Catalog
1. Array --"List
To convert an array into a collection, the array must be a reference data type. If an array of basic data types is converted into a collection, the whole array will be converted as an object.
Arrays.asList() returns a fixed-size list supported by a specified array. So you can't do Add, Remove, etc.
private static void arrToList(){
// Converting an array of basic data types into a collection takes the entire array as an object transformation
// Output: [[I@3830f1c0]
int[] arr = {1, 2, 3, 4, 5};
List<int[]> list = Arrays.asList(arr);
System.out.println(list);
// To convert an array into a collection, the array must be a reference data type
// Output: [1, 2, 3, 4, 5]
Integer[] arr2 = {1, 2, 3, 4, 5};
// Arrays.asList() returns a fixed-size list supported by a specified array. So you can't do Add, Remove, etc.
List<Integer> list2 = Arrays.asList(arr2);
// Just create a new List again.
List<Integer> list3 = new ArrayList<>(list2);
list3.add(7);
System.out.println(list2);
System.out.println(list3);
}2. List --"Array
private static void arrToList(){
// Arrays must be reference data types
List<String> list = new ArrayList<>();
for (int i = 0; i < 4; i++) {
list.add("1-"+i);
}
String[] arr = new String[list.size()];
list.toArray(arr);
System.out.println(Arrays.toString(arr));
}3,List —> Set / Set —> List
Converting a List set to a Set set can be directly de-duplicated.
private static void listToSet(){
List<String> list = new ArrayList<>();
for (int i = 0; i < 4; i++) {
list.add("1-"+i);
}
// Direct a new object, just put the list in
Set<String> set = new HashSet<>(list);
System.out.println(set.toString());
}Similar to a List collection, a set collection can be transferred directly to a List object and placed into it.
4,List -> Map
If our List stores an object, we want to convert it into a Map collection, with key as an attribute of the object. (Implemented using Stream streams)
Entity class:
/**
* @author MouFangCai
* @date 2019/8/2 10:41
*/
public class TestEntity {
private Integer id;
private String name;
@Override
public String toString() {
return "TestEntity{" +
"id=" + id +
", name='" + name + '\'' +
'}';
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
private static void listToMap(){
List<TestEntity> list = new ArrayList<>();
for (int i = 0; i < 4; i++) {
TestEntity testEntity = new TestEntity();
testEntity.setId(i);
testEntity.setName("name-"+i);
list.add(testEntity);
}
System.out.println(list.toString());
// Using stream stream stream to achieve high efficiency
// Here the key of the map is the id of TestEntity
Map<Integer, TestEntity> map = list.stream().collect
(Collectors.toMap(TestEntity::getId, Function.identity(), (key1, key2) -> key2));
System.out.println(map.toString());
}5,Map —> List / Set
private static void mapToList(){
Map<Integer, TestEntity> map = new HashMap<>();
for (int i = 0; i < 4; i++) {
TestEntity testEntity = new TestEntity();
testEntity.setId(i);
testEntity.setName("name-"+i);
map.put(i, testEntity);
}
// Get the set set set of map key s directly
Set<Integer> set1 = map.keySet();
// Converting map values to Set sets
Set<TestEntity> set = new HashSet<>(map.values());
// Converting map values to List collections
List<TestEntity> list = new ArrayList<>(map.values());
System.out.println(set.toString());
System.out.println(list.toString());
}6,List —> List / Set
Requirement: List1 collection store is an object, and we want to get a List2 collection of an object's attributes directly from the collection (implemented using Stream stream)
private static void listToList(){
List<TestEntity> list = new ArrayList<>();
for (int i = 0; i < 4; i++) {
TestEntity testEntity = new TestEntity();
testEntity.setId(i);
testEntity.setName("name-"+i);
list.add(testEntity);
}
// Get a list collection of name s
List<String> list1 = list.stream().map(TestEntity::getName).collect(Collectors.toList());
// Get the set set set set for name
Set<String> set = list.stream().map(TestEntity::getName).collect(Collectors.toSet());
System.out.println(list1.toString());
}