Detailed explanation of conversion between java list, set, map and arrays

java list, set, map, detailed explanation of mutual conversion between arrays

1.list to set

Set set = new HashSet( new ArrayList());

2.set to list

List list = new ArrayList( new HashSet());

3. Convert the array to a list

List stooges = Arrays.asList( "Larry" , "Moe" , "Curly" );

At this time, there are three elements in stooges. Note: The add operation cannot be performed on the list at this time, otherwise "java.lang.UnsupportedOperationException" will be reported. Arrays.asList() returns a List, and it is a fixed-length List, so it cannot be converted to an ArrayList and can only be converted to AbstractList

The reason is that the asList() method returns the list form of an array. The returned list is just another view of the array, and the array itself does not disappear. Any operation on the list will eventually Reflected on the array. Therefore, the remove and add methods are not supported

String[] arr = { "1" , "2" };
List list = Arrays.asList(arr);

4. The array is converted to set

int [] a = { 1 , 2 , 3 };
Set set = new HashSet(Arrays.asList(a));

5.map related operate.

Map map = new HashMap();
map.put("1" , "a" );
map.put('2' , 'b' );
map.put('3' , 'c' );
System.out.println(map);
// 输出所有的值 
System.out.println(map.keySet());
// 输出所有的键 
System.out.println(map.values());
// 将map的值转化为List 
List list = new ArrayList(map.values());
System.out.println(list);
// 将map的值转化为Set 
Set set = new HashSet(map.values());
System.out.println(set);

6. List to array

List list = Arrays.asList( "a" , "b" );
System.out.println(list);
      
String[] arr = (String[])list.toArray(new String[list.size()]);
System.out.println(Arrays.toString(arr));

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