Java Collection Framework
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Jha AvinashJava Collection Framework
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Jha AvinashOverview of Java Collection Framework
heJava Collection Frameworkis a unified architecturefor managing groups of objects. It
T
provides interfaces, classes, and algorithms to manipulate and store data efficiently.
Introduced in JDK 1.2, it eliminates the need for custom data structures by providing
reusable implementations.
Key Components
1. Interfaces: Define the contract or blueprint for collections.Examples:
○
Collection
○ List
○ Set
○ Queue
○ Map(not part of theCollectionhierarchy but a keycomponent)
2. Classes: Implementations of the interfaces, such as:
○
ArrayList ,
LinkedList ,
Vector(implement
List
)
○ HashSet,TreeSet,LinkedHashSet(implement
Set
)
○ PriorityQueue ,
Deque(implement
Queue)
○ HashMap,TreeMap,LinkedHashMap(implement
Map)
3. A
lgorithms: Common operations like sorting and searching,implemented via utility
classes like
Collections.
Hierarchy
Here’s a simplified hierarchy of JCF:
Vbnet code
java.util.Collection (Interface)
|- List (Interface)
|- ArrayList, LinkedList, Vector
|- Set (Interface)
|- HashSet, LinkedHashSet, TreeSet
|- Queue (Interface)
|- PriorityQueue, ArrayDeque
java.util.Map (Interface)
|- HashMap, TreeMap, LinkedHashMap
Key Interfaces in JCF
1. Collection Interface
R
● oot interfaceof the framework.
● Methods provided:
○ add(E e) ,
addAll(Collection<? extends E> c)
○ remove(Object o) ,
removeAll(Collection<?> c) ,
clear()
○ contains(Object o) ,
containsAll(Collection<?> c)
○ size(),
isEmpty()
○ iterator()
2. List Interface
A
● n ordered collection allows duplicate elements.
● Examples:ArrayList,LinkedList, Vector
● Methods in addition to
Collection
:
○ get(int index),set(int index, E element)
○ add(int index, E element)
○ remove(int index)
○ indexOf(Object o),
lastIndexOf(Object o)
○ subList(int fromIndex, int toIndex)
3. Set Interface
A
● collection that doesnot allow duplicate elements.
● Examples: HashSet ,
LinkedHashSet ,
TreeSet
● Key Characteristics:
○ No guarantee of order inHashSet .
○ Ordered in insertion sequence for
LinkedHashSet.
○ Sorted in natural or custom order for
TreeSet
.
4. Queue Interface
A
● collection forFIFO(First-In-First-Out) operations.
● Examples: PriorityQueue, ArrayDeque
● Key Methods:
○ offer(E e) : Adds an element.
○poll(): Retrieves and removes the head.
○ peek(): Retrieves the head without removing it.
5. Map Interface
A
● collection forkey-value pairs.
● Examples: HashMap ,
TreeMap
,
LinkedHashMap
● Methods:
○ put(K key, V value), putAll(Map<? extends K, ? extends V>
m)
○ get(Object key),remove(Object key)
○ containsKey(Object key), containsValue(Object value)
○ keySet()
,
values() ,
entrySet()
Detailed Analysis of ArrayList (Part 1: List)
Let’s start with
ArrayList
, a commonly used implementationof
List
.
Characteristics
● ses adynamic arrayto store elements.
U
● Allowsrandom access(via index).
● Not synchronized(unsuitable for multithreading withoutexternal synchronization).
● Allows duplicate elements.
Common Use Cases
S
● toring ordered data with frequent read operations.
● Maintaining a list of elements without worrying about duplicates.
Example Code
Java code
import java.util.ArrayList;
public class ArrayListExample {
public static void main(String[] args) {
// Create an ArrayList
ArrayList<String> names = new ArrayList<>();
// Add elements
names.add("Avinash");
names.add("Kumar");
names.add("Jha");
// Access elements
System.out.println("Name at index 1: " + names.get(1));
// Remove element
names.remove("Kumar");
// Iterate over elements
for (String name : names) {
System.out.println(name);
}
// Size of the ArrayList
System.out.println("Size: " + names.size());
}
}
Internal Working
1. D ynamic Resizing: When the internal array is full,the size is increased by50%(in
JDK 1.8 and above).
2. Complexity:
○
add(E e): O(1) (amortized)
○get(int index): O(1)
○ remove(int index) : O(n)
LinkedList
LinkedListis another implementation of the
Listinterface,but it is backed by a
oubly-linked listrather than a dynamic array. Thisgives it different characteristics
d
compared to
ArrayList .
Characteristics
. D
1 ynamic in size: No need to resize like an array.
2. Efficient insertions and deletions: Especially in the middle of the list (O(1) for
adding/removing nodes).
3. Sequential access: No random access (O(n) for get(intindex) ).
Key Differences: ArrayList vs. LinkedList
Feature ArrayList LinkedList
nderlying Data
U Dynamic Array Doubly Linked List
Structure
Access Time (1) for
O get(random (n) for
O get(sequential
access) access)
Insertion/Deletion O(n) in the middle O(1) in the middle
Memory Overhead Less (only elements) More (nodes + pointers)
Best Use Case Frequent reads Frequent insertions/deletions
Common Use Cases
Implementingqueues or stacks.
●
● Frequent insertions or deletions at the start, middle, or end.
Example Code
Java code
import java.util.LinkedList;
public class LinkedListExample {
public static void main(String[] args) {
// Create a LinkedList
LinkedList<String> tasks = new LinkedList<>();
// Add elements
tasks.add("Complete Java Assignment");
tasks.add("Prepare for interview");
tasks.add("Read psychology book");
// Add element at the beginning and end
tasks.addFirst("Wake up");
tasks.addLast("Go to bed");
// Remove elements
tasks.removeFirst();
tasks.removeLast();
// Iterate over elements
for (String task : tasks) {
System.out.println(task);
}
// Access specific element
System.out.println("Task at index 1: " + tasks.get(1));
// Check if it contains an element
System.out.println("Contains 'Prepare for interview': " +
tasks.contains("Prepare for interview"));
}
}
Internal Working
● Node Structure: Each node contains three parts:
prev(pointer to the previous
ode),
n data, and
next(pointer to the next node).
Operations:
●
○
add(E e) : O(1) for appending.
○get(int index): O(n), as it must traverse the listsequentially.
○ remove(Object o)orremove(int index) : O(1) if atthe start or end;
O(n) otherwise.
Set Interface
ext, let’s dive intoSet, an interface designed forcollections whereduplicates are not
N
allowed.
Key Implementations
1. HashSet:
○ Backed by aHashMapinternally.
○ No guarantee of order.
○ Fast operations (O(1) for add, remove, and contains).
2. LinkedHashSet:
○ Extends
HashSet, but maintainsinsertion order.
Slightly slower than
○ HashSetdue to additional overhead.
3. TreeSet:
○ Implements NavigableSet.
○ Backed by aRed-Black Tree.
○ Maintains elements insorted order.
○ Operations like add, remove, and contains takeO(logn)time.
Example Code for HashSet
Java code
import java.util.HashSet;
public class HashSetExample {
public static void main(String[] args) {
// Create a HashSet
HashSet<String> cities = new HashSet<>();
// Add elements
cities.add("Hyderabad");
cities.add("Kolkata");
cities.add("Mumbai");
cities.add("Hyderabad"); // Duplicate, won't be added
// Display elements
System.out.println("Cities: " + cities);
// Check for an element
System.out.println("Contains 'Kolkata': " +
cities.contains("Kolkata"));
// Remove an element
cities.remove("Mumbai");
// Iterate over elements
for (String city : cities) {
System.out.println(city);
}
}
}
Example Code for TreeSet
Java code
import java.util.TreeSet;
public class TreeSetExample {
public static void main(String[] args) {
// Create a TreeSet
TreeSet<Integer> numbers = new TreeSet<>();
// Add elements
numbers.add(10);
numbers.add(5);
numbers.add(20);
numbers.add(15);
// Display elements (sorted order)
System.out.println("Numbers: " + numbers);
// Access first and last elements
System.out.println("First: " + numbers.first());
System.out.println("Last: " + numbers.last());
// Remove an element
numbers.remove(10);
// Iterate over elements
for (Integer number : numbers) {
System.out.println(number);
}
}
}
Internal Working of HashSet
● Backed by a
HashMap.
● When you add an element to a
HashSet
, it is storedas a key in the underlying
HashMapwith a constant dummy value (
PRESENT).
Internal Working of TreeSet
U
● ses a self-balancingRed-Black Tree.
● Maintains abinary search treewhere the left subtreehas smaller values, and the
right subtree has larger values.
● Ensures logarithmic time complexity for key operations.
Map Interface
A
Maprepresents a collection ofkey-value pairswhere:
E
● ach key is unique.
● A key maps to exactly one value (no duplicate keys allowed, but duplicate values are
allowed).
Key Methods in
Map
Method Description
put(K key, V value)
Adds or updates a key-value pair.
get(Object key)
Retrieves the value associated with the key.
remove(Object key)
Removes the mapping for a key.
containsKey(Object key) Checks if a key exists.
ontainsValue(Object
c Checks if a value exists.
value)
keySet()
Returns a
Setview of all keys.
values()
Returns a
Collectionview of all values.
entrySet()
Returns a
Setview of all key-value pairs
(
Map.Entry).
Implementations of Map
1. HashMap
U
● ses ahash tablefor storing key-value pairs.
● Order of keys is not guaranteed.
● Allows one null key and multiple null values.
Example Code
Java code
import java.util.HashMap;
public class HashMapExample {
public static void main(String[] args) {
// Create a HashMap
HashMap<String, Integer> scores = new HashMap<>();
// Add key-value pairs
scores.put("Avinash", 85);
scores.put("Kumar", 92);
scores.put("Jha", 78);
scores.put(null, 50); // One null key allowed
// Access value
System.out.println("Score of Kumar: " +
scores.get("Kumar"));
// Remove a key-value pair
scores.remove("Jha");
// Iterate over entries
for (HashMap.Entry<String, Integer> entry :
scores.entrySet()) {
System.out.println(entry.getKey() + ": " +
entry.getValue());
}
// Check for a key or value
System.out.println("Contains key 'Avinash': " +
scores.containsKey("Avinash"));
}
}
Internal Working of HashMap
1. Bucket Structure:
○ A HashMapuses an array of buckets. Each bucket storesentries as a linked
list or tree (from JDK 1.8 onwards).
○ The index for a key is determined by its hash code ( hashCode()).
2. Load Factor and Rehashing:
○ When the number of entries exceeds capacity * load
factor , the map
resizes (default load factor is0.75).
○ Resizing involves creating a larger array and rehashing all keys.
3. Complexity:
○ Average-case time for getand put :O(1).
○ Worst-case (if many hash collisions):O(n).
2. LinkedHashMap
● Extends
HashMapand maintains theinsertion orderof keys.
lightly slower than
● S HashMapdue to the additionaloverhead of maintaining the
order.
Example Code
Java code
import java.util.LinkedHashMap;
public class LinkedHashMapExample {
public static void main(String[] args) {
// Create a LinkedHashMap
LinkedHashMap<String, String> capitals = new
LinkedHashMap<>();
// Add key-value pairs
capitals.put("India", "New Delhi");
capitals.put("USA", "Washington DC");
capitals.put("Japan", "Tokyo");
// Display entries in insertion order
for (LinkedHashMap.Entry<String, String> entry :
capitals.entrySet()) {
System.out.println(entry.getKey() + ": " +
entry.getValue());
}
}
}
3. TreeMap
Implements
● NavigableMap, backed by aRed-Black Tree.
● Maintains keys insorted order(natural order or asper a custom comparator).
● Doesnot allow null keysbut permits null values.
Example Code
Java code
import java.util.TreeMap;
public class TreeMapExample {
public static void main(String[] args) {
// Create a TreeMap
TreeMap<String, Integer> marks = new TreeMap<>();
// Add key-value pairs
marks.put("Physics", 88);
marks.put("Math", 95);
marks.put("Chemistry", 72);
// Display entries in sorted order of keys
for (TreeMap.Entry<String, Integer> entry :
marks.entrySet()) {
System.out.println(entry.getKey() + ": " +
entry.getValue());
}
// Access first and last keys
System.out.println("First Subject: " + marks.firstKey());
System.out.println("Last Subject: " + marks.lastKey());
}
}
Internal Working of TreeMap
1. Red-Black Tree:
○ Self-balancing binary search tree.
○ EnsuresO(log n)time complexity for operations.
2. Key Ordering:
○ By default, uses thenatural orderof keys (requires
Comparable
implementation).
○ A custom comparator can be provided during TreeMap construction.
Difference Between HashMap, LinkedHashMap, and TreeMap
Feature HashMap LinkedHashMap TreeMap
Ordering No order aintains insertion
M Sorted by key
order
Null Keys Allows one null key Allows one null key Does not allow null keys
erformanc O
P (1) for most (1) for most
O (log n) for all
O
e operations operations operations
Use Case ast lookup and
F redictable iteration
P orted data or range
S
insertion order queries
NavigableMap and Its Features
A NavigableMapis a subinterface of
SortedMapand provides methods to navigate
through the keys in a map.
Key Methods
Method Description
firstEntry()
Retrieves the first key-value pair.
lastEntry()
Retrieves the last key-value pair.
lowerKey(K key)
Returns the greatest key strictly less than
key.
higherKey(K key)
Returns the smallest key strictly greater than
key.
ubMap(K fromKey, K
s Returns a view of the map within a key range.
toKey)
Example
Java code
import java.util.TreeMap;
public class NavigableMapExample {
public static void main(String[] args) {
TreeMap<Integer, String> grades = new TreeMap<>();
grades.put(60, "Pass");
grades.put(75, "Good");
grades.put(90, "Excellent");
System.out.println("Lower Key (80): " +
grades.lowerKey(80));
System.out.println("Higher Key (80): " +
grades.higherKey(80));
System.out.println("SubMap (60 to 90): " + grades.subMap(60,
90));
}
}
Synchronization in Collections
y default, most classes in the Java Collection Framework arenot thread-safe. This means
B
multiple threads modifying or accessing a collection concurrently can lead to unexpected
behavior.
Why Synchronization Is Needed
When multiple threads interact with a collection:
. D
1 ata Corruption: Simultaneous updates to the collection can corrupt its state.
2. Concurrent Modification Exception: If one thread modifiesa collection while
another is iterating over it, this exception is thrown.
Thread-Safe Collections
Java provides multiple ways to make collections thread-safe.
1. Synchronized Wrappers (Legacy Approach)
he
T Collectionsutility class provides static methodsto create synchronized (thread-safe)
versions of collections.
Example Code
Java code
import java.util.*;
public class SynchronizedCollectionExample {
public static void main(String[] args) {
// Create a non-synchronized list
List<String> names = new ArrayList<>();
names.add("Avinash");
names.add("Kumar");
// Create a synchronized version of the list
List<String> synchronizedNames =
Collections.synchronizedList(names);
// Synchronized block for iteration
synchronized (synchronizedNames) {
for (String name : synchronizedNames) {
System.out.println(name);
}
}
}
}
Methods to Create Synchronized Collections
Method Description
ollections.synchronizedList(L R
C eturns a synchronized
ist)
List.
ollections.synchronizedSet(Se Returns a synchronized
C Set.
t)
ollections.synchronizedMap(Ma Returns a synchronized
C Map.
p)
Limitations
● P erformance Overhead: Synchronization affects performance, as it locks the entire
collection.
● Iteration: Must use explicit synchronization wheniterating over synchronized
collections.
2. Concurrent Collections (Modern Approach)
Introduced in Java 5 as part of the
java.util.concurrentpackage, these classes are
designed for high-performance concurrent access.
Key Classes
Class Description
ConcurrentHashMap
Thread-safe, highly efficient implementation of
Map.
opyOnWriteArrayLis Thread-safe implementation of
C List.
t
CopyOnWriteArraySet Thread-safe implementation of
Set.
oncurrentLinkedQue Thread-safe, non-blocking queue.
C
ue
BlockingQueue
ueue with blocking methods (
Q LinkedBlockingQueue
,
(Interface) etc.).
ConcurrentHashMap
1. How It Works:
○ Divides the map into segments (buckets) to reduce contention.
○ Only locks a specific segment during updates instead of the whole map.
○ Operations like
get()are non-blocking.
2. Key Methods:
○ putIfAbsent(K key, V value): Adds a value only if the key is not
already mapped.
○ compute(K key, BiFunction) : Updates the value atomically.
○ forEach() : Parallel traversal.
3. Example Code:
Java code
import java.util.concurrent.ConcurrentHashMap;
public class ConcurrentHashMapExample {
public static void main(String[] args) {
// Create a ConcurrentHashMap
ConcurrentHashMap<String, Integer> scores = new
ConcurrentHashMap<>();
// Add elements
scores.put("Java", 90);
scores.put("Python", 85);
// Perform atomic operations
scores.putIfAbsent("C++", 80); // Add only if absent
scores.compute("Java", (key, val) -> val + 10); // Increment
score of Java
// Iterate
scores.forEach((key, value) -> {
System.out.println(key + ": " + value);
});
}
}
CopyOnWriteArrayList
1. How It Works:
○ Every modification (add/remove) creates a new copy of the underlying array.
○ Ideal for scenarios withfrequent readsandinfrequentwrites.
2. Example Code:
Java code
import java.util.concurrent.CopyOnWriteArrayList;
public class CopyOnWriteArrayListExample {
public static void main(String[] args) {
// Create a CopyOnWriteArrayList
CopyOnWriteArrayList<String> items = new
CopyOnWriteArrayList<>();
// Add elements
items.add("Java");
items.add("Python");
// Modify list while iterating
for (String item : items) {
System.out.println(item);
items.add("C++"); // No ConcurrentModificationException
}
// Display updated list
System.out.println("Final List: " + items);
}
}
BlockingQueue
1. How It Works:
○ Designed for producer-consumer scenarios.
○ Blocks a thread if the queue is full or empty.
2. Common Implementations:
○
ArrayBlockingQueue : Fixed-size queue.
○LinkedBlockingQueue: Linked list-based queue.
○ PriorityBlockingQueue : Priority-based queue.
3. Example Code:
Java code
import java.util.concurrent.ArrayBlockingQueue;
public class BlockingQueueExample {
public static void main(String[] args) throws
InterruptedException {
// Create a blocking queue with capacity 2
ArrayBlockingQueue<String> queue = new
ArrayBlockingQueue<>(2);
// Producer thread
new Thread(() -> {
try {
queue.put("Task 1");
System.out.println("Added Task 1");
queue.put("Task 2");
System.out.println("Added Task 2");
} catch (InterruptedException e) {
e.printStackTrace();
}
}).start();
// Consumer thread
new Thread(() -> {
try {
Thread.sleep(1000); // Simulate processing delay
System.out.println("Removed: " + queue.take());
System.out.println("Removed: " + queue.take());
} catch (InterruptedException e) {
e.printStackTrace();
}
}).start();
}
}
When to Use Which Approach
Scenario Best Choice
Frequent updates to a collection ConcurrentHashMap
Frequent reads, infrequent writes CopyOnWriteArrayList/Set
Producer-consumer pattern BlockingQueue
implementations
mall applications with single-threaded
S Synchronized wrappers
use
Custom Comparators in Java
A Comparatoris an interface used to define customsorting logic for objects. It is typically
used when:
● You need a sorting order different from the natural order defined by
Comparable
.
● The objects being compared do not implement
Comparable
.
Key Points About Comparator
unctional Interface: Introduced in Java 8,
F Comparatoris a functional interface with a
single abstract method:
Java code
int compare(T o1, T o2);
1.
R
○ eturns a negative value if
o1is less than
o2
.
○ Returns zero if they are equal.
○ Returns a positive value if
o1is greater than
o2 .
2. Default Methods:
Comparatorhas several default methodsintroduced in Java 8:
○reversed()
: Reverses the sorting order.
○ thenComparing(): Chains multiple comparators for complexsorting.
Implementing Comparator
Example 1: Sorting Based on a Custom Field
Suppose we have a
Personclass with fields
nameand
age:
Java code
class Person {
String name;
int age;
/ Constructor
/
public Person(String name, int age) {
this.name = name;
this.age = age;
}
Override
@
public String toString() {
return name + " (" + age + ")";
}
}
We can create a custom comparator to sort people by age:
Comparator Implementation
Java code
import java.util.*;
public class CustomComparatorExample {
public static void main(String[] args) {
// List of persons
List<Person> people = Arrays.asList(
new Person("Avinash", 30),
new Person("Kumar", 25),
new Person("Jha", 35)
);
// Sort by age
Collections.sort(people, new Comparator<Person>() {
@Override
public int compare(Person p1, Person p2) {
return Integer.compare(p1.age, p2.age); // Compare
ages
}
});
// Display sorted list
System.out.println("Sorted by age: " + people);
}
}
Example 2: Using a Lambda Expression
In Java 8+, we can simplify the above code using a lambda expression:
Java code
ollections.sort(people, (p1, p2) -> Integer.compare(p1.age,
C
p2.age));
Or, using the
List.sort()method:
Java code
people.sort((p1, p2) -> Integer.compare(p1.age, p2.age));
Example 3: Sorting with
thenComparing
If we want to sort people by age and, for people with the same age, by name alphabetically:
Java code
import java.util.*;
public class MultiComparatorExample {
public static void main(String[] args) {
List<Person> people = Arrays.asList(
new Person("Avinash", 30),
new Person("Kumar", 25),
new Person("Jha", 30)
);
// Multi-level sorting
people.sort(
Comparator.comparingInt((Person p) -> p.age)
.thenComparing(p -> p.name)
);
System.out.println("Sorted by age, then name: " + people);
}
}
Example 4: Reverse Sorting
To sort the list in descending order of age:
Java code
eople.sort(Comparator.comparingInt((Person p) ->
p
p.age).reversed());
Advanced Customization with
TreeMapor
TreeSet
hen using
W TreeMaporTreeSet
, you can pass a customcomparator in their constructors
for automatic sorting.
Example: TreeMap with Custom Comparator
Java code
import java.util.*;
public class TreeMapComparatorExample {
public static void main(String[] args) {
// TreeMap with custom comparator (sort by descending key)
TreeMap<Integer, String> map = new
TreeMap<>(Comparator.reverseOrder());
map.put(1, "One");
map.put(3, "Three");
map.put(2, "Two");
System.out.println("Descending Order: " + map);
}
}
Advanced Topics: Iterators
nIteratoris an object used to traverse collections. It is part of the
A java.utilpackage
and is preferred for iterating over collections when:
W
● e need to remove elements during iteration.
● We want to traverse elements without exposing the underlying structure.
Types of Iterators
1. Iterator (Universal Iterator):
○ Available for all Collectionimplementations.
○ Allows element removal during iteration.
2. ListIterator(Only for List):
○ Bi-directional iteration.
○ Allows adding, modifying, and removing elements during traversal.
3. Spliterator(Introduced in Java 8):
○ Designed for parallel traversal of collections.
Iterator Example
Java code
import java.util.*;
public class IteratorExample {
public static void main(String[] args) {
List<String> names = new
ArrayList<>(Arrays.asList("Avinash", "Kumar", "Jha"));
// Create an iterator
Iterator<String> iterator = names.iterator();
// Traverse the list
while (iterator.hasNext()) {
String name = iterator.next();
System.out.println(name);
// Remove an element during iteration
if (name.equals("Kumar")) {
iterator.remove();
}
}
System.out.println("After removal: " + names);
}
}
ListIterator Example
Java code
import java.util.*;
public class ListIteratorExample {
public static void main(String[] args) {
List<String> names = new ArrayList<>(Arrays.asList("Java",
"Python", "C++"));
// Create a ListIterator
ListIterator<String> listIterator = names.listIterator();
// Forward traversal
while (listIterator.hasNext()) {
System.out.println(listIterator.next());
}
// Backward traversal
while (listIterator.hasPrevious()) {
System.out.println(listIterator.previous());
}
}
}
Spliterator Example
The
Spliteratorsplits the collection into parts for parallel processing. Example:
Java code
import java.util.*;
public class SpliteratorExample {
public static void main(String[] args) {
List<String> languages = Arrays.asList("Java", "Python",
"C++", "Ruby");
// Create a Spliterator
Spliterator<String> spliterator = languages.spliterator();
// Try splitting the list
Spliterator<String> secondHalf = spliterator.trySplit();
// Traverse the first half
spliterator.forEachRemaining(System.out::println);
// Traverse the second half
System.out.println("Second Half:");
secondHalf.forEachRemaining(System.out::println);
}
}
What Are Streams?
Astreamis a sequence of elements that supports:
. F
1 unctional-style operations(like
map
,
filter
, and
reduce).
2. Lazy evaluation, meaning intermediate operations are not executed until a terminal
operation is invoked.
3. Pipeline processing, where data flows through a seriesof operations.
Streams do not store data; they process data from collections or other sources.
Why Use Streams?
S
● implifies complex collection operations (like filtering or mapping).
● Enables parallel processing with minimal effort.
● Enhances readability and maintainability.
Stream Operations
Stream operations are divided into two categories:
1. Intermediate Operations: Return a new stream (lazyevaluation). Examples:
filter()
,
map(),sorted() .
2. Terminal Operations: Produce a result or side-effect(eager evaluation). Examples:
collect()
,
forEach() ,
reduce()
.
1. Creating Streams
Streams can be created from various data sources, such as:
ollections:
C
Java code
ist<String> names = List.of("Java", "Python", "C++");
L
Stream<String> stream = names.stream();
rrays:
A
Java code
tring[] languages = {"Java", "Python", "C++"};
S
Stream<String> stream = Arrays.stream(languages);
tatic Methods:
S
Java code
Stream<Integer> numbers = Stream.of(1, 2, 3, 4, 5);
2. Intermediate Operations
filter()
Filters elements based on a predicate.
Java code
ist<String> names = List.of("Avinash", "Kumar", "Jha", "Java");
L
names.stream()
.filter(name -> name.startsWith("J"))
.forEach(System.out::println); // Output: Jha, Java
map()
Transforms each element using a function.
Java code
ist<String> names = List.of("Avinash", "Kumar");
L
names.stream()
.map(String::toUpperCase)
.forEach(System.out::println); // Output: AVINASH, KUMAR
sorted()
Sorts elements in natural or custom order.
Java code
ist<Integer> numbers = List.of(5, 3, 8, 1);
L
numbers.stream()
.sorted()
.forEach(System.out::println); // Output: 1, 3, 5, 8
distinct()
Removes duplicate elements.
Java code
ist<Integer> numbers = List.of(1, 2, 2, 3, 4, 4);
L
numbers.stream()
.distinct()
.forEach(System.out::println); // Output: 1, 2, 3, 4
limit() and skip()
● limit(n): Limits the stream to the first
nelements.
● skip(n): Skips the first
nelements.
Java code
tream<Integer> stream = Stream.of(1, 2, 3, 4, 5);
S
stream.limit(3).forEach(System.out::println); // Output: 1, 2, 3
stream.skip(2).forEach(System.out::println); // Output: 3, 4, 5
3. Terminal Operations
forEach()
Performs an action for each element.
Java code
ist<String> names = List.of("Java", "Python");
L
names.stream().forEach(System.out::println);
collect()
Converts the stream into a collection or another structure.
Java code
ist<String> names = List.of("Java", "Python");
L
List<String> upperCaseNames = names.stream()
.map(String::toUpperCase)
.collect(Collectors.toList());
System.out.println(upperCaseNames); // Output: [JAVA, PYTHON]
reduce()
Aggregates elements into a single result.
Java code
ist<Integer> numbers = List.of(1, 2, 3, 4);
L
int sum = numbers.stream()
.reduce(0, Integer::sum); // Initial value is 0
System.out.println(sum); // Output: 10
count()
Counts the number of elements in the stream.
Java code
ong count = List.of("Java", "Python", "C++").stream().count();
l
System.out.println(count); // Output: 3
anyMatch(), allMatch(), noneMatch()
● nyMatch()
a : Returns trueif any element matches thepredicate.
● allMatch(): Returnstrueif all elements match thepredicate.
●
noneMatch() : Returns
trueif no elements match thepredicate.
Example:
Java code
ist<Integer> numbers = List.of(2, 4, 6, 8);
L
boolean allEven = numbers.stream().allMatch(n -> n % 2 == 0);
System.out.println(allEven); // Output: true
4. Parallel Streams
For large datasets, streams can be processed in parallel to improve performance:
Java code
ist<Integer> numbers = List.of(1, 2, 3, 4, 5);
L
numbers.parallelStream()
.map(n -> n * n)
.forEach(System.out::println); // Order is not guaranteed
Practical Examples
Example 1: Filtering and Collecting
Java code
ist<String> names = List.of("Java", "Python", "C++", "JavaScript");
L
List<String> longNames = names.stream()
.filter(name -> name.length() > 4)
.collect(Collectors.toList());
System.out.println(longNames); // Output: [Python, JavaScript]
Example 2: Grouping Data
Using
Collectors.groupingBy():
Java code
ist<String> items = List.of("Apple", "Banana", "Cherry",
L
"Avocado");
Map<Character, List<String>> groupedByFirstLetter = items.stream()
.collect(Collectors.groupingBy(item -> item.charAt(0)));
System.out.println(groupedByFirstLetter);
// Output: {A=[Apple, Avocado], B=[Banana], C=[Cherry]}
Example 3: Finding Max/Min
Java code
ist<Integer> numbers = List.of(10, 20, 30, 40);
L
int max = numbers.stream()
.max(Integer::compareTo)
.orElseThrow(); // Output: 40
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