How to understand the concept of collections in java

What is a collection?

The Java collection class is stored in the java.util package and is a container used to store objects.

Note:

1. Collections can only store objects. For example, if you store an int type data 66 into a collection, it is actually converted into an Integer class and then stored automatically. Each basic data type in Java has a corresponding reference type.

2. Collections store references to objects, not the objects themselves. So we call the objects in the collection the references to the objects in the collection. The object itself is still placed in the heap memory.

3. Collections can store different types of data types, with no limit on the number.

Java Collection Framework

I found a feature that all the above collection classes, except the map series collection, are The collections on the left all implement the Iterator interface.

Iterator is an interface used to traverse elements in a collection. It mainly has three methods: hashNext(), next(), and remove().

Its sub-interface ListIterator adds three more methods based on it, namely add(), previous(), and hasPrevious().

We can see from the picture:

1. Collections are mainly divided into two interfaces: Collection and Map.

2.Collection is inherited by List and Set respectively.

3.List is implemented by AbstractList and then divided into 3 subclasses, ArrayList, LinkList and VectorList.

4.Set is implemented by AbstractSet and is divided into 2 subclasses, HashSet and TreeSet.

5.Map is implemented by AbstractMap and is divided into 2 subclasses, HashMap and TreeMap.

6.Map is implemented by Hashtable.

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