Exploring the Java Collections Framework Hierarchy
Jul 07, 2025 am 02:39 AM
The core of the Java collection framework is the Collection interface and the Map interface, which form the basis of the entire framework. 1. The Collection interface is the root interface of all collection classes. Its three sub-interfaces List, Set and Queue are used to process ordered and repeatable data (such as ArrayList and LinkedList), unordered and unrepeatable data (such as HashSet and TreeSet), and first-in-first-out queue operations (such as LinkedList and PriorityQueue). 2. Although the Map interface does not belong to the Collection system, it is also an important part of the framework and is used to store key-value pair data. Common implementations include HashMap, TreeMap and LinkedHashMap. 3. The Iterator and Iterable interfaces provide support for collection traversal, where Iterable ensures that all collection classes can be traversed through enhanced for loops. 4. The Collections tool class encapsulates many practical methods, such as sorting, inversion, generating read-only or thread-safe collections, which greatly simplifies the operation and maintenance of collections. Mastering these core structures and their implementation classes will help to use the Java collection framework more efficiently.
Java's Collections Framework looks a bit complicated, especially when you first come across it. In fact, it has a clear structure. As long as you understand the relationship between classes and interfaces, it will be much easier to use. This article will help you clarify the inheritance system of the Java collection framework, and focus on how to organize common interfaces and their implementation classes and why they are designed in this way.
1. The Collection interface is the root, List, Set, and Queue are the three major branches
The core of the Java collection framework is the Collection interface, which is the basis of all collection classes. Although you may not use it directly in normal times, its subinterface defines the basic behavior of the collection.
- List : Orderly and repeatable, representing a string of elements arranged in order. For example,
ArrayListandLinkedList. - Set : Unordered, non-repeatable, used to store non-repeatable data. For example,
HashSetandTreeSet. - Queue : Queue, usually used in first-in-first-out (FIFO) scenarios, such as task scheduling. Common ones are
LinkedListandPriorityQueue.
These interfaces have different implementation classes and are suitable for different scenarios. For example, ArrayList supports fast random access, but insertion and deletion are slow; while LinkedList insertion and deletion are fast, but access efficiency is low.
2. Map is not a subinterface of Collection, but it is part of the collection framework
Many people mistakenly think that Map is a subclass of Collection , but it is not. Map is an independent interface that stores key-value pairs (Key-Value). Although it does not belong to the Collection branch, it is an important part of the collection framework.
Common implementations include:
-
HashMap: Most commonly used, based on hash table implementation, fast search speed -
TreeMap: key sorting, suitable for scenes where orderly output is required -
LinkedHashMap: Keep insertion order or access order
If you want to traverse the content of the Map, you can get a collection view through entrySet() or keySet() , and you can traverse with for-each.
3. Iterator and Iterable are the key interfaces for traversing collections
In the collection framework, traversal operations are completed through Iterator . The Iterable interface is one of the parent interfaces of Collection , so all collection classes support iteration.
You can use this:
List<String> list = new ArrayList<>();
list.add("A");
list.add("B");
for (String item : list) {
System.out.println(item);
} Behind this "enhanced for loop" is the call to iterator() method. If you want to manually control the traversal process, you can also explicitly get the Iterator object and use hasNext() and next() methods.
It should be noted that modifying the collection during traversal may throw ConcurrentModificationException unless you use Iterator.remove() .
4. The Collections tool class provides practical methods to simplify collection operations
In addition to interfaces and implementation classes, there is also a tool class called Collections (note that it is a plural), which provides a series of static methods to operate on collections, such as:
-
Collections.sort(list): Sort List -
Collections.reverse(list): Invert the list -
Collections.unmodifiableList(list): Returns read-only list wrapper -
Collections.synchronizedList(list): Returns a thread-safe list
These methods are very convenient and can avoid repeated wheel making. However, it should be noted that some methods return the packaging of the original collection, and modification will affect the original data.
Basically that's it. The Java collection framework seems huge, but as long as you grasp the backbone structure and several core interfaces, and select the appropriate implementation class based on actual needs, it will be used smoothly.
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