[Java] 用 Comparator 实现排序

最近正好用到Comparator，发现能对不同类型的对象进行排序(当然排序依据还是基本类型)，也不用自己实现排序算法，用起来很方便，所以简单记录一下。

本文地址：http://www.cnblogs.com/rossoneri/p/4549529.html

Interface Comparator<T>

对任意类型集合对象进行整体排序，排序时将此接口的实现传递给Collections.sort方法或者Arrays.sort方法排序。

实现int compare(T o1, T o2);方法，返回正数，零，负数各代表大于，等于，小于。具体看代码。

简单例子：

public class Test {

	private final class CompareName implements Comparator<Milan> {
		boolean is_Ascend;

		public CompareName(boolean b) {
			// TODO Auto-generated constructor stub
			is_Ascend = b;
		}

		@Override
		public int compare(Milan o1, Milan o2) {
			// TODO Auto-generated method stub
			if (is_Ascend)
				return o1.p_Name.compareTo(o2.p_Name);
			else
				return o2.p_Name.compareTo(o1.p_Name);
		}
	}

	private final class CompareId implements Comparator<Milan> {
		boolean is_Ascend;

		public CompareId(boolean b) {
			// TODO Auto-generated constructor stub
			is_Ascend = b;
		}

		@Override
		public int compare(Milan o1, Milan o2) {
			// TODO Auto-generated method stub
			int a, b;
			if (is_Ascend) {
				a = o1.p_Id;
				b = o2.p_Id;
			} else {
				a = o2.p_Id;
				b = o1.p_Id;
			}

			if (a > b)
				return 1;
			else if (a == b)
				return 0;
			else
				return -1;
		}
	}

	public static void main(String[] args) {
		Test t = new Test();

		Milan p1 = new Milan(1, "Dida");
		Milan p2 = new Milan(2, "Cafu");
		Milan p3 = new Milan(3, "Maldini");
		Milan P4 = new Milan(6, "Baresi");
		Milan p5 = new Milan(9, "Inzaghi");
		Milan P6 = new Milan(10, "Costa");

		List<Milan> mList = new ArrayList<Milan>();
		mList.add(p1);
		mList.add(P6);
		mList.add(P4);
		mList.add(p2);
		mList.add(p5);
		mList.add(p3);

		System.out.println("初始顺序");
		System.out.println("姓名	|	号码");
		for (Milan p : mList) {
			System.out.println(p.p_Name + "	|	" + p.p_Id);
		}
		System.out.println();
		System.out.println("对号码降序");
		System.out.println("姓名	|	号码");

		Collections.sort(mList, t.new CompareId(false));

		for (Milan p : mList) {
			System.out.println(p.p_Name + "	|	" + p.p_Id);
		}
		System.out.println();
		System.out.println("对姓名升序");
		System.out.println("姓名	|	号码");

		Collections.sort(mList, t.new CompareName(true));

		for (Milan p : mList) {
			System.out.println(p.p_Name + "	|	" + p.p_Id);
		}
	}

}

输出结果：

初始顺序

姓名 | 号码

Dida | 1

Costa | 10

Baresi | 6

Cafu | 2

Inzaghi | 9

Maldini | 3

对号码降序

姓名 | 号码

Costa | 10

Inzaghi | 9

Baresi | 6

Maldini | 3

Cafu | 2

Dida | 1

对姓名升序

姓名 | 号码

Baresi | 6

Cafu | 2

Costa | 10

Dida | 1

Inzaghi | 9

Maldini | 3

sort方法第一个参数是一个list对象，排序结束后的结果就继续保存在这个list中，可以直接使用。

附上文档，不用再去查了

public interface Comparator<T>

A comparison function, which imposes a total ordering on some collection of objects. Comparators can be passed to a sort method (such as Collections.sort or Arrays.sort) to allow precise control over the sort order. Comparators can also be used to control the order of certain data structures (such as sorted sets or sorted maps), or to provide an ordering for collections of objects that don't have a natural ordering.
The ordering imposed by a comparator c on a set of elements S is said to be consistent with equals if and only if c.compare(e1, e2)==0 has the same boolean value as e1.equals(e2) for every e1 and e2 in S.

Caution should be exercised when using a comparator capable of imposing an ordering inconsistent with equals to order a sorted set (or sorted map). Suppose a sorted set (or sorted map) with an explicit comparator c is used with elements (or keys) drawn from a set S. If the ordering imposed by c on S is inconsistent with equals, the sorted set (or sorted map) will behave "strangely." In particular the sorted set (or sorted map) will violate the general contract for set (or map), which is defined in terms of equals.

For example, suppose one adds two elements a and b such that (a.equals(b) && c.compare(a, b) != 0) to an empty TreeSet with comparator c. The second add operation will return true (and the size of the tree set will increase) because a and b are not equivalent from the tree set's perspective, even though this is contrary to the specification of the Set.add method.

Note: It is generally a good idea for comparators to also implement java.io.Serializable, as they may be used as ordering methods in serializable data structures (like TreeSet, TreeMap). In order for the data structure to serialize successfully, the comparator (if provided) must implement Serializable.

For the mathematically inclined, the relation that defines the imposed ordering that a given comparator c imposes on a given set of objects S is:

       {(x, y) such that c.compare(x, y) <= 0}.

The quotient for this total order is:
       {(x, y) such that c.compare(x, y) == 0}.

It follows immediately from the contract for compare that the quotient is an equivalence relation on S, and that the imposed ordering is a total order on S. When we say that the ordering imposed by c on S is consistent with equals, we mean that the quotient for the ordering is the equivalence relation defined by the objects' equals(Object) method(s):
     {(x, y) such that x.equals(y)}.
Unlike Comparable, a comparator may optionally permit comparison of null arguments, while maintaining the requirements for an equivalence relation.

---

int compare(T o1, T o2)

Compares its two arguments for order. Returns a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second.
In the foregoing description, the notation sgn(expression) designates the mathematical signum function, which is defined to return one of -1, 0, or 1 according to whether the value of expression is negative, zero or positive.

The implementor must ensure that sgn(compare(x, y)) == -sgn(compare(y, x)) for all x and y. (This implies that compare(x, y) must throw an exception if and only if compare(y, x) throws an exception.)

The implementor must also ensure that the relation is transitive: ((compare(x, y)>0) && (compare(y, z)>0)) implies compare(x, z)>0.

Finally, the implementor must ensure that compare(x, y)==0 implies that sgn(compare(x, z))==sgn(compare(y, z)) for all z.

It is generally the case, but not strictly required that (compare(x, y)==0) == (x.equals(y)). Generally speaking, any comparator that violates this condition should clearly indicate this fact. The recommended language is "Note: this comparator imposes orderings that are inconsistent with equals."

Parameters:
o1 - the first object to be compared.
o2 - the second object to be compared.
Returns:
a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second.

---

boolean equals(Object obj)

Indicates whether some other object is "equal to" this comparator. This method must obey the general contract of Object.equals(Object). Additionally, this method can return true only if the specified object is also a comparator and it imposes the same ordering as this comparator. Thus, comp1.equals(comp2) implies that sgn(comp1.compare(o1, o2))==sgn(comp2.compare(o1, o2)) for every object reference o1 and o2.
Note that it is always safe not to override Object.equals(Object). However, overriding this method may, in some cases, improve performance by allowing programs to determine that two distinct comparators impose the same order.

Overrides:
equals in class Object
Parameters:
obj - the reference object with which to compare.
Returns:
true only if the specified object is also a comparator and it imposes the same ordering as this comparator.

---

public static <T> void sort(List<T> list, Comparator<? super T> c)

Sorts the specified list according to the order induced by the specified comparator. All elements in the list must be mutually comparable using the specified comparator (that is, c.compare(e1, e2) must not throw a ClassCastException for any elements e1 and e2 in the list).
This sort is guaranteed to be stable: equal elements will not be reordered as a result of the sort.

The sorting algorithm is a modified mergesort (in which the merge is omitted if the highest element in the low sublist is less than the lowest element in the high sublist). This algorithm offers guaranteed n log(n) performance. The specified list must be modifiable, but need not be resizable. This implementation dumps the specified list into an array, sorts the array, and iterates over the list resetting each element from the corresponding position in the array. This avoids the n2 log(n) performance that would result from attempting to sort a linked list in place.

Parameters:
list - the list to be sorted.
c - the comparator to determine the order of the list. A null value indicates that the elements' natural ordering should be used.