Arrays工具类的方法源码解析

public static String toString(int[] a)
public static void sort(int[] a) 底层是快速排序，知道就可以了。
public static int binarySearch(int[] a,int key)

开发原则：
 只要是对象，我们就要判断该对象是否为null。

int[] arr = { 24, 69, 80, 57, 13 };
System.out.println("排序前：" + Arrays.toString(arr));

public static String toString(int[] a) {
 //a -- arr -- { 24, 69, 80, 57, 13 }

    if (a == null)
        return "null"; //说明数组对象不存在
    int iMax = a.length - 1; //iMax=4;
    if (iMax == -1)
        return "[]"; //说明数组存在,但是没有元素。

    StringBuilder b = new StringBuilder();
    b.append('['); //"["
    for (int i = 0; ; i++) {
        b.append(a[i]); //"[24, 69, 80, 57, 13"
        if (i == iMax)
         //"[24, 69, 80, 57, 13]"
            return b.append(']').toString();
        b.append(", "); //"[24, 69, 80, 57, "
    }
}
-----------------------------------------------------

int[] arr = {13, 24, 57, 69, 80};
System.out.println("binarySearch:" + Arrays.binarySearch(arr, 577));

public static int binarySearch(int[] a, int key) {
 //a -- arr -- {13, 24, 57, 69, 80}
 //key -- 577
    return binarySearch0(a, 0, a.length, key);
}

private static int binarySearch0(int[] a, int fromIndex, int toIndex,
                                 int key) {
    //a -- arr --  {13, 24, 57, 69, 80}
    //fromIndex -- 0
    //toIndex -- 5
    //key -- 577                          
                                
                                
    int low = fromIndex; //low=0
    int high = toIndex - 1; //high=4

    while (low <= high) {
        int mid = (low + high) >>> 1; //mid=2,mid=3,mid=4
        int midVal = a[mid]; //midVal=57,midVal=69,midVal=80

        if (midVal < key)
            low = mid + 1; //low=3,low=4,low=5
        else if (midVal > key)
            high = mid - 1;
        else
            return mid; // key found
    }
    return -(low + 1);  // key not found.
}