Introduction to Java Part 4 Collections

1. What is a set?

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In Java, a collection is a container class used to store and manipulate a set of objects. It provides a series of methods and functions for conveniently managing and operating collections of objects. The collection framework is a very important and commonly used part of Java. It contains multiple interfaces and classes, such as List, Set, Map, etc.

The functions of collections have the following aspects:

Dynamic storage: Collections can dynamically store and manage objects without specifying the capacity in advance. In contrast to arrays, collections can automatically expand and shrink as needed.
Flexible operations: Collections provide a wealth of methods and functions, making it easy to perform operations such as adding, deleting, searching, and sorting. They encapsulate complex data structures and algorithms, making operations simpler and more efficient.
Generic support: The collection framework supports generics, which can specify the types of objects stored in the collection, improving type safety and code readability.
Provides multiple collection types: The collection framework provides many different types of collections, such as List, Set, Map, etc. Each type has different characteristics and applicable scenarios. This allows you to choose the appropriate collection type based on your specific needs.

Although arrays are similar to sets in some ways, sets have additional advantages and capabilities that make them a more commonly used data structure. Compared with arrays, collections have the characteristics of dynamic storage and flexible operations, and can easily handle an uncertain number of objects without the need to manually manage capacity. In addition, collections provide more functions and algorithms, allowing developers to process and manipulate data more efficiently.

In summary, a collection is a container for storing and manipulating a set of objects, which provides the advantages of dynamic storage, flexible operations, generic support, and multiple collection types. By using collections, we can process and operate data more conveniently, improving the readability and maintainability of the code.

In Java, the collection framework provides multiple interfaces and classes for representing and manipulating different types of collections. Following is a list of commonly used collection interfaces and classes in Java:

Collection interface:
- List interface: ordered, repeatable collection, such as ArrayList, LinkedList, Vector, etc.
- Set interface: unordered, non-repeatable collections, such as HashSet, TreeSet, LinkedHashSet, etc.
Map interface:
- HashMap class: an unordered collection of key-value pairs, for quick search based on the keys, and the keys cannot be repeated.
- TreeMap class: An ordered collection of key-value pairs, sorted according to the natural order of the keys.
- LinkedHashMap class: An ordered collection of key-value pairs, maintaining insertion order or access order.
- Hashtable class: Similar to the HashMap class, but thread-safe (deprecated, ConcurrentHashMap is recommended).
- ConcurrentHashMap class: Thread-safe HashMap implementation.
Queue interface:
- LinkedList class: bidirectional queue, which can be used as a queue, stack or double-ended queue.
- PriorityQueue class: Priority queue, sorted according to the priority of elements.

In addition to the above-mentioned commonly used collection interfaces and classes, Java also provides some other collection classes, such as BitSet, Stack, etc. Additionally, Java 8 introduced the Stream API, which provides a new way to work with collections and data streams.

It should be noted that the collection interfaces and classes are located under the java.util package. They are all part of the Java standard library and can be used directly without additional imports.

To summarize, there are multiple collection interfaces and classes in Java for representing and manipulating different types of collections. These collections provide a rich set of methods and functions to meet different needs. According to specific scenarios and needs, choosing the appropriate collection type can improve the efficiency and readability of the code.

2、ArrayList

In Java, ArrayList is a dynamic array that implements the List interface. ArrayList automatically resizes as needed and can store any type of object. The following is an introduction and explanation of the syntax of Java ArrayList:

Import the ArrayList class:
Before using ArrayList, you need to import the java.util.ArrayList class in the Java code. You can import it through the following statement:
```
import java.util.ArrayList;
```
Create an ArrayList object:
To create an ArrayList object, you can use the following syntax:
```
ArrayList<数据类型> arrayList = new ArrayList<>();
```
Specify the data type stored in the ArrayList within angle brackets. For example, to create an ArrayList that stores integers, you would use the following statement:
```
ArrayList<Integer> arrayList = new ArrayList<>();
```
Add elements:
Use the add() method to add elements to the ArrayList. For example, to add an integer to an ArrayList, use the following statement:
```
arrayList.add(10);
```
You can call theadd() method multiple times in succession to add multiple elements.
Accessing elements:
You can use indexes to access elements in an ArrayList. The index starts from 0, use the get() method to get the element at the specified index position. For example, to get the first element in an ArrayList, you can use the following statement:
```
int element = arrayList.get(0);
```
Modify elements:
You can use the index and set() methods to modify elements in the ArrayList. For example, to modify the first element in the ArrayList to a new value, you can use the following statement:
```
arrayList.set(0, newValue);
```
Delete elements:
You can use indexes or objects to delete elements in an ArrayList. Use the remove() method and specify the index or object to delete. For example, to delete the first element in an ArrayList, you can use the following statement:
```
arrayList.remove(0);
```
If you want to delete a specific object, you can use the object as argument:
```
arrayList.remove(object);
```
Get the size of ArrayList:
Use the size() method to get the number of elements in the ArrayList. For example, to get the size of an ArrayList, you can use the following statement:
```
int size = arrayList.size();
```
Traverse ArrayList:
You can use a for loop or iterator to traverse the elements in ArrayList. The following is an example of using a for loop to iterate over an ArrayList:
```
for (int i = 0; i < arrayList.size(); i++) {
      
      
    int element = arrayList.get(i);
    // 处理元素
}
```
Corresponding operations can be performed in the loop as needed.

These are the basic syntax introduction and explanation of ArrayList in Java. ArrayList provides a set of convenient methods to operate and manage dynamic arrays, making it more convenient when processing collection data.

3. HashMap

In Java, HashMap is a commonly used collection class that implements the Map interface and is used to store key-value pairs. The following is the syntax code tutorial and explanation of Java HashMap:

Import the HashMap class:
Before using HashMap, you need to import the java.util.HashMap class in the Java code. You can import it through the following statement:
```
import java.util.HashMap;
```
Create a HashMap object:
To create a HashMap object, you can use the following syntax:
```
HashMap<键的数据类型, 值的数据类型> hashMap = new HashMap<>();
```
Specify the data types of keys and values within angle brackets. For example, to create a HashMap that stores strings as keys and integers as values, you would use the following statement:
```
HashMap<String, Integer> hashMap = new HashMap<>();
```
Add key-value pairs:
Use the put() method to add key-value pairs to HashMap. For example, to add a key-value pair with the key "key" and the value 10 to the HashMap, you can use the following statement:
```
hashMap.put("key", 10);
```
You can call theput() method multiple times in succession to add multiple key-value pairs.
Get the value:
You can use the key to get the value in the HashMap. Use the get() method and specify the key to get the corresponding value. For example, to get the value with key "key", you can use the following statement:
```
int value = hashMap.get("key");
```
If the key does not exist, the get() method will return null.
Modify value:
You can use the key and put() methods to modify the value in HashMap. For example, to change the value of key "key" to a new value, you can use the following statement:
```
hashMap.put("key", newValue);
```
Delete key-value pairs:
Keys can be used to delete key-value pairs in a HashMap. Use the remove() method and specify the key to delete. For example, to delete the key-value pair with the key "key", you can use the following statement:
```
hashMap.remove("key");
```
Determine whether the key exists:
You can use the containsKey() method to determine whether the HashMap contains the specified key. For example, to check whether a key-value pair with key "key" exists, you can use the following statement:
```
boolean contains = hashMap.containsKey("key");
```
Get the size of HashMap:
Use the size() method to get the number of key-value pairs in the HashMap. For example, to get the size of a HashMap, you can use the following statement:
```
int size = hashMap.size();
```
Traverse HashMap:
You can use a for-each loop or iterator to traverse the key-value pairs in the HashMap. The following is an example of using a for-each loop to iterate over a HashMap:
```
for (Map.Entry<键的数据类型, 值的数据类型> entry : hashMap.entrySet()) {
      
      
    键的数据类型 key = entry.getKey();
    值的数据类型 value = entry.getValue();
    // 处理键值对
}
```
In the loop, you can get the key through entry.getKey() and the value through entry.getValue().

These are the basic syntax code tutorials and explanations of HashMap in Java. HashMap provides a convenient way to store and manage key-value data, with fast lookup and insertion performance, suitable for many common programming tasks.

Four, Set

In Java, Set is a collection interface that represents a set of unique elements. The Set interface does not define a specific order and does not allow duplicate elements. Java provides multiple Set implementation classes, such as HashSet, LinkedHashSet and TreeSet. The following is the syntax code tutorial and explanation of Java Set:

Import the Set class:
Before using Set, you need to import the java.util.Set class in the Java code. You can import it through the following statement:
```
import java.util.Set;
```
Create a Set object:
Set is an interface and cannot be instantiated directly. You need to use the implementation class of Set to create objects. For example, to create a HashSet object, you can use the following syntax:
```
Set<数据类型> set = new HashSet<>();
```
Specify the data type stored in the Set within angle brackets. You can choose other Set implementation classes as needed, such as LinkedHashSet or TreeSet.
Add elements:
Use the add() method to add elements to the Set. For example, to add an element to a Set, you can use the following statement:
```
set.add(element);
```
Set will automatically ensure the uniqueness of elements. If duplicate elements are added, Set will ignore the duplicate elements.
Delete elements:
Use the remove() method to delete elements from the Set. For example, to delete an element in a Set, you can use the following statement:
```
set.remove(element);
```
Determine whether the element exists:
Use the contains() method to determine whether the Set contains the specified element. For example, to check whether an element exists in a Set, you can use the following statement:
```
boolean contains = set.contains(element);
```
Get the size of the Set:
Use the size() method to get the number of elements in the Set. For example, to get the size of a Set, you can use the following statement:
```
int size = set.size();
```
Traverse Set:
You can use a for-each loop or iterator to traverse the elements in the Set. The following is an example of using a for-each loop to iterate over a Set:
```
for (数据类型 element : set) {
      
      
    // 处理元素
}
```
In the loop, each element in the Set can be accessed through theelement variable.

The Set interface provides a convenient way to store and manage unique collections of elements. According to specific needs, you can choose different Set implementation classes and use Set methods to operate and process elements in the collection.

Five, Queue

In Java, Queue is an interface that represents a queue of elements. The queue is a first-in-first-out (FIFO) data structure. The elements are arranged in the order of insertion. New elements are inserted at the end of the queue, and when elements are obtained from the queue, they are obtained from the head. Java provides multiple Queue implementation classes, such as LinkedList and PriorityQueue. The following is the syntax code tutorial and explanation of Java Queue:

Import the Queue class:
Before using Queue, you need to import the java.util.Queue class in the Java code. You can import it through the following statement:
```
import java.util.Queue;
```
Create a Queue object:
Queue is an interface and cannot be instantiated directly. You need to use the implementation class of Queue to create an object. For example, to create a LinkedList object as a Queue, you can use the following syntax:
```
Queue<数据类型> queue = new LinkedList<>();
```
Specify the data type stored in the Queue in angle brackets. You can choose other Queue implementation classes as needed, such as PriorityQueue.
Add elements:
Use the offer() method to add elements to the Queue. For example, to add an element to the Queue, you can use the following statement:
```
queue.offer(element);
```
Get and delete the head element of the queue:
Use the poll() method to get and delete the head element of the queue. For example, to get and delete the head element, you can use the following statement:
```
数据类型 element = queue.poll();
```
If the queue is empty, the poll() method will return null.
Get but not delete the head element of the queue:
Use the peek() method to get but not delete the head element of the queue. For example, to get but not delete the head element, you can use the following statement:
```
数据类型 element = queue.peek();
```
If the queue is empty, the peek() method will return null.
Determine whether the queue is empty:
Use the isEmpty() method to determine whether the Queue is empty. For example, to check if the Queue is empty, you can use the following statement:
```
boolean isEmpty = queue.isEmpty();
```
Get the size of the Queue:
Use the size() method to get the number of elements in the Queue. For example, to get the size of the Queue, you can use the following statement:
```
int size = queue.size();
```

The Queue interface provides a convenient way to implement a queue data structure that processes elements in first-in, first-out order. According to specific needs, you can choose different Queue implementation classes and use Queue methods to operate and process elements in the queue.

Six, queue

ArrayBlockingQueue is a FIFO blocking queue implementation class in Java. It is based on array implementation and has a fixed capacity. The following is the grammar and explanation about ArrayBlockingQueue:

Import related classes:
Before using ArrayBlockingQueue, you need to import related classes in Java code. The required classes can be imported using the following statement:
```
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.BlockingQueue;
```
CreateArrayBlockingQueue objects:
Use ArrayBlockingQueue classes to create ArrayBlockingQueue objects. For example, to create an object with a capacity of capacity, use the following syntax: ArrayBlockingQueue
```
BlockingQueue<数据类型> queue = new ArrayBlockingQueue<>(capacity);
```
Specify the data type stored in the queue in angle brackets, and specify the capacity of the queue in brackets.
Add elements:
Use the put() method to add elements to ArrayBlockingQueue. This method blocks when the queue is full until space becomes available on the queue. For example, to add an element to the queue, you would use the following statement:
```
queue.put(element);
```
Get and delete the head element:
Use the take() method to get and delete the head element from ArrayBlockingQueue. This method blocks when the queue is empty until a new element is available in the queue. For example, to get and delete the head element, you can use the following statement:
```
数据类型 element = queue.take();
```
Get but not delete the head element:
Use the peek() method to get but not delete the head element from ArrayBlockingQueue element. For example, to get but not delete the head element, you can use the following statement:
```
数据类型 element = queue.peek();
```
If the queue is empty, the peek() method will returnnull.
Determine whether the queue is empty:
Use the isEmpty() method to determine whether ArrayBlockingQueue is empty. For example, to check if the queue is empty, you can use the following statement:
```
boolean isEmpty = queue.isEmpty();
```
Get the size of the queue:
Use the size() method to get the number of elements in ArrayBlockingQueue. For example, to get the size of a queue, you can use the following statement:
```
int size = queue.size();
```

ArrayBlockingQueue is a thread-safe queue implementation suitable for multi-threaded environments. It provides blocking waiting features, which can block threads when the queue is empty or full to achieve synchronization and coordination between threads. By using ArrayBlockingQueue, you can easily implement multi-threaded scenarios such as the producer-consumer model.