What is the data structure of Java collection framework

1. What is a collection framework?

In Java, there is a set of ready-made data structures, such as sequence lists, linked lists, queues, stacks, priority queues, hash tables, etc., which are encapsulated into corresponding interfaces/classes for programmers to directly To use it, you only need to create the relevant objects to use, without the need to implement its internal structure.

A collection is a data structure that stores and manages multiple elements, where these elements are placed in a single unit and can be processed through operations such as addition, deletion, modification, and query. For example, a set of playing cards (a collection of cards), an address book (a set of mapping relationships between names and phone numbers), etc. can be used as examples.

The picture below is very important! ! ! You need to remember its commonly used interfaces and classes! !

Because the map interface does not implement the Iterable interface, how to traverse the elements in it?

		Map<Integer,String> map = new HashMap();
        map.put(1,"jack");
        map.put(2,"tom");
        Set<Map.Entry<Integer, String>> entries = map.entrySet();
        // 使用迭代器进行遍历 ,增强 for同理
        Iterator<Map.Entry<Integer, String>> iterator = entries.iterator();
        while (iterator.hasNext()) {
            Map.Entry<Integer, String> entry =  iterator.next();
            System.out.println(entry.getKey() + " " + entry.getValue());
        }

Taking HashMap as an example, you can call its entrySet() method to encapsulate each key-value pair in the map into a Map.Entry object. Because the Set interface is used to receive it, you can use iterators or for-each() to traverse, and each entry object has getKey() and getValue() methods to obtain the key value and value value respectively.

Basic relationship (simplified version)

2. Collection interface

is generally accepted by an interface or class that implements the Collection interface Specifically implement the object of the class, because as can be seen from the above figure, the Collection interface is the parent interface of a series of interfaces and classes. It has relatively few internally implemented methods, so it cannot call some common methods of subclasses.

1. Specify the object type in the corresponding collection through generics

Note: The type passed in here can only be a reference type. If it is a basic data type, its wrapper class should be used. Specify

		Collection<String> collection1 = new ArrayList();
        collection1.add("haha");
        collection1.add("world");
        Collection<Integer> collection2 = new ArrayList();
        collection2.add(1);
        collection2.add(2);
        //collection2.add("hh");// 这里会报错，不符合传入的指定类型Integer

2.Collection common methods use

Note: In the last Object[] toArray() method, an array of type Object[] is returned. The underlying method is: take out the elements in the collection one by one, convert them into Object objects, and store them in the array to be returned. , and finally returns an array of type Object[]. A type conversion exception is thrown when converting to a String[] array.

Because there is no guarantee that every element in the array is converted to String, but it is only forced to be converted into an array of type String[], so if you have to To convert, you need to first traverse the returned results, convert them to String type one by one, and finally assign them to an array of String[] type. It is not recommended to convert the array type as a whole in Java.

		Object[] objects = collection1.toArray();
        String[] strings = new String[objects.length];
        for (int i = 0; i < objects.length; i++) {
            strings[i] = (String)objects[i];// 一个一个转，但是没啥必要
        }

3. Map interface

stores data in the form of < k, v > key-value pairs. The key value here is unique, and each key value can correspond to its The corresponding value value. Different key values ​​can correspond to the same value. HashMap: When storing elements, the internal hashCode function is called based on its key value to find the location where the element should be placed. Therefore, the elements in the hash table are not stored in the order in which they are stored.

Map common methods use

		HashMap<Integer, String> map = new HashMap<>();

        // put()
        map.put(1,"张飞");// 这里的 key 值唯一
        map.put(1,"宋江");// 如果二次插入的 key 值之前有，则替换其 value值
        map.put(2,"Jack");
        System.out.println(map);

        // get()
        String s1 = map.get(1);// 返回 宋江
        String s3 = map.getOrDefault(3,"三团");// 未找到，返回 三团

        // entrySet()
        // 该方法返回一个 Set<Map.Entry<Integer, String>> 对象
        Set<Map.Entry<Integer, String>> entries = map.entrySet();
        for (Map.Entry<Integer, String> entry : entries) {
            // 通过 entry.getKey() 和 entry.getValue() 获取每个entry对应的 k, v值
            System.out.println(entry.getKey() + " " + entry.getValue());
        }

4、具体的实现类

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