数组扩容：

        int arr1[] = {1,2,3};
        int arr2[] = new int[arr1.length+1];
        for (int i = 0; i < arr2.length-1; i++) {
            arr2[i]=arr1[i];
            arr2[arr2.length-1]= 4;
        }
        System.out.println(Arrays.toString(arr2));

输出结果：
[1, 2, 3, 4]

数组缩减：

        int[] arr1 = {1, 2, 3, 4, 5};
        for (int i = 0; i <= arr1.length; i++) {
            if (arr1.length - i==0){
                System.out.println("不能再进行缩减");
                break;
            }
            int arr2[] = new int[arr1.length - i];
            for (int j = 0; j <= arr2.length-1; j++) {
                arr2[j] = arr1[j];
            }
            System.out.println(Arrays.toString(arr2));
        }

输出结果：
[1, 2, 3, 4, 5]
[1, 2, 3, 4]
[1, 2, 3]
[1, 2]
[1]
不能再进行缩减