How much do you know about java collections

After using Java collections for so long, I have never systematically studied the collection structure of Java. Today I personally drew the following class diagram, and finally gained something.

1. All collections implement the Iterable interface.

The Iterable interface contains an abstract method: Iterator iterator(); each collection that implements this method will return an Iterator object.

Iterator: It contains three methods, hashNext(), next(), and remove(), through which collection traversal and element deletion can be achieved, for example:

Collection list = new ArrayList();

list.add("a");

list.add("b");

list.add("c");

Iterator iterator = list.iterator();

while (iterator .hasNext()) {

　 　 　 String element = iterator.next();

    System.out.println(element);//a b c

}

System.out.println(list);//[a, b, c] iterator = list.iterator(); iterator.next();

iterator.remove();

System.out.println(list);//[b, c]

ps: Why not What about directly implementing the Iterator interface? Because: Iterator will carry the position information of the current collection, and it will not start from 0 the next time it is used; and the Iterable interface returns an Iterator object each time (Iterator is implemented through an internal class), and each iterator is independent of each other. Influence.

2. ListIterator

Starting from AbstractList, the public ListIterator listIterator() method is encapsulated, returning a ListIterator, which adds add(), previous(), hasPrevious() and other methods based on Iterator. Bidirectional traversal is possible.

AbstractList list = new ArrayList();

list.add("a");

list.add("b");

list.add("c");

ListIterator iterator = list.listIterator(3);

while (iterator.hasPrevious()) {

String element = iterator.previous();

System.out.println(element);//c,b,a

}

3. Comparison

ArrayList: allows to store duplicate elements, and the elements are ordered, making random access more convenient.

LinkedList: Linked list implementation, better than ArrayList when inserting and deleting.

HashSet: Duplicate elements and unordered elements are not allowed (the hash function sorts the elements and can be queried quickly), and null values ​​are allowed.

TreeSet: Red-black tree sorting, can be sorted, the elements contained must implement the Comparable interface and define the compareTo method, and null values ​​​​are not allowed.

HashMap: Thread-unsafe, both key and value are allowed to be null. To determine whether the key is included, use the containsKey() method. Repeated keys are not allowed. The default size of the hash array is 16, and it must be an exponent of 2. Recalculate the hash value. .

HashTable: Thread-safe, null values ​​are not allowed in key and value, and duplicate keys are not allowed. The default size of the array is 11, and the increasing method is old*2+1, using the hash value of the containing object.

4. Collections and Arrays

Collections: a special class under java.util, which contains various static methods related to collection operations, and can implement operations such as searching, sorting, and thread safety for various collections. .

Arrays: A special class under java.util, used to operate arrays, providing static methods such as search, sorting, copying, and conversion.