16 数组

16.2 数组是第一级对象

ArrayOptions

class BerylliumSphere {
    private static long counter;
    private final long id = counter++;
    public String toString() {
        return "Sphere:" + id;
    }
}
class ArrayOptions {
    public static void chapter16_2() {
        //(1)对象数组
        //(i)默认初始化
        BerylliumSphere[] a = new BerylliumSphere[3];
        System.out.println(Arrays.toString(a));
        //(ii)列表初始化
        BerylliumSphere[] b = new BerylliumSphere[] {new BerylliumSphere(),new BerylliumSphere()};
        System.out.println(Arrays.toString(b));

        //(2)基本数组
        //(i)默认初始化
        int[] c = new int[5];
        System.out.println(Arrays.toString(c));
        //(ii)列表初始化
        int[] d = new int[5];
        for(int i=0;i<5;i++){
            d[i] = 7;
        }
        System.out.println(Arrays.toString(d));
    }
}

对象数组保存的是引用，基本类型数组直接保存基本类型的值

16.3 返回一个数组

返回一个数组与返回其他对象(实际上返回引用)没什么区别。垃圾回收器会自动清理数组，不必操心。

16.4 多维数组

MultiArray

class MultiArray {
    public static void chapter16_4() {
        Integer[][] a = new Integer[3][];
        for(int i=0;i<a.length;i++) {
            a[i] = new Integer[5];
            for(int j=0;j<a[i].length;j++) {
                a[i][j] = i+j;
            }
        }
        System.out.println(Arrays.deepToString(a));
    }
}

Arrays.deepToString()可以将多维数组转换为多个String

16.7 Arrays实用功能

Arrays类一套用于数组的static实用方法:

equals 数组的比较

deepEquals 多维数组的比较

fill 填充数组

sort 数组排序

binarySearch 在已经排序的数组中查找元素

hashCode 产生数组散列码

Arrays.asList 接受任意的序列或数组作为其参数，并将其转变为List容器

CopyingArrays

class CopyingArrays {
    public static void chapter16_7() {
        int[] src = new int[] {1,2,3,4,5};
        int[] dst = new int[10];
        System.arraycopy(src,0,dst,0,src.length);
        System.out.println(Arrays.toString(dst));

        Integer[] a = new Integer[5];
        Arrays.fill(a,7);
        Integer[] b = new Integer[10];
        System.arraycopy(a,0,b,5,a.length);
        System.out.println(Arrays.toString(b));
        a[4] = 11;
        System.out.println(Arrays.toString(a));
        System.out.println(Arrays.toString(b));
    }
}

System.arraycopy()，用它复制数组比用for循环要快很多。

源数组，从源数组什么位置开始复制的偏移量，目标数组，从目标数组什么位置开始复制的偏移量，需要复制的元素个数

StringSorting

class StringSorting {
    public static void chapter16_7() {
        String[] str = new String[]{"Abcde","AAAAb","HxxHv","aaaaa"};
        System.out.println(Arrays.toString(str));

        Arrays.sort(str);
        System.out.println(Arrays.toString(str));
        int location = Arrays.binarySearch(str,"AAAAb");
        System.out.println("location:" + location);

        Arrays.sort(str,Collections.reverseOrder());
        System.out.println(Arrays.toString(str));
        //wrong
        //location = Arrays.binarySearch(str,"AAAAb");
        location = Arrays.binarySearch(str,"AAAAb", Collections.reverseOrder());
        System.out.println("location:" + location);

        Arrays.sort(str,String.CASE_INSENSITIVE_ORDER);
        System.out.println(Arrays.toString(str));
        location = Arrays.binarySearch(str,"AAAAb", String.CASE_INSENSITIVE_ORDER);
        System.out.println("location:" + location);
    }
}

数组排序：词典序，反词典序，忽略大小写词典序

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