The newSetFromMap() method of java.util.Collections class is used to return a set backed by the specified map. The resulting set displays the same ordering, concurrency, and performance characteristics as the backing map. In essence, this factory method provides a Set implementation corresponding to any Map implementation. There is no need to use this method on a Map implementation that already has a corresponding Set implementation (such as HashMap or TreeMap).
Syntax:
public static Set newSetFromMap(Map map)
Parameters: This method takes the backing map as a parameter
Return Value: This method returns the set backed by the map
Exception: This method throws IllegalArgumentException, if map is not empty.
Below are the examples to illustrate the newSetFromMap() method
Example 1:
// Java program to demonstrate// newSetFromMap() method import java.util.*; public class GFG1 { public static void main(String[] argv) throws Exception { try { // creating object of Map<String, Boolean> Map<String, Boolean> map = new WeakHashMap<String, Boolean>(); // creating object of LinkedList Set<String> set = Collections.newSetFromMap(map); // add values in set set.add("A"); set.add("B"); set.add("C"); set.add("D"); // set and map values are System.out.println("Map is: " + map); System.out.println("Set from Map is: " + set); } catch (IllegalArgumentException e) { System.out.println("Exception thrown : " + e); } }} |
Map is: {C=true, B=true, A=true, D=true}
Set from Map is: [C, B, A, D]
Example 2: for IllegalArgumentException
// Java program to demonstrate// newSetFromMap() method// for IllegalArgumentException import java.util.*;<div class = 'outputDiv'><b>Output:</b><pre>Set is: [C, B, A, D]Map is: {C=true, B=true, A=true, D=true}</pre></div> public class GFG1 { public static void main(String[] argv) throws Exception { try { // creating object of Map<String, Boolean> Map<String, Boolean> map = new WeakHashMap<String, Boolean>(); // putting value in map map.put("1", true); // creating object of LinkedList Set<String> set = Collections.newSetFromMap(map); // add values in set set.add("A"); set.add("B"); set.add("C"); set.add("D"); // set and map values are System.out.println("Map is: " + map); System.out.println("Set from Map is: " + set); } catch (IllegalArgumentException e) { System.out.println("Exception thrown : " + e); } }} |
Exception thrown : java.lang.IllegalArgumentException: Map is non-empty
