0. Utils

  • 字符串数组的排序:

    Set<String> set = new TreeSet<String>();
    Collections.addAll(set, args);	// args 为 String[] 类型
    
  • 数组复制:

    • system.arraycopy:必须明确自行建立一个数组对象,
    int[] arr1 = {1, 2, 3, 4, 5}; 
    int[] arr2 = new int[5];
    
    System.arraycopy(arr1, 0, arr2, 0, arr1.length);
    
    • Arrays.copyOf:可直接传回一个数组对象:
    int[] arr1 = {1, 2, 3, 4, 5}; 
    int[] arr2 = Arrays.copyOf(arr1, arr1.length);
    // 如果新数组的长度超过原数组的长度,则保留数组默认值
    int[] arr3 = Arrays.copyOf(arr1, 10);
    		// 1 2 3 4 5 0 0 0 0 0
    

1. List <–> Array

  • list 实例转换为 String[],使用 list 的实例方法:toArray

    List<String> list = new ArrayList<String>();  
    list.add("China");  
    list.add("Switzerland");  
    list.add("Italy");  
    list.add("France");  
    String [] countries = list.toArray(new String[list.size()]);  
    
  • String[] => list,使用 Arrays 的静态方法 asList

    String[] countries = {"China", "Switzerland", "Italy", "France"};  
    List list = Arrays.asList(countries);  
    

2. Array => list => Set

  • 其实是先将 array 转换为 list 再转换为 set:

    String [] countries = {"India", "Switzerland", "Italy"};        
    Set<String> set = new HashSet<String>(Arrays.asList(countries));  
    

3. Map => Set/List

  • Map => List

    List<Value> list = new ArrayList<Value>(map.values());
    
  • Map => Set

    Set<String> targetSet = new HashSet<>(sourceMap.values());  
    

4. Collections

  • shuffle:置乱一个数组(list)

    public static void shuffle(List<?> list, Random rnd) {
            int size = list.size();
            if (size < SHUFFLE_THRESHOLD || list instanceof RandomAccess) {
                for (int i=size; i>1; i--)
                    swap(list, i-1, rnd.nextInt(i));
            } else {
                Object arr[] = list.toArray();
    
                // Shuffle array
                for (int i=size; i>1; i--)
                    swap(arr, i-1, rnd.nextInt(i));
    
                ListIterator it = list.listIterator();
                for (int i=0; i<arr.length; i++) {
                    it.next();
                    it.set(arr[i]);
                }
            }
        }
    
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