background
First, let's look at a piece of code:
List<Member> memberList = listMember();
Iterator<Member> iterator = memberList.iterator();
while (iterator.hasNext()) {
Member next = iterator.next();
...
}
The above code uses an iterator to traverse the List. Let's take a look at the following traversal method:
for (Member member : memberList) {
...
}
The above traversal is a direct traversal.
Direct traversal is the traversal process in which the collection directly participates. The traversal method here is too coupled with the collection object. If we manipulate the collection in direct traversal, an operation exception occurs.
Using iterator traversal, we separate the collection object and its traversal behavior. When the client traverses the collection, it is operated through the iterator. For the client, the internal details of the collection are hidden. With iterator traversal, we will have no problem operating the collection while traversing the collection.
In fact, iterator traversal uses the iterator pattern.
What is the iterator pattern
“Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation. (It provides a way to access the elements of a container object without exposing the internal details of the object.)
”
The iterator mode mainly consists of the following 4 elements:
“”
Abstract aggregation (Aggregate) role: defines the interface for storing, adding, and deleting aggregate objects, and creating iterator objects.
Concrete Aggregate (ConcreteAggregate) role: implements an abstract aggregation class and returns an instance of a concrete iterator.
Abstract Iterator role: defines the interface for accessing and traversing aggregate elements, usually including methods such as hasNext(), first(), next(), etc.
Concrete iterator (Concretelterator) role: implement the methods defined in the abstract iterator interface, complete the traversal of the aggregate object, and record the current position of the traversal.
The structure diagram of the iterator mode is as follows:
Code
Aggregate
public interface Aggregate {
void add(Object obj);
void remove(Object obj);
Iterator getIterator();
}
Iterator
public interface Iterator {
Object first();
Object next();
boolean hasNext();
}
ConcreteAggregate
ConcreteIterator
Test code:
Test Results:
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Thoughts on the iterator pattern
In Java, collections provide iterator classes. If we want to write a collection-like aggregation class ourselves, we can use the iterator class. Using the iterator pattern, the aggregated object may be accessed without exposing internal details, which protects the security of the aggregated object's data.
In fact, in Java development, you may not be able to use the iterator mode, because in the Java class library, all libraries that need to use the iterator mode already provide an iterator class.
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