Overview - collections and generics

Java Collections and Utility Classes – Overview

The Java Collections Framework (JCF) provides efficient and organized ways to store and manage data. It includes Lists, Sets, Queues, and Maps along with utility classes and methods.

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1. Generics in Java

Generics allow defining type-safe classes and methods.

Generic Class Example:

class Box<T> {
    private T value;
    public void set(T value) { this.value = value; }
    public T get() { return value; }
}

• Ensures type safety
• Eliminates explicit type casting

Generic Method Example:

public static <T> void printArray(T[] array) {
    for (T item : array) System.out.print(item + " ");
}

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2. Collection Hierarchy

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3. Lists: Ordered Collections

ArrayList (Fast retrieval, slow insertion/deletion)

ArrayList<String> list = new ArrayList<>();
list.add("Apple");
list.add("Banana");
System.out.println(list); // [Apple, Banana]

LinkedList (Fast insertions, slow random access)

LinkedList<Integer> numbers = new LinkedList<>();
numbers.add(10);
numbers.addFirst(5);
System.out.println(numbers); // [5, 10]

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4. Sets: Unique Elements

HashSet (Unordered, Unique Values)

Set<String> set = new HashSet<>();
set.add("Apple");
set.add("Banana");
System.out.println(set); // Unordered Output

TreeSet (Sorted Order)

TreeSet<Integer> set = new TreeSet<>();
set.add(20);
set.add(10);
System.out.println(set); // [10, 20]

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5. Iterators for Traversing Collections

Using Iterator

Iterator<Integer> itr = list.iterator();
while (itr.hasNext()) {
    System.out.println(itr.next());
}

Using ListIterator (Forward & Backward)

ListIterator<String> itr = list.listIterator();
while (itr.hasNext()) System.out.println(itr.next());

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6. Utility Methods

Overriding toString()

class Person {
    String name;
    public String toString() { return name; }
}

Sorting with Comparator

class AgeComparator implements Comparator<Person> {
    public int compare(Person p1, Person p2) {
        return Integer.compare(p1.age, p2.age);
    }
}

Generating Unique HashCodes

public int hashCode() { return Objects.hash(name, age); }

Concept	Description
Generics	Type-safe collections & methods.
Lists	Ordered, duplicates allowed (ArrayList, LinkedList).
Sets	Unique values (HashSet, TreeSet).
Iterators	Traverse collections (Iterator, ListIterator).
toString()	Converts object to a string.
Comparator	Custom sorting logic.
hashCode()	Generates unique ID for objects.