Java中List，Set，數組的互相轉換

List轉數組

• 通過toArray()推薦

public static void main(String[] args) {
        //1. 通過 toArray()
        List<String> list = new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            list.add("value" + i);
        }
        String[] arrays = list.toArray(new String[0]);
        System.out.println(Arrays.toString(arrays));
    }
           

• jdk1.8 stream

public static void main(String[] args) {
        //2. jdk1.8 stream
        List<String> list = new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            list.add("value" + i);
        }
        String[] arrays = list.stream().toArray(String[]::new);
        System.out.println(Arrays.toString(arrays));
    }
           

數組轉List

• Arrays.asList()

public static void main(String[] args) {
        /*
         * 此種方法生成的List不可進行add和remove操作
         * 因為它是一個定長的List集合，跟數組長度一緻
         */
        String[] array = new String[]{"value1", "value2", "value3"};
        List<String> stringList = Arrays.asList(array);
        System.out.println(stringList.toString());
    }
           

• 通過Collections.addAll(list, arrays)

public static void main(String[] args) {
        //2.通過Collections.addAll(list, arrays);
        List<String> stringList=new ArrayList<>();
        String[] array=new String[]{"value1","value2","value3"};
        Collections.addAll(stringList, array);
        System.out.println(stringList.toString());
    }
           

• jdk1.8 通過Stream

public static void main(String[] args) {
        //3. jdk1.8 通過Stream
        String[] arrays = new String[]{"value1", "value2", "value3"};
        List<String> listStrings = Stream
                .of(arrays)
                .collect(Collectors.toList());
        System.out.println(listStrings.toString());
    }
           

Set轉數組

• 通過 toArray()

private static void setToArray1() {
        Set<String> set = new HashSet<>();
        set.add("value1");
        set.add("value2");
        set.add("value3");
        //Set-->數組
        String[] array=set.toArray(new String[0]);
        System.out.println(Arrays.toString(array));
    }
           

• 通過Stream jdk1.8

private static void setToArray2() {
        Set<String> set = new HashSet<>();
        set.add("value1");
        set.add("value2");
        set.add("value3");
        //Set-->數組
        String[] array= set.stream().toArray(String[]::new);
        System.out.println(Arrays.toString(array));
    }
           

數組轉Set

• 通過先轉List之後引入Set

//數組-->Set
    private static void arrayToSet() {
        String[] array = {"value1","value2","value3"};
        Set<String> set = new HashSet<>(Arrays.asList(array));
        System.out.println(set);
    }
           

• jdk1.8之後通過Stream

private static void arrayToSet2() {
        String[] array = {"value1","value2","value3"};
        Set<String> set = Stream.of(array).collect(Collectors.toSet());
        System.out.println(set);
    }
           

Set轉List

- 普遍使用
    //set為null會報錯
    private static void setToList() {
        Set<String> set = new HashSet<>();
        set.add("value1");
        set.add("value2");
        set.add("value3");
        set=null;
        List<String> list=new ArrayList<>(set);
        System.out.println(list.toString());
    }
           

• jdk1.8通過Stream

//效率低不推薦
    private static void setToList2() {
       Set<String> set = new HashSet<>();
        set.add("value1");
        set.add("value2");
        set.add("value3");
        List list=Stream.of(set.toArray(new String[0])).collect(Collectors.toList());
        System.out.println(list.toString());
    }
           

List轉Set

//list為null會報錯
    private static void listToSet() {
        List<String> list = new ArrayList<>();
        list.add("value1");
        list.add("value2");
        list.add("value3");
        Set<String> set=new HashSet<>(list);
        System.out.println(set.toString());
    }