java复习1之数组（进阶）

悠悠 2023-07-13 04:25 62阅读 0赞

**二分查找** 排序在前一篇文章中讲过了，所以二分查找也就有了基础，那么在代码中有middle = start + (end + start)/2;请问你还有哪些方法不？（提示：位运算？为啥不直接（start+end)/2呢？），请在评论区留下你的思考吧！

package 二分查找;
    
    public class test {
        
    	public static void main(String[] args) {
        
    		//二分查找的前提是 数列要有序
    		int[] num = {
        10,20,50,65,88,90};
    		int index = binarySearch(num,66);
    		System.out.println(index);
    	}
    	
    	public static int binarySearch(int[] num,int key) {
        
    		int start = 0;
    		int end = num.length-1;
    		while(start <= end) {
        
    			int middle = start + (end-start)/2;
    			if(num[middle] > key) {
        
    				end = middle-1;
    			}else if(num[middle] < key){
        
    				start = middle+1;
    			}else if(num[middle] == key) {
        
    				return middle;
    			}
    		}
    		return -1;
    	}
    }

**Arrays工具类中的几个方法：**

package Arrays工具类;
    
    import java.util.Scanner;
    import java.util.Arrays;
    
    public class 二分查找 {
        
    	public static void main(String[] args) {
        
    		
    		int[] num = {
        45, 65, 76, 87, 98, 90};
    		
    		//二分查找
    		int index = Arrays.binarySearch(num, 98);
    		System.out.println("找到的下标点：" + index);
    		
    		//输出数组
    		for(int n : num) {
        
    			System.out.println(n);
    		}
    		System.out.println(Arrays.toString(num));
    		//测试输出数据时使用会更简洁方便
    		
    		//排序
    		int[] num2 = {
        10,32,11,44,5,32,65,56};
    		Arrays.sort(num2);
    		System.out.println(Arrays.toString(num2));
    		
    		
    		//数组的复制
    		int[] num3 = Arrays.copyOf(num2, 20);//第一个参数是要copy的数组，第二个参数是新数组的长度
    		System.out.println(Arrays.toString(num3));
    		
    		int num4[] = Arrays.copyOfRange(num2, 2, 5);//前闭后开区间
    		System.out.println(Arrays.toString(num4));
    		
    		int[] newnum = new int[num2.length];
    		System.arraycopy(num2,0,newnum,0,num2.length);
    		System.out.println(Arrays.toString(newnum));
    		
    		//判断两个数组的值是否相等
    		System.out.println(Arrays.equals(num2, newnum));
    		
    		//填充数组
    		Arrays.fill(newnum, 0);
    		System.out.println(Arrays.toString(newnum));
    	}
    }

可以顺便看一看jdk的具体实现：

// 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;
                else
                    return mid; // key found
            }
            return -(low + 1);  // key not found.
        }

我们能平时输出数组是for，这里我们看一下Arrays里toString的实现：

int\[\]型数组为例（long\[\]型，short\[\]型，char\[\]型，byte\[\]型，boolean\[\]型，float\[\]型，double\[\]型,Object\[\]型除了传进来的参数不一样，其他完全一样）：

public static String toString(int[] a) {
        
            if (a == null)
                return "null";
            int iMax = a.length - 1;
            if (iMax == -1)
                return "[]";
    
            StringBuilder b = new StringBuilder();
            b.append('[');
            for (int i = 0; ; i++) {
        
                b.append(a[i]);
                if (i == iMax)
                    return b.append(']').toString();
                b.append(", ");
            }
        }

咱们再看看Arrays里的sort（现在看不懂没关系，之后我会出一份解析源码的专栏）:

private static void sort(int[] a, int left, int right, boolean leftmost) {
        
            int length = right - left + 1;
    
            // Use insertion sort on tiny arrays
            if (length < INSERTION_SORT_THRESHOLD) {
        
                if (leftmost) {
        
                    /*
                     * Traditional (without sentinel) insertion sort,
                     * optimized for server VM, is used in case of
                     * the leftmost part.
                     */
                    for (int i = left, j = i; i < right; j = ++i) {
        
                        int ai = a[i + 1];
                        while (ai < a[j]) {
        
                            a[j + 1] = a[j];
                            if (j-- == left) {
        
                                break;
                            }
                        }
                        a[j + 1] = ai;
                    }
                } else {
        
                    /*
                     * Skip the longest ascending sequence.
                     */
                    do {
        
                        if (left >= right) {
        
                            return;
                        }
                    } while (a[++left] >= a[left - 1]);
    
                    /*
                     * Every element from adjoining part plays the role
                     * of sentinel, therefore this allows us to avoid the
                     * left range check on each iteration. Moreover, we use
                     * the more optimized algorithm, so called pair insertion
                     * sort, which is faster (in the context of Quicksort)
                     * than traditional implementation of insertion sort.
                     */
                    for (int k = left; ++left <= right; k = ++left) {
        
                        int a1 = a[k], a2 = a[left];
    
                        if (a1 < a2) {
        
                            a2 = a1; a1 = a[left];
                        }
                        while (a1 < a[--k]) {
        
                            a[k + 2] = a[k];
                        }
                        a[++k + 1] = a1;
    
                        while (a2 < a[--k]) {
        
                            a[k + 1] = a[k];
                        }
                        a[k + 1] = a2;
                    }
                    int last = a[right];
    
                    while (last < a[--right]) {
        
                        a[right + 1] = a[right];
                    }
                    a[right + 1] = last;
                }
                return;
            }
    
            // Inexpensive approximation of length / 7
            int seventh = (length >> 3) + (length >> 6) + 1;
    
            /*
             * Sort five evenly spaced elements around (and including) the
             * center element in the range. These elements will be used for
             * pivot selection as described below. The choice for spacing
             * these elements was empirically determined to work well on
             * a wide variety of inputs.
             */
            int e3 = (left + right) >>> 1; // The midpoint
            int e2 = e3 - seventh;
            int e1 = e2 - seventh;
            int e4 = e3 + seventh;
            int e5 = e4 + seventh;
    
            // Sort these elements using insertion sort
            if (a[e2] < a[e1]) {
         int t = a[e2]; a[e2] = a[e1]; a[e1] = t; }
    
            if (a[e3] < a[e2]) {
         int t = a[e3]; a[e3] = a[e2]; a[e2] = t;
                if (t < a[e1]) {
         a[e2] = a[e1]; a[e1] = t; }
            }
            if (a[e4] < a[e3]) {
         int t = a[e4]; a[e4] = a[e3]; a[e3] = t;
                if (t < a[e2]) {
         a[e3] = a[e2]; a[e2] = t;
                    if (t < a[e1]) {
         a[e2] = a[e1]; a[e1] = t; }
                }
            }
            if (a[e5] < a[e4]) {
         int t = a[e5]; a[e5] = a[e4]; a[e4] = t;
                if (t < a[e3]) {
         a[e4] = a[e3]; a[e3] = t;
                    if (t < a[e2]) {
         a[e3] = a[e2]; a[e2] = t;
                        if (t < a[e1]) {
         a[e2] = a[e1]; a[e1] = t; }
                    }
                }
            }
    
            // Pointers
            int less  = left;  // The index of the first element of center part
            int great = right; // The index before the first element of right part
    
            if (a[e1] != a[e2] && a[e2] != a[e3] && a[e3] != a[e4] && a[e4] != a[e5]) {
        
                /*
                 * Use the second and fourth of the five sorted elements as pivots.
                 * These values are inexpensive approximations of the first and
                 * second terciles of the array. Note that pivot1 <= pivot2.
                 */
                int pivot1 = a[e2];
                int pivot2 = a[e4];
    
                /*
                 * The first and the last elements to be sorted are moved to the
                 * locations formerly occupied by the pivots. When partitioning
                 * is complete, the pivots are swapped back into their final
                 * positions, and excluded from subsequent sorting.
                 */
                a[e2] = a[left];
                a[e4] = a[right];
    
                /*
                 * Skip elements, which are less or greater than pivot values.
                 */
                while (a[++less] < pivot1);
                while (a[--great] > pivot2);
    
                /*
                 * Partitioning:
                 *
                 *   left part           center part                   right part
                 * +--------------------------------------------------------------+
                 * |  < pivot1  |  pivot1 <= && <= pivot2  |    ?    |  > pivot2  |
                 * +--------------------------------------------------------------+
                 *               ^                          ^       ^
                 *               |                          |       |
                 *              less                        k     great
                 *
                 * Invariants:
                 *
                 *              all in (left, less)   < pivot1
                 *    pivot1 <= all in [less, k)     <= pivot2
                 *              all in (great, right) > pivot2
                 *
                 * Pointer k is the first index of ?-part.
                 */
                outer:
                for (int k = less - 1; ++k <= great; ) {
        
                    int ak = a[k];
                    if (ak < pivot1) {
         // Move a[k] to left part
                        a[k] = a[less];
                        /*
                         * Here and below we use "a[i] = b; i++;" instead
                         * of "a[i++] = b;" due to performance issue.
                         */
                        a[less] = ak;
                        ++less;
                    } else if (ak > pivot2) {
         // Move a[k] to right part
                        while (a[great] > pivot2) {
        
                            if (great-- == k) {
        
                                break outer;
                            }
                        }
                        if (a[great] < pivot1) {
         // a[great] <= pivot2
                            a[k] = a[less];
                            a[less] = a[great];
                            ++less;
                        } else {
         // pivot1 <= a[great] <= pivot2
                            a[k] = a[great];
                        }
                        /*
                         * Here and below we use "a[i] = b; i--;" instead
                         * of "a[i--] = b;" due to performance issue.
                         */
                        a[great] = ak;
                        --great;
                    }
                }
    
                // Swap pivots into their final positions
                a[left]  = a[less  - 1]; a[less  - 1] = pivot1;
                a[right] = a[great + 1]; a[great + 1] = pivot2;
    
                // Sort left and right parts recursively, excluding known pivots
                sort(a, left, less - 2, leftmost);
                sort(a, great + 2, right, false);
    
                /*
                 * If center part is too large (comprises > 4/7 of the array),
                 * swap internal pivot values to ends.
                 */
                if (less < e1 && e5 < great) {
        
                    /*
                     * Skip elements, which are equal to pivot values.
                     */
                    while (a[less] == pivot1) {
        
                        ++less;
                    }
    
                    while (a[great] == pivot2) {
        
                        --great;
                    }
    
                    /*
                     * Partitioning:
                     *
                     *   left part         center part                  right part
                     * +----------------------------------------------------------+
                     * | == pivot1 |  pivot1 < && < pivot2  |    ?    | == pivot2 |
                     * +----------------------------------------------------------+
                     *              ^                        ^       ^
                     *              |                        |       |
                     *             less                      k     great
                     *
                     * Invariants:
                     *
                     *              all in (*,  less) == pivot1
                     *     pivot1 < all in [less,  k)  < pivot2
                     *              all in (great, *) == pivot2
                     *
                     * Pointer k is the first index of ?-part.
                     */
                    outer:
                    for (int k = less - 1; ++k <= great; ) {
        
                        int ak = a[k];
                        if (ak == pivot1) {
         // Move a[k] to left part
                            a[k] = a[less];
                            a[less] = ak;
                            ++less;
                        } else if (ak == pivot2) {
         // Move a[k] to right part
                            while (a[great] == pivot2) {
        
                                if (great-- == k) {
        
                                    break outer;
                                }
                            }
                            if (a[great] == pivot1) {
         // a[great] < pivot2
                                a[k] = a[less];
                                /*
                                 * Even though a[great] equals to pivot1, the
                                 * assignment a[less] = pivot1 may be incorrect,
                                 * if a[great] and pivot1 are floating-point zeros
                                 * of different signs. Therefore in float and
                                 * double sorting methods we have to use more
                                 * accurate assignment a[less] = a[great].
                                 */
                                a[less] = pivot1;
                                ++less;
                            } else {
         // pivot1 < a[great] < pivot2
                                a[k] = a[great];
                            }
                            a[great] = ak;
                            --great;
                        }
                    }
                }
    
                // Sort center part recursively
                sort(a, less, great, false);
    
            } else {
         // Partitioning with one pivot
                /*
                 * Use the third of the five sorted elements as pivot.
                 * This value is inexpensive approximation of the median.
                 */
                int pivot = a[e3];
    
                /*
                 * Partitioning degenerates to the traditional 3-way
                 * (or "Dutch National Flag") schema:
                 *
                 *   left part    center part              right part
                 * +-------------------------------------------------+
                 * |  < pivot  |   == pivot   |     ?    |  > pivot  |
                 * +-------------------------------------------------+
                 *              ^              ^        ^
                 *              |              |        |
                 *             less            k      great
                 *
                 * Invariants:
                 *
                 *   all in (left, less)   < pivot
                 *   all in [less, k)     == pivot
                 *   all in (great, right) > pivot
                 *
                 * Pointer k is the first index of ?-part.
                 */
                for (int k = less; k <= great; ++k) {
        
                    if (a[k] == pivot) {
        
                        continue;
                    }
                    int ak = a[k];
                    if (ak < pivot) {
         // Move a[k] to left part
                        a[k] = a[less];
                        a[less] = ak;
                        ++less;
                    } else {
         // a[k] > pivot - Move a[k] to right part
                        while (a[great] > pivot) {
        
                            --great;
                        }
                        if (a[great] < pivot) {
         // a[great] <= pivot
                            a[k] = a[less];
                            a[less] = a[great];
                            ++less;
                        } else {
         // a[great] == pivot
                            /*
                             * Even though a[great] equals to pivot, the
                             * assignment a[k] = pivot may be incorrect,
                             * if a[great] and pivot are floating-point
                             * zeros of different signs. Therefore in float
                             * and double sorting methods we have to use
                             * more accurate assignment a[k] = a[great].
                             */
                            a[k] = pivot;
                        }
                        a[great] = ak;
                        --great;
                    }
                }
    
                /*
                 * Sort left and right parts recursively.
                 * All elements from center part are equal
                 * and, therefore, already sorted.
                 */
                sort(a, left, less - 1, leftmost);
                sort(a, great + 1, right, false);
            }
        }

Arrays里copyOf的实现：

public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
        
            @SuppressWarnings("unchecked")
            T[] copy = ((Object)newType == (Object)Object[].class)
                ? (T[]) new Object[newLength]
                : (T[]) Array.newInstance(newType.getComponentType(), newLength);
            System.arraycopy(original, 0, copy, 0,
                             Math.min(original.length, newLength));
            return copy;
        }

然后看一下arraycopy的实现（看不到，除非SUN授权查看源码的地方可以查看，这里就暂时不看了，日后在源码分析的专栏里进行分析）

public static native void arraycopy(Object src,  int  srcPos,
                                            Object dest, int destPos,
                                            int length);