Collections中min和max工具方法详解

以下是java.util.Collections.min()方法和java.util.Collections.min()的声明

public static <T extends Object & Comparable<? super T>> T min(Collection<? extends T> cl)

public static <T extends Object & Comparable<? super T>> T max(Collection<? extends T> coll)

min和max方法的实现原理都是使用迭代器模式，如下源码：

    /*** Returns the maximum element of the given collection, according to the* <i>natural ordering</i> of its elements.  All elements in the* collection must implement the <tt>Comparable</tt> interface.* Furthermore, all elements in the collection must be <i>mutually* comparable</i> (that is, <tt>e1.compareTo(e2)</tt> must not throw a* <tt>ClassCastException</tt> for any elements <tt>e1</tt> and* <tt>e2</tt> in the collection).<p>** This method iterates over the entire collection, hence it requires* time proportional to the size of the collection.** @param  coll the collection whose maximum element is to be determined.* @return the maximum element of the given collection, according*         to the <i>natural ordering</i> of its elements.* @throws ClassCastException if the collection contains elements that are*         not <i>mutually comparable</i> (for example, strings and*         integers).* @throws NoSuchElementException if the collection is empty.* @see Comparable*/public static <T extends Object & Comparable<? super T>> T max(Collection<? extends T> coll) {Iterator<? extends T> i = coll.iterator();T candidate = i.next();while (i.hasNext()) {T next = i.next();if (next.compareTo(candidate) > 0)candidate = next;}
        return candidate;}

    /*** Returns the minimum element of the given collection, according to the* order induced by the specified comparator.  All elements in the* collection must be <i>mutually comparable</i> by the specified* comparator (that is, <tt>comp.compare(e1, e2)</tt> must not throw a* <tt>ClassCastException</tt> for any elements <tt>e1</tt> and* <tt>e2</tt> in the collection).<p>** This method iterates over the entire collection, hence it requires* time proportional to the size of the collection.** @param  coll the collection whose minimum element is to be determined.* @param  comp the comparator with which to determine the minimum element.*         A <tt>null</tt> value indicates that the elements' <i>natural*         ordering</i> should be used.* @return the minimum element of the given collection, according*         to the specified comparator.* @throws ClassCastException if the collection contains elements that are*         not <i>mutually comparable</i> using the specified comparator.* @throws NoSuchElementException if the collection is empty.* @see Comparable*/public static <T> T min(Collection<? extends T> coll, Comparator<? super T> comp) {if (comp==null)
            return (T)min((Collection<SelfComparable>) (Collection) coll);Iterator<? extends T> i = coll.iterator();T candidate = i.next();while (i.hasNext()) {T next = i.next();if (comp.compare(next, candidate) < 0)candidate = next;}
        return candidate;}

coll-方法参数是实现了Collection接口的集合，Collection接口实现了Iterator接口
示例如下：

        List<String> list = new ArrayList<String>();list.add("a");list.add("b");list.add("c");list.add("d");list.add("1687");list.add("e");String result = Collections.min(list);String result1 = Collections.max(list);System.out.println(result);System.out.println(result1);

输出结果是:

1687
e

上面的实例会按照自然排序输出集合的最小值1687和最大值e

Collections中的max和min方法除了按照默认自然排序方式取得最大值和最小值外还可以传入一个比较器Comparator定制比较。

        List<String> list = new ArrayList<String>();list.add("a");list.add("b");list.add("c");list.add("d");list.add("1687");list.add("e");String result = Collections.min(list, new Comparator<String>() {@Overridepublic int compare(String o1, String o2) {return o1.compareTo(o2);}});String result1 = Collections.max(list, new Comparator<String>() {@Overridepublic int compare(String o1, String o2) {return o1.compareTo(o2);}});System.out.println(result);System.out.println(result1);

min和max方法中新增一个比较器可以灵活的定制我们的比较规则。

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Collections中min和max工具方法详解

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