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Collection

Common Methods in Collection Interfaces

• add(): Adds an element to the collection.
• remove(): Removes an element from the collection.
• size(): Returns the number of elements in the collection.
• contains(): Checks if the collection contains a specific element.
• clear(): Removes all elements from the collection.
• isEmpty(): Checks if the collection is empty.
• iterator(): Returns an iterator to iterate over the elements.
• forEach(): Iterates over each element in the collection (Java 8+).

Iterating Through Collections

You can iterate over collections using Iterator or enhanced for loop.

1. Using Iterator:

   Iterator<String> iterator = fruits.iterator();
   while (iterator.hasNext()) {
       System.out.println(iterator.next());
   }

2. Using Enhanced for-loop:

   for (String fruit : fruits) {
       System.out.println(fruit);
   }

Collections Class Utility Methods

The Collections class provides several utility methods to work with collections, such as:

• sort(List list): Sorts the list.
• reverse(List<?> list): Reverses the list.
• shuffle(List<?> list): Randomizes the order of elements in the list.
• max(Collection<? extends T> coll): Returns the maximum element from the collection.
• min(Collection<? extends T> coll): Returns the minimum element from the collection.

List

A List is an ordered collection (or sequence) that allows duplicate elements. Elements in a List are indexed, meaning each element is positioned at a specific index and can be accessed via its index.

Unique Methods in List:

• add(int index, E element) – Insert at a specific index.
• remove(int index) – Remove by index.
• set(int index, E element) – Modify element at specific index.
• get(int index) – Access element by index.
• indexOf(Object o) – Find the index of the first occurrence.
• lastIndexOf(Object o) – Find the index of the last occurrence.
• subList(int fromIndex, int toIndex) – Get a portion of the list.
• listIterator() – Get an iterator that can traverse both forward and backward.
• addAll(Collection<? extends E> c) - Adds all the elements from the specified collection to the list.

Implementations of the List Interface:

ArrayList

• A dynamically resizing array implementation of the List interface.
• Provides fast random access (constant time for get() and set() operations).
• Slower for insertions and deletions in the middle of the list because it may need to shift elements.

List<String> fruits = new ArrayList<>();
fruits.add("Apple");
fruits.add("Banana");
fruits.add("Cherry");
System.out.println(fruits);  // Output: [Apple, Banana, Cherry]

LinkedList

• A doubly-linked list implementation of the List interface.
• Provides fast insertions and deletions, but slower random access because it needs to traverse the list.
• It implements both List and Queue interfaces, so it can be used as a queue or stack.

List<String> fruits = new LinkedList<>();
fruits.add("Apple");
fruits.add("Banana");
fruits.add("Cherry");
System.out.println(fruits);  // Output: [Apple, Banana, Cherry]

ArrayList vs LinkedList

Feature	ArrayList	LinkedList
Memory structure	Stored in a contiguous block of memory (like an array).	Stored as nodes in heap memory, each holding data + pointers.
Resizing	Automatically resizes when full by copying elements to a new, 1.5x larger array.	No resizing — just adds/removes nodes, updating pointers.
Memory overhead	Less overhead (stores only data).	More overhead (stores data + pointers for next/prev nodes).
Cache performance	Cache-friendly — elements are stored sequentially, helping CPU fetch data faster.	Cache-unfriendly — nodes are scattered in memory, causing pointer chasing.
Insertion/Deletion	Slower — needs shifting elements when adding/removing (except at the end).	Faster — adding/removing nodes is quick (only pointer updates).
Access time	Faster (O(1)) — can directly access any index.	Slower (O(n)) — must traverse nodes from the head/tail to find an element.
Best use case	Frequent access to elements by index.	Frequent additions/removals (especially at the start/middle).

Vector

• A legacy class that is similar to ArrayList but synchronized, making it thread-safe.
• Generally, it’s not recommended to use Vector in modern Java code due to performance issues with synchronization.

Stack

• A subclass of Vector that implements the Stack data structure (LIFO - Last In First Out).
• Provides methods like push(), pop(), and peek() for stack operations.

Stack<String> stack = new Stack<>();
stack.push("Apple");
stack.push("Banana");
stack.push("Cherry");
System.out.println(stack);  // Output: [Apple, Banana, Cherry]
System.out.println(stack.pop());  // Output: Cherry

Queue

In Java, a Queue is a collection used to store elements in a particular order, typically following the FIFO (First-In-First-Out) principle. This means the first element added to the queue will be the first one to be removed.

Methods of Queue:

1. add(E e): Adds the specified element to the queue. If the element cannot be added (due to capacity restrictions), it throws an IllegalStateException.
2. offer(E e): Adds the specified element to the queue. Unlike add(), it does not throw an exception if the queue is full; it simply returns false if the element cannot be added.
3. remove(): Removes and returns the head of the queue. Throws NoSuchElementException if the queue is empty.
4. poll(): Removes and returns the head of the queue. Returns null if the queue is empty.
5. element(): Returns the head of the queue without removing it. Throws NoSuchElementException if the queue is empty.
6. peek(): Returns the head of the queue without removing it. Returns null if the queue is empty.

Queue Implementations:

1. LinkedList:

   • LinkedList implements the Queue interface and can be used as a basic queue. It is efficient for adding/removing elements from both ends.

   Queue<String> queue = new LinkedList<>();
   queue.add("Apple");

2. PriorityQueue:

   • Unlike other queues, PriorityQueue orders its elements based on their natural ordering or a specified comparator. It does not follow FIFO strictly; instead, it processes elements based on their priority.

   Queue<Integer> queue = new PriorityQueue<>();
   queue.add(10);
   queue.add(20);
   queue.add(5);  // The head will be 5 because it's the lowest priority
   System.out.println(queue.peek());  // Output: 5

Set

In Java, a Set is a collection that does not allow duplicate elements and does not guarantee any specific order of elements. It models the mathematical set abstraction, where each element in the collection must be unique.

Common Methods in Set:

1. add(E e): Adds the specified element to the set. If the element already exists, the set remains unchanged, and it returns false.

2. remove(Object o): Removes the specified element from the set. If the element exists, it is removed, and the method returns true. Otherwise, it returns false.

3. contains(Object o): Returns true if the set contains the specified element, otherwise false.

4. size(): Returns the number of elements in the set.

5. isEmpty(): Returns true if the set contains no elements, otherwise false.

6. clear(): Removes all elements from the set.

7. iterator(): Returns an iterator over the elements in the set. The iteration order is typically not predictable (unless using LinkedHashSet or TreeSet).

   Iterator<String> iterator = set.iterator();
   while (iterator.hasNext()) {
       System.out.println(iterator.next());
   }

Common Implementations of Set:

1. HashSet:

   • Implements the Set interface and does not maintain any order of its elements.
   • It uses a hash table to store the elements and provides constant-time performance for basic operations (add, remove, contains).

   Set<String> set = new HashSet<>();
   set.add("Apple");
   set.add("Banana");
   set.add("Cherry");
   set.add("Apple");  // This will not be added again.
   System.out.println(set);  // Output: [Apple, Banana, Cherry] (Order may vary)

2. LinkedHashSet:

   • Implements the Set interface and maintains the order in which elements were inserted.
   • It provides predictable iteration order (insertion order).

   Set<String> set = new LinkedHashSet<>();
   set.add("Apple");
   set.add("Banana");
   set.add("Cherry");
   set.add("Apple");  // This will not be added again.
   System.out.println(set);  // Output: [Apple, Banana, Cherry] (Order preserved)

3. TreeSet:

   • Implements the Set interface and stores elements in a sorted order according to their natural ordering or a specified comparator.
   • It provides logarithmic time complexity for basic operations (add, remove, contains).

   Set<Integer> set = new TreeSet<>();
   set.add(10);
   set.add(5);
   set.add(20);
   set.add(15);
   System.out.println(set);  // Output: [5, 10, 15, 20] (Sorted in natural order)

Map

A Map in Java is a collection that stores key-value pairs — kind of like a dictionary! It’s part of the java.util package but does not extend Collection. Let’s break it down into key points:

Key Characteristics

• Stores key-value pairs: Each key is unique, and each key maps to one value.

• No duplicate keys: If you insert a new entry with an existing key, it overwrites the old value.

• Allows null values

  (depending on the implementation).

  • HashMap allows 1 null key and multiple null values.
  • TreeMap and Hashtable don’t allow null keys.

Common Implementations

Here are the most used Map implementations and their differences:

Implementation	Ordering	Null Keys/Values	Thread-Safe	Performance
HashMap	❌ Unordered	✅ 1 null key, multiple null values	❌ No	⚡ Fast (O(1) get/put)
LinkedHashMap	✅ Insertion order	✅ Same as HashMap	❌ No	⚡ Slightly slower than HashMap
TreeMap	✅ Sorted (natural/comparator)	❌ No null keys, ✅ null values	❌ No	🐢 Slower (O(log n)) due to sorting
Hashtable	❌ Unordered	❌ No null keys/values	✅ Yes	🐌 Slower (legacy)
ConcurrentHashMap	❌ Unordered	❌ No null keys/values	✅ Yes (better than Hashtable)	⚡ Good for concurrency

Essential Methods

Method	Description
put(key, value)	Inserts or updates a key-value pair
get(key)	Returns the value for the given key
remove(key)	Removes the key-value pair
containsKey(key)	Checks if the map has the given key
containsValue(value)	Checks if the map has the given value
keySet()	Returns a Set of all keys
values()	Returns a Collection of all values
entrySet()	Returns a Set of Map.Entry (key-value pairs)
putIfAbsent(key, value)	Inserts only if the key doesn’t exist
replace(key, value)	Replaces the value for an existing key

Traversing a Map

Here are 3 popular ways to loop through a map:

Using for-each and entrySet()

for (Map.Entry<String, Integer> entry : map.entrySet()) {
    System.out.println(entry.getKey() + " = " + entry.getValue());
}

Using keySet() (keys only)

for (String key : map.keySet()) {
    System.out.println(key + " -> " + map.get(key));
}

Using forEach() (Java 8 Lambda)

map.forEach((key, value) -> System.out.println(key + " : " + value));

Special Map Features

• Immutable Maps (Java 9+)

  Map<String, String> map = Map.of("key1", "value1", "key2", "value2");

• Sorted Maps:
  TreeMap keeps keys sorted by natural order or a custom Comparator.

• Thread-safe Maps:

  • Hashtable (legacy, slow).
  • ConcurrentHashMap (modern, faster alternative).

HashCode()

• hashCode() is a method in Java that returns an integer value that is used to identify objects in hash-based collections (like HashMap, HashSet, Hashtable).
• It is used primarily in hashing algorithms to locate objects in buckets within collections.
• The goal of hashCode() is to efficiently map objects to hash buckets for fast lookups.

Why Override hashCode()

• If you override the equals() method, you must override hashCode() as well. This ensures that objects that are considered equal by equals() will return the same hash code.

The General Contract of hashCode()

• Equal objects must have the same hash code. This means:
  • If a.equals(b) is true, then a.hashCode() must be equal to b.hashCode().
• Unequal objects can have different hash codes, but it’s not required. It’s just desirable for better performance (i.e., fewer collisions in hash-based collections).
• Consistency: The hashCode() value should remain consistent throughout the lifetime of the object, unless a property used in equals() and hashCode() is modified.

Implements of HashMap

Array of Buckets: A HashMap is implemented using an array of buckets (typically an array of Node or Entry objects). Each bucket is a linked list (or a tree if collisions become too high) that stores key-value pairs.

When a key is added, the HashMap computes a hash code for the key using the key’s hashCode() method.

The hash code is then transformed into an index for the bucket array using a hash function (e.g., index = hashCode % array.length).

Collision Handling:

• Chaining (Linked List): If two keys have the same hash code (hash collision), the HashMap uses chaining, where each bucket stores a linked list of key-value pairs. Each node in the list contains the key, value, and a reference to the next node in the same bucket.
• Tree Nodes: In case of a large number of collisions, Java 8 introduced tree-based collision resolution (using TreeNode) for better performance (O(log n) instead of O(n) in case of long chains).

When the number of elements exceeds a certain threshold (based on the load factor, typically 0.75), the HashMap doubles the array size and rehashes the existing entries to new bucket locations.

Functional Interface: Comparable vs Comparator

In Java, both Comparable and Comparator are functional interfaces used for comparing objects, but they serve different purposes and are used in different contexts.

Comparable Interface

• Purpose: Used to define the natural ordering of objects. It allows a class to specify how its instances should be compared.

• Location: It is implemented by the class whose instances need to be compared.

• Method: It has a single method:

  int compareTo(T o);

  • Parameter: It compares the current object (this) with another object (o).
  • Return value:
    • A negative value if this is less than o.
    • Zero if this is equal to o.
    • A positive value if this is greater than o.

• Example:

  class Student implements Comparable<Student> {
      int rollNo;
      String name;
  
      public Student(int rollNo, String name) {
          this.rollNo = rollNo;
          this.name = name;
      }
  
      @Override
      public int compareTo(Student other) {
          return this.rollNo - other.rollNo; // Compare by roll number
      }
  }
  
  public class Main {
      public static void main(String[] args) {
          List<Student> students = new ArrayList<>();
          students.add(new Student(3, "Alice"));
          students.add(new Student(1, "Bob"));
          students.add(new Student(2, "Charlie"));
  
          Collections.sort(students);  // Sorts based on compareTo
      }
  }

• When to Use: Use Comparable when the class has a natural order (e.g., sorting by age, name, or any primary field).

Comparator Interface

• Purpose: Used to define custom ordering of objects, separate from the natural order defined by Comparable. A Comparator is useful when you want to compare objects based on multiple fields or in different ways.

• Location: A separate class implements the Comparator interface, not the object being compared.

• Method: It has two methods:

  • int compare(T o1, T o2);
  • boolean equals(Object obj); (optional for Comparator interface, not always used)

  The compare method compares two objects and returns:

  • A negative value if o1 is less than o2.
  • Zero if o1 is equal to o2.
  • A positive value if o1 is greater than o2.

• Example:

  class Student {
      int rollNo;
      String name;
  
      public Student(int rollNo, String name) {
          this.rollNo = rollNo;
          this.name = name;
      }
  }
  
  class StudentNameComparator implements Comparator<Student> {
      @Override
      public int compare(Student s1, Student s2) {
          return s1.name.compareTo(s2.name); // Compare by name
      }
  }
  
  public class Main {
      public static void main(String[] args) {
          List<Student> students = new ArrayList<>();
          students.add(new Student(3, "Alice"));
          students.add(new Student(1, "Bob"));
          students.add(new Student(2, "Charlie"));
  
          // Sort students by name using Comparator
          Collections.sort(students, new StudentNameComparator());
      }
  }

• When to Use: Use Comparator when you need to define multiple ways to compare objects or if you want to compare objects based on fields that don’t represent the natural ordering.

Key Differences

Feature	Comparable	Comparator
Implemented by	The class whose instances are being compared.	A separate class or anonymous class.
Purpose	Defines the natural ordering of objects.	Defines custom orderings of objects.
Method	compareTo(T o)	compare(T o1, T o2)
When to Use	When you want to define a default ordering (natural order).	When you need multiple or different ways to compare objects.

Functional Interface

A functional interface in Java is an interface that has exactly one abstract method, but it can have multiple default or static methods. Functional interfaces are used primarily in lambda expressions and method references.

The @FunctionalInterface annotation is optional, but it helps to clarify that the interface is intended to be functional. The compiler will also check that the interface conforms to the rules of a functional interface.

Common Built-in Functional Interfaces:

Predicate<T>

• Purpose: Represents a boolean-valued function of one argument. It is commonly used for filtering or matching.
• Method:
  • boolean test(T t): Evaluates the predicate on the given argument.
• Common Use: Used when you want to test whether an object satisfies a condition.

Example:

javaCopy codeimport java.util.function.Predicate;

public class Main {
    public static void main(String[] args) {
        Predicate<String> isLongString = (s) -> s.length() > 5;
        
        System.out.println(isLongString.test("Hello")); // false
        System.out.println(isLongString.test("Hello World")); // true
    }
}

Function<T, R>

• Purpose: Represents a function that takes one argument of type T and returns a result of type R. It is useful for transformations and mappings.
• Methods:
  • R apply(T t): Applies the function to the given argument.
  • default <V> Function<T, V> andThen(Function<? super R, ? extends V> after): Composes the function to apply after the current function.
  • default <V> Function<V, R> compose(Function<? super V, ? extends T> before): Composes the function to apply before the current function.
• Common Use: It is commonly used in mapping, transforming values, or converting data types.

Example:

javaCopy codeimport java.util.function.Function;

public class Main {
    public static void main(String[] args) {
        Function<String, Integer> stringLength = (s) -> s.length();
        
        System.out.println(stringLength.apply("Hello")); // 5
    }
}

Consumer<T>

• Purpose: Represents an operation that accepts a single input argument and returns no result. It is used for operations that consume input, but don’t return any output (e.g., printing or modifying a value).
• Method:
  • void accept(T t): Performs the operation on the given argument.
  • default Consumer<T> andThen(Consumer<? super T> after): Combines two Consumer operations where the second is executed after the first one.
• Common Use: Useful for performing side effects, such as modifying a value, printing to the console, or updating collections.

Example:

javaCopy codeimport java.util.function.Consumer;

public class Main {
    public static void main(String[] args) {
        Consumer<String> printMessage = (message) -> System.out.println(message);
        
        printMessage.accept("Hello, World!"); // Prints "Hello, World!"
    }
}

Supplier<T>

• Purpose: Represents a supplier of results, which takes no arguments but returns a result of type T. It is often used to generate or supply values.
• Method:
  • T get(): Returns a result.
• Common Use: Used when you need a lazy evaluation, such as generating random values, or obtaining data from a source (like a database or a file).

Example:

javaCopy codeimport java.util.function.Supplier;

public class Main {
    public static void main(String[] args) {
        Supplier<Double> randomValue = () -> Math.random();
        
        System.out.println(randomValue.get()); // Prints a random number between 0 and 1
    }
}

Author: o_oyao

Link: http://dyingdown.github.io/2025/03/06/Java-Collections/

License: All articles in this blog are licensed under CC BY-NC-SA 4.0 unless stating additionally.