[Java] Create a collection with only one element

The Collections class provides methods to create collections that have only one element.

Returns Set, List, and Map with one element for each.

import java.util.Collections;
import java.util.Set;
import java.util.List;
import java.util.Map;

import java.util.Collections.ImmutableSet;

class CollectionsSingleton {
    public static void main(String args[]) {
        Set<String> s = Collections.singleton("a");
        // s.add("b"); // UnsupportedOperationException

        List<String> l = Collections.singletonList("a");

        Map<Integer, String> m = Collections.singletonMap(0, "a");

The returned collection is immutable. An UnsupportedOperationException is thrown when trying to add an element or the like.


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