Example of Collections.singleton in Java
November 12, 2023
Collections.singleton, Collections.singletonList and Collections.singletonMap. They return serialized and immutable Java Set, List and Map respectively containing the specified object.
Find the example.
1. Collections.singleton
Collections.singleton returns a serialized and immutable Set containing given object.
CollectionsSingleton.java
package com.concretepage.util;
import java.util.Collections;
import java.util.Set;
public class CollectionsSingleton {
public static void main(String[] args) {
User user1 = new User("Mahesh");
Set<User> set= Collections.singleton(user1);
User user2 = new User("Ramesh");
//will throw error
set.add(user2);
}
}
package com.concretepage.util;
public class User {
private String name;
public User(String name) {
this.name = name;
}
public String getName() {
return name;
}
}
Exception in thread "main" java.lang.UnsupportedOperationException at java.util.AbstractCollection.add(AbstractCollection.java:262) at com.concretepage.util.CollectionsSingleton.main(CollectionsSingleton.java:11)
2. Collections.singletonList
Collections.singletonList returns a serialized and immutable List containing given object.
CollectionsSingletonList.java
package com.concretepage.util;
import java.util.Collections;
import java.util.List;
public class CollectionsSingletonList {
public static void main(String[] args) {
User user1 = new User("Mahesh");
List<User> list= Collections.singletonList(user1);
System.out.println(list.get(0).getName());
User user2 = new User("Ramesh");
//will throw error
list.add(user2);
}
}
3. Collections.singletonMap
Collections.singletonMap returns a serialized and immutable Map containing given key and value.
CollectionsSingletonMap.java
package com.concretepage.util;
import java.util.Collections;
import java.util.Map;
public class CollectionsSingletonMap {
public static void main(String[] args) {
User user1 = new User("Mahesh");
Map<Integer, User> map= Collections.singletonMap(1, user1);
System.out.println(map.get(1).getName());
User user2 = new User("Ramesh");
//will throw error
map.put(2, user2);
}
}
