Detailed explanation of conversion between list, set and array in JAVA

To convert between JAVA lists, sets, and arrays, you mainly use Apache Jakarta Commons Collections. The specific methods are as follows:

import org.apache.commons.collections.CollectionUtils;

String[] strArray = {"aaa", "bbb", "ccc"};

List strList = new ArrayList();

Set strSet = new HashSet();

CollectionUtils.addAll(strList, strArray);

CollectionUtils.addAll(strSet, strArray);

The implementation of the CollectionUtils.addAll() method is very simple, it just uses the add() method of Collection in a loop.

If you just want to convert an array into a List, you can use the java.util.Arrays class in the JDK:

import java.util.Arrays;

String[] strArray = {"aaa", "bbb", "ccc"};

List strList = Arrays.asList(strArray);

However, the List returned by the Arrays.asList() method cannot add objects, because the implementation of this method uses the size of the array referenced by the parameter to create a new ArrayList.

★ Collection to array

Use Collection's toArray() method directly, which has two overloaded versions:

Object[] toArray();

T[] toArray(T[] a);

★ Map to Collection

Use Map's values() method directly.

★ List and Set conversion

List list = new ArrayList(new Hashset());// Fixed-size list

List list = Arrays.asList(array);//Growable

list list = new LinkedList(Arrays.asList(array));//Duplicate elements are discarded

Set set = new HashSet(Arrays.asList(array));