Java中List，Set，数组的互相转换-蒲公英云

Java中List，Set，数组的互相转换

List转数组

通过toArray()推荐

//1. 通过 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));
    }

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<String> 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<String> 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<String> listStrings = Stream
                .of(arrays)
                .collect(Collectors.toList());
        System.out.println(listStrings.toString());
    }

Set转数组

通过 toArray()

private static void setToArray1() { 
        Set<String> 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<String> 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));
    }

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<String> 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<String> set = Stream.of(array).collect(Collectors.toSet());
        System.out.println(set);
    }

Set转List

普遍使用

//set为null会报错
    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通过Stream

//效率低不推荐
    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转Set

//list为null会报错
    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());
    }