Utilities

Introduction

java.util contains the collections framework, legacy collection classes, event model, date and time facilities, internationalization, and miscellaneous utility classes (a string tokenizer, a random-number generator, and a bit array).

Java Collection simply means a single unit of objects. Java Collection framework provides many interfaces (Set, List, Queue, Deque etc.) and classes (ArrayList, Vector, LinkedList, PriorityQueue, HashSet, LinkedHashSet, TreeSet etc).

Methods of Collection interface

Method	Description
public boolean add(Object element)	This method is used to insert an element in a collection.
public boolean addAll(Collection c)	This method is used to insert all elements of Collection c in the invoking collection.
public boolean remove(Object element)	This method is used to delete an element from a collection.
public boolean removeAll(Collection c)	This method is used to delete all the elements of Collection c from the invoking collection.
public boolean retainAll(Collection c)	This method is used to delete all the elements of invoking collection except the elements which are present in specified Collection c.
public int size()	This method returns the total number of elements in the collection.
public void clear()	This method removes all the elements from the collection.
public boolean contains(Object element)	This method is used to search an element in invoking collection.
public boolean containsAll(Collection c)	This method is used to check if all the elements of Collection c are present in invoking collection or not.
public Iterator iterator()	This method returns an iterator.
public boolean isEmpty()	This method checks if collection is empty.
public boolean equals(Object element)	This method is used to compare objects of collections.

NOTE!- The above methods work for Set and List Interfaces.

Methods of Iterator interface

Method	Description
public boolean hasNext()	It returns true if iterator has more elements.
public Object next()	It returns the element and moves the cursor pointer to the next element.
public void remove()	It removes the last elements returned by the iterator. It is rarely used.

Example

//This example demonstrates the use of all methods listed above
import java.util.*;

public class Main
{
    public static void main(String args[])
    {
        ArrayList <Integer> num= new ArrayList<Integer>();
        ArrayList <Integer> even_num= new ArrayList<Integer>();
        ArrayList <Integer> odd_num= new ArrayList<Integer>();
        
        //inserting elements using add() method
        for(int i=1;i<=10;i++)
            num.add(i);
        
        System.out.println("Original list of numbers is: ");
        //Print numbers using iterator
        Iterator it = num.iterator(); 
        while(it.hasNext())
        System.out.println(it.next());
                
        for(int i=2;i<=10;i=i+2)
            even_num.add(i);
        
        System.out.println("List of even numbers is: ");
        Iterator even_it = even_num.iterator(); 
        while(even_it.hasNext())
        System.out.println(even_it.next());
        
        
        //inserting all elements of num in odd_num
        odd_num.addAll(num);
        
        //removing elements of even_num from odd_num
        odd_num.removeAll(even_num);
        
        System.out.println("List of odd numbers is: ");
        Iterator odd_it = odd_num.iterator(); 
        while(odd_it.hasNext())
        System.out.println(odd_it.next());
        
        
        num.clear();
        int n =  num.size();
        int e = even_num.size();
        System.out.println("Size of num list is "+n+" and that of even_num list is "+e);
        
        //check if even_num list contains 10
        boolean check = even_num.contains(10);
        System.out.println("even_num contains 10? "+check);
        
        //check if even_num contains all elements of odd_num
        check = even_num.containsAll(odd_num);
        System.out.println("even_num contains odd_num? "+check);
        boolean empty = num.isEmpty();
        System.out.println("num is empty? "+empty);
        //even if any one element of odd_num is not present in even_num, the method will return false
        
        //check if temp is equl to odd_num
        ArrayList <Integer> temp= new ArrayList<Integer>();
        temp.addAll(odd_num);
        check= temp.equals(odd_num);
        System.out.println("temp list equal to odd_num? "+check);
        
        
    }
}

Output

Original list of numbers is: 
1
2
3
4
5
6
7
8
9
10
List of even numbers is: 
2
4
6
8
10
List of odd numbers is: 
1
3
5
7
9
Size of num list is 0 and that of even_num list is 5
even_num contains 10? true
even_num contains odd_num? false
num is empty? true
temp list equal to odd_num? true

Methods of Map Interface

Method | Description ———— | ————- Object put(Object key, Object value) | It is used to insert an entry in this map. void putAll(Map map) | It is used to insert the specified map in this map. Object remove(Object key) | It is used to delete an entry for the specified key. Object get(Object key) | It is used to return the value for the specified key. boolean containsKey(Object key) | It is used to search the specified key from this map. Set keySet() | It is used to return the Set view containing all the keys. Set entrySet() | It is used to return the Set view containing all the keys and values.

Methods of Map.Entry Interface

Entry is the sub interface of Map. So we will be accessed it by Map.Entry name. It provides methods to get key and value.

Method	Description
Object getKey()	It is used to obtain key.
Object getValue()	It is used to obtain value.

Maps can also be traversed using Iterator.

Example

//This example demonstrates use of all Map Interface methods
import java.util.*;  
class Main
{  
  public static void main(String args[])
  {  
   Map<Integer,String> map=new HashMap<Integer,String>(); 
   Map<Integer,String> copy=new HashMap<Integer,String>();
   map.put(100,"ABC");  
   map.put(101,"XYZ");  
   map.put(102,"PQR");  
   
   //traversing map
   System.out.println("Traversing map:");
   for(Map.Entry m:map.entrySet())  
    System.out.println(m.getKey()+" "+m.getValue());
   
   
   System.out.println("Value of key 100 in map is " +map.get(100));
   
   //inserting all elements of map into copy
   copy.putAll(map);
   System.out.println("Traversing copy: ");
   for(Map.Entry c:copy.entrySet())  
    System.out.println(c.getKey()+" "+c.getValue());
   
   
   map.remove(102);
   System.out.println("Removed key 102 from map.");
   boolean check = map.containsKey(102);
   
   System.out.println("map contains key 102? "+check);
   
 }  
}  

Output

Traversing map:
100 ABC
101 XYZ
102 PQR
Value of key 100 in map is ABC
Traversing copy: 
100 ABC
101 XYZ
102 PQR
Removed key 102 from map.
map contains key 102? false