其实这个题目并不难，但是却是很实用，因为很多同学对于异常条件吃不准的话就会有问题。
看看JDK自带的二分查找吧
/*** Searches a range of* the specified array of longs for the specified value using the* binary search algorithm.* The range must be sorted (as* by the {@link #sort(long[], int, int)} method)* prior to making this call. If it* is not sorted, the results are undefined. If the range contains* multiple elements with the specified value, there is no guarantee which* one will be found.** @param a the array to be searched* @param fromIndex the index of the first element (inclusive) to be* searched* @param toIndex the index of the last element (exclusive) to be searched* @param key the value to be searched for* @return index of the search key, if it is contained in the array* within the specified range;* otherwise, <tt>(-(<i>insertion point</i>) - 1)</tt>. The* <i>insertion point</i> is defined as the point at which the* key would be inserted into the array: the index of the first* element in the range greater than the key,* or <tt>toIndex</tt> if all* elements in the range are less than the specified key. Note* that this guarantees that the return value will be >= 0 if* and only if the key is found.* @throws IllegalArgumentException* if {@code fromIndex > toIndex}* @throws ArrayIndexOutOfBoundsException* if {@code fromIndex < 0 or toIndex > a.length}* @since 1.6*/public static int binarySearch(long[] a, int fromIndex, int toIndex,long key) {rangeCheck(a.length, fromIndex, toIndex);return binarySearch0(a, fromIndex, toIndex, key);}/*** Checks that {@code fromIndex} and {@code toIndex} are in* the range and throws an exception if they aren't.*/private static void rangeCheck(int arrayLength, int fromIndex, int toIndex) {if (fromIndex > toIndex) {throw new IllegalArgumentException("fromIndex(" + fromIndex + ") > toIndex(" + toIndex + ")");}if (fromIndex < 0) {throw new ArrayIndexOutOfBoundsException(fromIndex);}if (toIndex > arrayLength) {throw new ArrayIndexOutOfBoundsException(toIndex);}}// Like public version, but without range checks.private static int binarySearch0(long[] a, int fromIndex, int toIndex,long key) {int low = fromIndex;int high = toIndex - 1;while (low <= high) {int mid = (low + high) >>> 1;long midVal = a[mid];if (midVal < key)low = mid + 1;else if (midVal > key)high = mid - 1;elsereturn mid; // key found}return -(low + 1); // key not found.}