Collections Framework : MCQs & PYQs with Explanation

Option B

In Java, a Set is a collection that does not allow duplicate elements. Among the given options, TreeSet is the collection that maintains its elements in sorted order.
By default, TreeSet uses the elements natural ordering. For example, numbers are arranged from smaller to larger, while strings are arranged according to their natural lexicographical ordering.
E.g.:
import java.util.TreeSet;
class TestExample
{
    public static void main(String args[])
    {
        TreeSet<Integer> set = new TreeSet<Integer>();
        set.add(44);
        set.add(11);
        set.add(33);
        set.add(22);
        set.add(55);
        System.out.println(set);
    }
}
OUTPUT
[11, 22, 33, 44, 55]
Although the values were inserted as 44, 11, 33, 22, 55, TreeSet displays them in sorted order.

Natural Ordering vs Comparator in TreeSet:
TreeSet always maintains its elements according to some ordering rule. That ordering can come either from the elements themselves (Natural Ordering) or from a separate Comparator supplied by the programmer.

Java Collections:

Option C

In Java, the Collections Framework provides different collection classes for storing and managing groups of objects. These classes differ in important properties such as insertion order, duplicate elements, sorting, and uniqueness.
ArrayList is a List implementation. A List generally maintains the order in which elements are inserted and allows duplicate elements.
E.g.:
import java.util.ArrayList;
class TestExample
{
    public static void main(String args[])
    {
        ArrayList<String> list = new ArrayList<String>();
        list.add("Suresh");
        list.add("Banjara");
        list.add("Suresh");
        System.out.println(list);
    }
}
OUTPUT
[Suresh, Banjara, Suresh]
The first "Suresh" and the second "Suresh" are both retained. This shows that ArrayList permits duplicates. It also preserves their insertion order.

Note:

• List, generally maintains element order and allows duplicates.
• Set, stores unique elements and does not allow duplicate elements.

Option B

In the Java Collections Framework, Collection is the main root interface for collections that store individual elements. Interfaces such as List, Set, and Queue are derived from Collection.

Collection vs Collections vs Map:

Option D

In Java, the Map interface is used when data needs to be stored as key-value pairs. Each key is associated with a value, allowing the value to be retrieved using its corresponding key.
The TreeMap class is an important implementation of the Map interface. It stores entries in a sorted order based on the natural ordering of their keys, unless a custom Comparator is supplied.
E.g.:
import java.util.TreeMap;
class Example
{
    public static void main(String args[])
    {
        TreeMap<Integer, String> students = new TreeMap<>();
        students.put(103, "Suresh");
        students.put(101, "Banjara");
        students.put(102, "Nirmohi");
        System.out.println(students);
    }
}
OUTPUT
{101=Banjara, 102=Nirmohi, 103=Suresh}

Option C

In Java, a Map stores data in the form of key-value pairs. Different Map implementations provide different ordering behaviors.
LinkedHashMap is a Map implementation that maintains a predictable insertion order of its entries by default. This means that when entries are retrieved or iterated, they appear in the same order in which they were inserted.
E.g.:
import java.util.LinkedHashMap;
class TestExample
{
    public static void main(String args[])
    {
        LinkedHashMap<Integer, String> map = new LinkedHashMap<Integer, String>();
        map.put(3, "Sangari");
        map.put(1, "Banjara");
        map.put(4, "Suresh");
        map.put(2, "Nirmohi");
        System.out.println(map);
    }
}
OUTPUT
{3=Sangari, 1=Banjara, 4=Suresh, 2=Nirmohi}
The entries are maintained in their insertion order {3=Sangari, 1=Banjara, 4=Suresh, 2=Nirmohi}. Notice that the keys are not sorted numerically. They remain in the order 3, 1, 4, 2 because that was the order in which they were inserted.

Option D

In Java, a Vector is a class that implements a dynamic array, which means it can grow or shrink automatically as elements are added or removed. 

• It is part of the java.util, not java.io package. 
• The Vector class is synchronized, which means it is thread-safe and can be used safely in a multithreaded environment.

Option B

HashMap is a commonly used implementation of the Java Map interface. It stores data in the form of key-value pairs, where each key is unique.
One important property of HashMap is that it allows one null key and can also contain multiple null values.
E.g.:
import java.util.HashMap;
class TestExample
{
    public static void main(String args[])
    {
        HashMap<Integer, String> map = new HashMap<Integer, String>();
        map.put(null, "Suresh");
        map.put(1, "Banjara");
        map.put(2, null);
        map.put(3, null);
        System.out.println(map);
    }
}
OUTPUT
{null=Suresh, 1=Banjara, 2=null, 3=null}
Here, null is used as a key once, while null can appear as a value for multiple different keys. So, a HashMap can have one null key and multiple null values.
If another entry is inserted using null as the key as map.put(null, "Nirmohi"); it does not create a second null key. Instead, the value associated with the existing null key is replaced.
E.g.:
import java.util.HashMap;
class TestExample
{
    public static void main(String args[])
    {
        HashMap<Integer, String> map = new HashMap<Integer, String>();
        map.put(null, "Suresh");
        map.put(1, "Banjara");
        map.put(2, null);
        map.put(3, null);
        map.put(null, "Nirmohi"); 
        System.out.println(map);
    }
}
OUTPUT
{null=Nirmohi, 1=Banjara, 2=null, 3=null}