#What is the difference between collections in java?
In Java, collections are mainly divided into three types: List, Set, and Map. List and Set inherit from Collection, but Map does not.
The difference between List and Set: The elements in List have a storage order, and can store repeated elements. The retrieval efficiency is high, but the insertion and deletion efficiency is low. Set has no storage order, and cannot store repeated elements. Later elements It will replace the previously repeated elements, which results in low retrieval efficiency and high insertion and deletion efficiency. (Set storage location is determined by its HashCode code, so the object it stores must have the equals() method, and Set traversal can only use iteration because it has no subscript.)
1 , The most commonly used collection: ArrayList
Features: The storage locations of elements in the ArrayList collection are continuous, so the query will be faster, but the insertion and deletion operations will be more troublesome and will cause other elements to Changes in location.
Note: What is stored in the list is a reference to the object, not the object itself. If you are not clear about this, there will be a pitfall here. For example:
Some people write code like this in order to save optimization code and save virtual machine memory:
List> list=new ArrayList<>(); List list2=new ArrayList<>(); for(int i=0;i<10;i++){ list2.add(""+i); list.add(list2); list2.clear(); }
Writing like this can be used repeatedly as an object , However, list stores references to objects. When list2.clear(); is called, list2 in the list will also be cleared, so that the final result can only be a bunch of empty collections.
2. The opposite set to the most commonly used collection: LinkedList
LinkedList and ArrayList are complementary, so the advantages of ArrayList are the disadvantages of LinkedList, and the disadvantages of ArrayList are LinkedList The advantages.
Features: The position of elements in LinkedList is arbitrary, so the insertion and deletion operations are more efficient and the query efficiency is lower.
3. A set that is the opposite of a general set: Vector
Why is it said to be the opposite of a general set? Because it is an old dynamic array, many methods are modified with synchonized, so it is thread-safe, while general collections are thread-unsafe.
Features: Simultaneous access by multiple threads will not produce uncertain results, but its efficiency will be relatively low. You can use it if you want to consider thread safety.
4. The most commonly used collection in Set: HashSet
When I use Set collections, I almost always use HashSet. HashSet is implemented using a Hash table. The elements in the set are unordered and can have null values, but there cannot be duplicate elements.
Features: Because the same elements have the same hashCode, there cannot be duplicate elements
5. The second most commonly used set in Set: TreeSet
TreeSet is a set implemented using a binary tree structure
Features: The elements in the set are in order, null is not allowed, and duplicate elements are not allowed.
6. The second most commonly used collection: HashMap
We often encounter scenarios where key-value pairs need to be stored, and HashMap is the most commonly used key-value. to the stored collection.
Features: HashMap allows empty key values, and it is non-thread-safe, so inserting, deleting and positioning elements will be faster.
7. Some less commonly used Map collections: TreeMap, HashTable
TreeMap is implemented based on red-black trees and is suitable for ordering according to natural order. Traverse key.
HashTable is implemented based on HashCode, but it is thread-safe, so it is less efficient than HashMap, and null values are not allowed.
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