Java中List，Set，数组的互相转换

1. List转数组

• 通过toArray()推荐

    public static void main(String[] args) {
        //1. 通过 toArray()
        List 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 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));
    }

2.数组转List

• Arrays.asList()

    public static void main(String[] args) {
        /*
         * 此种方法生成的List不可进行add和remove操作
         * 因为它是一个定长的List集合，跟数组长度一致
         */
        String[] array = new String[]{"value1", "value2", "value3"};
        List stringList = Arrays.asList(array);
        System.out.println(stringList.toString());
    }

• 通过Collections.addAll(list, arrays)

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

• jdk1.8 通过Stream

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

3. Set转数组

• 通过 toArray()

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

• 通过Stream jdk1.8

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

4. 数组转Set

-通过先转List之后引入Set

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

• jdk1.8之后通过Stream

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

5. Set转List

• 普遍使用

    //set为null会报错
    private static void setToList() {
        Set set = new HashSet<>();
        set.add("value1");
        set.add("value2");
        set.add("value3");
        set=null;
        List list=new ArrayList<>(set);
        System.out.println(list.toString());
    }

• jdk1.8通过Stream

    //效率低不推荐
    private static void setToList2() {
       Set 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());
    }

6. List转Set

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

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