
            return x.Age - y.Age;
        }

        #endregion
}

测试代码:

static void Main(string[] args)
{
            Employee anders = new Employee(35, "Anders Wang", "Developer Division");
            Employee jeffrey = new Employee(28, "Jeffrey Chen", "Developer Division");
            Employee steve = new Employee(35, "Anders Wang", "Developer Division");
            Console.WriteLine(new EmployeeComparer().Compare(anders, jeffrey)); //output > 0
}

以上代码也同样实现了比较功能。

通过实现接口IEqualityComparer<Employee>:

要判断两个对象是否相等?我们可以通过实现IEqualityComparer<T>接口或者通过调用基类object的Equals方法。但是在大部分情况下,调用基类object的Equals方法需要进行强制转换,含有类型不安全的问题。下面我们通过EmployeeEqualityComparer来实现IEqualityComparer<T>。代码如下:

public class EmployeeEqualityComparer:IEqualityComparer<Employee>
{
        #region IEqualityComparer<Employee> Members

        public bool Equals(Employee x, Employee y)
        {
            if (x.Age == y.Age && x.Name == y.Name && x.Department == y.Department)
            {
                return true;
            }
            return false;
        }

        public int GetHashCode(Employee obj)
        {
            //由于是根据Age,Name,Department来进行判等的,
            //所以哈希值的求取过程也要进行一定的修改,
            //确保相等对象的哈希值也是相等的。
            //在类型Employee内部覆盖GetHashCode方法即可。
            return obj.GetHashCode();
        }

        #endregion
}

测试代码:

static void Main(string[] args)
{
            Employee anders = new Employee(35, "Anders Wang", "Developer Division");
            Employee jeffrey = new Employee(28, "Jeffrey Chen", "Developer Division");
            Employee steve = new Employee(35, "Anders Wang", "Developer Division");
            Console.WriteLine(new EmployeeEqualityComparer().Equals(anders, jeffrey)); //output false
            Console.WriteLine(new EmployeeEqualityComparer().Equals(anders, steve)); //output true
            Console.ReadKey();
}

以上代码通过实现IEqualityComparer<T>接口说明了如何判断两个对象相等。

总结

.NET框架提供了IComparable、IComparable<T>、IComparer<T>、IEqualityComparer<T>,这几个接口对于实现排序算法有着直接的作用,在泛型的帮助下,可以编写出通用的排序算法,只需要你根据特定的类型实现以上几个接口中的方法。正是因为接口,使得大家都有了规约,进而使得通用的排序算法实现变得可能。另外在处理和集合相关的类型时,比如字典,哈希表等数据结构时, 当你因为具体的语义覆盖了键类型的Equals方法后,务必必也要覆盖GetHashCode方法。

posted @ 2009-08-13 15:34  Kevin Dai  阅读(...)  评论(...编辑  收藏