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Factory method to create Immutable Set in Java SE 9
With Java 9, new factory methods are added to Set interface to create immutable instances. These factory methods are convenience factory methods to create a collection in less verbose and in concise way.
Old way to create collections
Example
import java.util.Collections;
import java.util.HashSet;
import java.util.Set;
public class Tester {
public static void main(String []args) {
Set<String> set = new HashSet<>();
set.add("A");
set.add("B");
set.add("C");
Set<String> readOnlySet = Collections.unmodifiableSet(set);
System.out.println(readOnlySet);
try {
readOnlySet.remove(0);
} catch (Exception e) {
e.printStackTrace();
}
}
}Output
It will print the following output.
[A, B, C] java.lang.UnsupportedOperationException at java.util.Collections$UnmodifiableCollection.remove(Collections.java:1058) at Tester.main(Tester.java:15)
New Methods
With java 9, following methods are added to Set interface along with their overloaded counterparts.
static <E> Set<E> of(); // returns immutable set of zero element static <E> Set<E> of(E e1); // returns immutable set of one element static <E> Set<E> of(E e1, E e2); // returns immutable set of two elements static <E> Set<E> of(E e1, E e2, E e3); //... static <E> Set<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9, E e10); static <E> Set<E> of(E... elements);// returns immutable set of arbitrary number of elements.
Points to Note
For Set interface, of(...) method is overloaded to have 0 to 10 parameters and one with var args parameter.
These methods returns immutable set and elements cannot be added, removed, or replaced. Calling any mutator method will always cause UnsupportedOperationException to be thrown.
New way to create immutable collections
Example
import java.util.Collections;
import java.util.HashSet;
import java.util.Set;
public class Tester {
public static void main(String []args) {
Set<String> set = Set.of("A","B","C");
Set<String> readOnlySet = Collections.unmodifiableSet(set);
System.out.println(readOnlySet);
try {
readOnlySet.remove(0);
} catch (Exception e) {
e.printStackTrace();
}
}
}Output
It will print the following output.
[A, B, C] java.lang.UnsupportedOperationException at java.util.Collections$UnmodifiableCollection.remove(Collections.java:1058) at Tester.main(Tester.java:10)
Updated on: 21-Jun-2020
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