Java Collections Framework: Lists, Sets and Maps

1. Introduction to Java Collections Framework

In this tutorial, you will learn about the Java collections framework and how to use different data structures such as lists, sets, and maps in your Java programs. The Java collections framework is a set of classes and interfaces that provide common functionality for storing, manipulating, and accessing data. The Java collections framework is part of the java.util package and consists of the following components:

• Collection: The root interface of the Java collections framework. It defines the basic operations that all collections support, such as adding, removing, and iterating over elements.
• List: A subinterface of Collection that represents an ordered sequence of elements. Lists allow duplicate elements and provide access to elements by their index.
• Set: A subinterface of Collection that represents a collection of unique elements. Sets do not allow duplicate elements and do not guarantee any order of elements.
• Map: An interface that represents a mapping of keys to values. Maps do not implement the Collection interface, but they are considered part of the Java collections framework. Maps allow one unique key to map to one value, and provide access to values by their keys.

Each of these interfaces has several implementations that provide different characteristics and performance. For example, some implementations are thread-safe, some are synchronized, some are sorted, and some are concurrent. In this tutorial, you will learn about the most commonly used implementations of lists, sets, and maps, and how to choose the best one for your needs.

Why should you use the Java collections framework? The Java collections framework provides many benefits, such as:

• It reduces the complexity of coding by providing ready-made data structures and algorithms.
• It improves the performance and efficiency of your programs by using optimized and tested implementations.
• It enhances the readability and maintainability of your code by using consistent and standard interfaces and methods.
• It facilitates interoperability and compatibility between different classes and libraries by using common data types and protocols.

Are you ready to explore the Java collections framework and learn how to use lists, sets, and maps in your Java programs? Let’s get started!

2. Java Lists: ArrayList, LinkedList, and Vector

In this section, you will learn about the Java lists, which are one of the most commonly used data structures in Java. A list is a collection of elements that are ordered and can be accessed by their index. Lists allow duplicate elements and can grow or shrink dynamically. The Java collections framework provides three main implementations of the List interface: ArrayList, LinkedList, and Vector. Each of these implementations has its own advantages and disadvantages, depending on the use case and the performance requirements. In this section, you will learn about the differences between these three implementations and how to choose the best one for your needs.

The ArrayList class is the most popular implementation of the List interface. It uses an array internally to store the elements. The array size is automatically adjusted as the list grows or shrinks. The ArrayList class offers fast random access to the elements by their index, but it is slow to insert or delete elements in the middle of the list, as it requires shifting the elements. The ArrayList class is not thread-safe, meaning that it cannot be safely used by multiple threads without external synchronization.

The LinkedList class is another implementation of the List interface. It uses a doubly-linked list internally to store the elements. Each element in the list has a reference to the previous and the next element. The LinkedList class offers fast insertion and deletion of elements at any position in the list, as it only requires updating the references. However, it is slow to access the elements by their index, as it requires traversing the list. The LinkedList class is also not thread-safe, meaning that it cannot be safely used by multiple threads without external synchronization.

The Vector class is the oldest implementation of the List interface. It is similar to the ArrayList class, as it also uses an array internally to store the elements. However, the Vector class is synchronized, meaning that it can be safely used by multiple threads without external synchronization. The synchronization comes at a cost of performance, as it adds overhead to each operation. The Vector class is considered to be deprecated, as there are better alternatives for thread-safe lists, such as the CopyOnWriteArrayList class.

How do you decide which implementation of the List interface to use in your Java programs? Here are some general guidelines:

• If you need fast random access to the elements and you do not need to insert or delete elements frequently, use the ArrayList class.
• If you need fast insertion and deletion of elements at any position and you do not need to access the elements by their index frequently, use the LinkedList class.
• If you need a thread-safe list and you do not care about the performance, use the Vector class. However, you should consider using other concurrent collections instead, such as the CopyOnWriteArrayList class.

In the next section, you will learn how to create, iterate, and modify lists in Java using the List interface and its methods.

2.1. ArrayList vs LinkedList vs Vector

In this section, you will learn about the differences between the three main implementations of the List interface in Java: ArrayList, LinkedList, and Vector. You will also learn how to choose the best one for your needs, depending on the performance and concurrency requirements of your program. As you have learned in the previous section, each of these implementations has its own advantages and disadvantages, which are summarized in the table below:

As you can see, the ArrayList and the Vector classes use an array internally to store the elements, while the LinkedList class uses a doubly-linked list. This affects the performance of the operations on the list, such as accessing, inserting, and deleting elements. The ArrayList and the Vector classes offer fast random access to the elements by their index, but they are slow to insert or delete elements in the middle of the list, as they require shifting the elements. The LinkedList class offers fast insertion and deletion of elements at any position in the list, as it only requires updating the references, but it is slow to access the elements by their index, as it requires traversing the list.

Another difference between these implementations is the thread-safety. The Vector class is synchronized, meaning that it can be safely used by multiple threads without external synchronization. However, this comes at a cost of performance, as it adds overhead to each operation. The ArrayList and the LinkedList classes are not synchronized, meaning that they cannot be safely used by multiple threads without external synchronization. If you need a thread-safe list, you should consider using other concurrent collections instead, such as the CopyOnWriteArrayList class, which offers better performance and scalability than the Vector class.

How do you decide which implementation of the List interface to use in your Java programs? Here are some general guidelines:

• If you need fast random access to the elements and you do not need to insert or delete elements frequently, use the ArrayList class.
• If you need fast insertion and deletion of elements at any position and you do not need to access the elements by their index frequently, use the LinkedList class.
• If you need a thread-safe list and you do not care about the performance, use the Vector class. However, you should consider using other concurrent collections instead, such as the CopyOnWriteArrayList class.

In the next section, you will learn how to create, iterate, and modify lists in Java using the List interface and its methods.

2.2. How to Create, Iterate, and Modify Lists in Java

In this section, you will learn how to create, iterate, and modify lists in Java using the List interface and its methods. The List interface is a subinterface of the Collection interface, which means that it inherits all the methods of the Collection interface, such as size(), isEmpty(), contains(), add(), remove(), and clear(). In addition, the List interface provides some methods that are specific to lists, such as get(), set(), add() with index, remove() with index, indexOf(), lastIndexOf(), and subList(). You can use these methods to manipulate the elements of a list according to their order and index.

To create a list in Java, you need to use one of the implementations of the List interface, such as ArrayList, LinkedList, or Vector. You can either create an empty list or a list with some initial elements. For example, you can create an empty list of strings using the ArrayList class as follows:

List list = new ArrayList<>();

Alternatively, you can create a list of strings with some initial elements using the Arrays.asList() method, which returns a fixed-size list backed by an array. For example, you can create a list of strings with three elements as follows:

List list = Arrays.asList("Java", "Python", "C++");

To iterate over a list in Java, you can use different ways, such as a for loop, a for-each loop, an iterator, or a stream. For example, you can use a for loop to iterate over a list of strings and print each element as follows:

for (int i = 0; i < list.size(); i++) {
  String element = list.get(i);
  System.out.println(element);
}

To modify a list in Java, you can use the methods of the List interface, such as set(), add(), and remove(). For example, you can use the set() method to replace an element at a given index, the add() method to insert an element at a given index, and the remove() method to delete an element at a given index. For example, you can modify a list of strings as follows:

list.set(0, "JavaScript"); // replace the first element with "JavaScript"
list.add(1, "Ruby"); // insert "Ruby" as the second element
list.remove(3); // delete the fourth element

In this section, you have learned how to create, iterate, and modify lists in Java using the List interface and its methods. In the next section, you will learn about the Java sets, which are another type of collection that store unique elements without any order.

3. Java Sets: HashSet, LinkedHashSet, and TreeSet

In this section, you will learn about the Java sets, which are another type of collection that store unique elements without any order. A set is a collection of elements that cannot contain duplicate elements. Sets do not guarantee any order of elements, meaning that the elements can be stored in any order. The Java collections framework provides three main implementations of the Set interface: HashSet, LinkedHashSet, and TreeSet. Each of these implementations has its own characteristics and performance. In this section, you will learn about the differences between these three implementations and how to choose the best one for your needs.

The HashSet class is the most common implementation of the Set interface. It uses a hash table internally to store the elements. A hash table is a data structure that maps each element to a hash code, which is a unique integer value that represents the element. The hash code is used to determine the position of the element in the hash table. The HashSet class offers fast insertion, deletion, and lookup of elements, as it only requires computing the hash code of the element. However, the HashSet class does not maintain any order of elements, and it may change the order of elements over time. The HashSet class is not thread-safe, meaning that it cannot be safely used by multiple threads without external synchronization.

The LinkedHashSet class is a subclass of the HashSet class. It uses a hash table and a linked list internally to store the elements. A linked list is a data structure that connects each element to the next element by a reference. The LinkedHashSet class preserves the insertion order of elements, meaning that the elements are stored in the same order as they are added to the set. The LinkedHashSet class offers the same performance as the HashSet class for insertion, deletion, and lookup of elements, but it also provides predictable iteration over the elements. The LinkedHashSet class is also not thread-safe, meaning that it cannot be safely used by multiple threads without external synchronization.

The TreeSet class is another implementation of the Set interface. It uses a balanced binary tree internally to store the elements. A balanced binary tree is a data structure that organizes the elements in a hierarchical way, such that each element has at most two children, and the left child is smaller than the parent, and the right child is larger than the parent. The TreeSet class sorts the elements according to their natural order, or according to a custom comparator that can be provided. The TreeSet class offers fast insertion, deletion, and lookup of elements, as it only requires traversing the tree. However, the TreeSet class also provides additional operations, such as finding the minimum, maximum, or closest element, or getting a subset of elements within a range. The TreeSet class is also not thread-safe, meaning that it cannot be safely used by multiple threads without external synchronization.

How do you decide which implementation of the Set interface to use in your Java programs? Here are some general guidelines:

• If you need a fast and simple set that does not require any order of elements, use the HashSet class.
• If you need a fast and simple set that preserves the insertion order of elements, use the LinkedHashSet class.
• If you need a sorted set that provides additional operations on the elements, use the TreeSet class.

In the next section, you will learn how to create, iterate, and modify sets in Java using the Set interface and its methods.

3.1. HashSet vs LinkedHashSet vs TreeSet

In this section, you will learn about the differences between the three main implementations of the Set interface in Java: HashSet, LinkedHashSet, and TreeSet. You will also learn how to choose the best one for your needs, depending on the performance and concurrency requirements of your program. As you have learned in the previous section, each of these implementations has its own characteristics and performance, which are summarized in the table below:

As you can see, the HashSet and the LinkedHashSet classes use a hash table internally to store the elements, while the TreeSet class uses a balanced binary tree. This affects the order of the elements in the set, as well as the performance of the operations on the set. The HashSet class does not maintain any order of elements, and it may change the order of elements over time. The LinkedHashSet class preserves the insertion order of elements, meaning that the elements are stored in the same order as they are added to the set. The TreeSet class sorts the elements according to their natural order, or according to a custom comparator that can be provided. The HashSet, LinkedHashSet, and TreeSet classes all offer fast insertion, deletion, and lookup of elements, as they only require a constant or logarithmic time complexity. However, the TreeSet class also provides additional operations, such as finding the minimum, maximum, or closest element, or getting a subset of elements within a range.

Another difference between these implementations is the thread-safety. None of these classes are synchronized, meaning that they cannot be safely used by multiple threads without external synchronization. If you need a thread-safe set, you should consider using other concurrent collections instead, such as the ConcurrentSkipListSet class, which offers a scalable and concurrent alternative to the TreeSet class.

How do you decide which implementation of the Set interface to use in your Java programs? Here are some general guidelines:

• If you need a fast and simple set that does not require any order of elements, use the HashSet class.
• If you need a fast and simple set that preserves the insertion order of elements, use the LinkedHashSet class.
• If you need a sorted set that provides additional operations on the elements, use the TreeSet class.

In the next section, you will learn how to create, iterate, and modify sets in Java using the Set interface and its methods.

3.2. How to Create, Iterate, and Modify Sets in Java

In this section, you will learn how to create, iterate, and modify sets in Java using the Set interface and its methods. The Set interface is a subinterface of the Collection interface, which means that it inherits all the methods of the Collection interface, such as size(), isEmpty(), contains(), add(), remove(), and clear(). However, the Set interface does not allow duplicate elements, and it does not guarantee any order of elements. Therefore, some methods of the Collection interface, such as get(), set(), add() with index, remove() with index, indexOf(), lastIndexOf(), and subList(), are not applicable to sets. You can use other methods of the Set interface, such as equals(), hashCode(), containsAll(), addAll(), retainAll(), and removeAll(), to compare, combine, or manipulate sets.

To create a set in Java, you need to use one of the implementations of the Set interface, such as HashSet, LinkedHashSet, or TreeSet. You can either create an empty set or a set with some initial elements. For example, you can create an empty set of integers using the HashSet class as follows:

Set set = new HashSet<>();

Alternatively, you can create a set of integers with some initial elements using the Set.of() method, which returns an immutable set of the given elements. For example, you can create a set of integers with three elements as follows:

Set set = Set.of(1, 2, 3);

To iterate over a set in Java, you can use different ways, such as a for-each loop, an iterator, or a stream. For example, you can use a for-each loop to iterate over a set of integers and print each element as follows:

for (Integer element : set) {
  System.out.println(element);
}

To modify a set in Java, you can use the methods of the Set interface, such as add() and remove(). For example, you can use the add() method to insert an element to the set, and the remove() method to delete an element from the set. For example, you can modify a set of integers as follows:

set.add(4); // insert 4 to the set
set.remove(2); // delete 2 from the set

In this section, you have learned how to create, iterate, and modify sets in Java using the Set interface and its methods. In the next section, you will learn about the Java maps, which are another type of collection that store key-value pairs.

4. Java Maps: HashMap, LinkedHashMap, and TreeMap

In this section, you will learn about the Java maps, which are another type of collection that store key-value pairs. A map is a collection of entries, where each entry consists of a key and a value. Maps do not allow duplicate keys, but they allow duplicate values. Maps provide access to values by their keys, and they also provide methods to manipulate the entries. The Java collections framework provides three main implementations of the Map interface: HashMap, LinkedHashMap, and TreeMap. Each of these implementations has its own characteristics and performance. In this section, you will learn about the differences between these three implementations and how to choose the best one for your needs.

The HashMap class is the most common implementation of the Map interface. It uses a hash table internally to store the entries. A hash table is a data structure that maps each key to a hash code, which is a unique integer value that represents the key. The hash code is used to determine the position of the entry in the hash table. The HashMap class offers fast insertion, deletion, and lookup of entries, as it only requires computing the hash code of the key. However, the HashMap class does not maintain any order of entries, and it may change the order of entries over time. The HashMap class is not thread-safe, meaning that it cannot be safely used by multiple threads without external synchronization.

The LinkedHashMap class is a subclass of the HashMap class. It uses a hash table and a linked list internally to store the entries. A linked list is a data structure that connects each entry to the next entry by a reference. The LinkedHashMap class preserves the insertion order of entries, meaning that the entries are stored in the same order as they are added to the map. The LinkedHashMap class offers the same performance as the HashMap class for insertion, deletion, and lookup of entries, but it also provides predictable iteration over the entries. The LinkedHashMap class is also not thread-safe, meaning that it cannot be safely used by multiple threads without external synchronization.

The TreeMap class is another implementation of the Map interface. It uses a balanced binary tree internally to store the entries. A balanced binary tree is a data structure that organizes the entries in a hierarchical way, such that each entry has at most two children, and the left child is smaller than the parent, and the right child is larger than the parent. The TreeMap class sorts the entries according to their natural order, or according to a custom comparator that can be provided. The TreeMap class offers fast insertion, deletion, and lookup of entries, as it only requires traversing the tree. However, the TreeMap class also provides additional operations, such as finding the minimum, maximum, or closest entry, or getting a submap of entries within a range. The TreeMap class is also not thread-safe, meaning that it cannot be safely used by multiple threads without external synchronization.

How do you decide which implementation of the Map interface to use in your Java programs? Here are some general guidelines:

• If you need a fast and simple map that does not require any order of entries, use the HashMap class.
• If you need a fast and simple map that preserves the insertion order of entries, use the LinkedHashMap class.
• If you need a sorted map that provides additional operations on the entries, use the TreeMap class.

In the next section, you will learn how to create, iterate, and modify maps in Java using the Map interface and its methods.

4.1. HashMap vs LinkedHashMap vs TreeMap

In this section, you will learn about the differences between the three main implementations of the Map interface in Java: HashMap, LinkedHashMap, and TreeMap. You will also learn how to choose the best one for your needs, depending on the performance and concurrency requirements of your program. As you have learned in the previous section, each of these implementations has its own characteristics and performance, which are summarized in the table below:

As you can see, the HashMap and the LinkedHashMap classes use a hash table internally to store the entries, while the TreeMap class uses a balanced binary tree. This affects the order of the entries in the map, as well as the performance of the operations on the map. The HashMap class does not maintain any order of entries, and it may change the order of entries over time. The LinkedHashMap class preserves the insertion order of entries, meaning that the entries are stored in the same order as they are added to the map. The TreeMap class sorts the entries according to their natural order, or according to a custom comparator that can be provided. The HashMap, LinkedHashMap, and TreeMap classes all offer fast insertion, deletion, and lookup of entries, as they only require a constant or logarithmic time complexity. However, the TreeMap class also provides additional operations, such as finding the minimum, maximum, or closest entry, or getting a submap of entries within a range.

Another difference between these implementations is the thread-safety. None of these classes are synchronized, meaning that they cannot be safely used by multiple threads without external synchronization. If you need a thread-safe map, you should consider using other concurrent collections instead, such as the ConcurrentHashMap class, which offers a scalable and concurrent alternative to the HashMap class.

How do you decide which implementation of the Map interface to use in your Java programs? Here are some general guidelines:

• If you need a fast and simple map that does not require any order of entries, use the HashMap class.
• If you need a fast and simple map that preserves the insertion order of entries, use the LinkedHashMap class.
• If you need a sorted map that provides additional operations on the entries, use the TreeMap class.

In the next section, you will learn how to create, iterate, and modify maps in Java using the Map interface and its methods.

4.2. How to Create, Iterate, and Modify Maps in Java

In this section, you will learn how to create, iterate, and modify maps in Java using the Map interface and its methods. The Map interface is a collection of entries, where each entry consists of a key and a value. Maps do not allow duplicate keys, but they allow duplicate values. Maps provide access to values by their keys, and they also provide methods to manipulate the entries. The Map interface is not a subinterface of the Collection interface, which means that it does not inherit all the methods of the Collection interface. However, you can use some methods of the Collection interface, such as size(), isEmpty(), containsKey(), containsValue(), and clear(), to perform basic operations on the map. You can also use other methods of the Map interface, such as get(), put(), remove(), equals(), hashCode(), keySet(), values(), entrySet(), containsAll(), putAll(), retainAll(), and removeAll(), to access, modify, or compare the entries in the map.

To create a map in Java, you need to use one of the implementations of the Map interface, such as HashMap, LinkedHashMap, or TreeMap. You can either create an empty map or a map with some initial entries. For example, you can create an empty map of strings to integers using the HashMap class as follows:

Map map = new HashMap<>();

Alternatively, you can create a map of strings to integers with some initial entries using the Map.of() method, which returns an immutable map of the given key-value pairs. For example, you can create a map of strings to integers with three entries as follows:

Map map = Map.of("one", 1, "two", 2, "three", 3);

To iterate over a map in Java, you can use different ways, such as a for-each loop, an iterator, or a stream. However, unlike lists and sets, you cannot directly iterate over the map, as the map is not an iterable collection. Instead, you need to iterate over the keys, values, or entries of the map, which are returned by the keySet(), values(), or entrySet() methods of the Map interface. For example, you can use a for-each loop to iterate over the entries of a map of strings to integers and print each key-value pair as follows:

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

To modify a map in Java, you can use the methods of the Map interface, such as put() and remove(). For example, you can use the put() method to insert or update an entry to the map, and the remove() method to delete an entry from the map. For example, you can modify a map of strings to integers as follows:

map.put("four", 4); // insert a new entry with key "four" and value 4
map.put("one", 10); // update the value of the existing entry with key "one" to 10
map.remove("two"); // delete the entry with key "two"

In this section, you have learned how to create, iterate, and modify maps in Java using the Map interface and its methods. In the next section, you will learn about the conclusion and further resources of this tutorial.

5. Conclusion and Further Resources

In this tutorial, you have learned about the Java collections framework and how to use different data structures such as lists, sets, and maps in your Java programs. You have learned about the differences between the main implementations of the List, Set, and Map interfaces, and how to choose the best one for your needs. You have also learned how to create, iterate, and modify collections in Java using the methods of the Collection and Map interfaces.

The Java collections framework is a powerful and versatile tool that can help you solve many problems that involve storing, manipulating, and accessing data. By using the Java collections framework, you can benefit from the following advantages:

• You can reduce the complexity of coding by using ready-made data structures and algorithms.
• You can improve the performance and efficiency of your programs by using optimized and tested implementations.
• You can enhance the readability and maintainability of your code by using consistent and standard interfaces and methods.
• You can facilitate interoperability and compatibility between different classes and libraries by using common data types and protocols.

However, the Java collections framework is not the only option for working with data in Java. There are other alternatives that you can explore, such as arrays, streams, and custom data structures. Depending on your specific problem and requirements, you may find that these alternatives offer better solutions than the Java collections framework. Therefore, you should always evaluate the pros and cons of each option and choose the one that best suits your needs.

If you want to learn more about the Java collections framework and its features, you can check out the following resources:

• The official documentation of the Collections class, which provides static methods for operating on collections.
• The official documentation of the Collection interface, which is the root interface of the Java collections framework.
• The official documentation of the List interface, which represents an ordered sequence of elements.
• The official documentation of the Set interface, which represents a collection of unique elements.
• The official documentation of the Map interface, which represents a mapping of keys to values.
• The official tutorial of the Java collections framework, which provides a comprehensive and detailed introduction to the concepts and features of the Java collections framework.
• The Baeldung website, which provides many articles and examples on how to use the Java collections framework in various scenarios and applications.

We hope you have enjoyed this tutorial and learned something new and useful. Thank you for reading and happy coding!