I have above code write by java:

import java.util.Map;
import java.util.function.Function;
import java.util.stream.Stream;

public interface Document {
    void put(String key, Object value);

    Object get(String key);

    <T> Stream<T> children(String key, Function<Map<String, Object>, T> constructor);
}

What's the Document mean?

The Document interface in this Java code represents a generic data structure that allows the storage and retrieval of key-value pairs, similar to a dictionary or a hash table. The interface defines three methods:

  1. put(String key, Object value): This method takes a key and a value as input and stores the key-value pair in the Document.
  2. get(String key): This method takes a key as input and returns the value associated with that key in the Document, if it exists.
  3. <T> Stream<T> children(String key, Function<Map<String, Object>, T> constructor): This method takes a key and a constructor function as input. It returns a stream of objects of type T that are the "children" of the key-value pair associated with the input key. The constructor function is used to create each child object from a Map of key-value pairs representing the child's properties.

The Document interface can be implemented by various classes that represent different kinds of documents or data structures. By providing a common set of methods for storing and accessing key-value pairs, the Document interface allows for generic programming and easy interchangeability of different implementations.

If i have a abstract class AbstractDocument and it rewrite children method like below code :

@Override
public <T> Stream<T> children(String key, Function<Map<String, Object>, T> constructor) {
    return Stream.ofNullable(get(key))
            .filter(Objects::nonNull)
            .map(el -> (List<Map<String, Object>>) el)
            .findAny()
            .stream()
            .flatMap(Collection::stream)
            .map(constructor);
}

Can you analyze it?

Sure! The children method in the AbstractDocument class overrides the children method of the Document interface. Here is an analysis of the code:

  1. The method takes a key and a constructor function as input, similar to the original children method in the Document interface.
  2. The get method is called with the input key to retrieve the value associated with that key in the document. The ofNullable method is then called to wrap the value in an Optional object to avoid a NullPointerException if the value is null.
  3. The filter method is called on the Optional object to discard any null values.
  4. The map method is called to convert the value to a List<Map<String, Object>>, assuming that the value is a list of maps. If the value is not a list of maps, the map method will return an empty Optional.
  5. The findAny method is called to return the first element of the stream, if it exists. This will return an Optional<List<Map<String, Object>>>.
  6. The stream method is called on the Optional object to get a stream of Map<String, Object> objects, or an empty stream if the Optional is empty.
  7. The flatMap method is called to flatten the stream of Map<String, Object> objects into a single stream of Map<String, Object>.
  8. Finally, the map method is called to apply the constructor function to each Map<String, Object> object in the stream, producing a stream of objects of type T.

The code essentially retrieves a list of maps from the Document associated with the input key, and then applies the constructor function to each map to produce a stream of child objects. The AbstractDocument class assumes that the value associated with the input key is a list of maps, and will return an empty stream if the value is not a list of maps.

What's the Stream.ofNullable(get(key)) mean？

Stream.ofNullable(get(key)) is a method call that creates a stream of a single element, which is the result of calling the get method of the current Document object with the input key.

The ofNullable method is a static factory method of the Stream class that creates a stream containing a single non-null element if the provided value is non-null, and an empty stream if the provided value is null.

So, Stream.ofNullable(get(key)) will return a stream containing a single element if the value associated with the input key is non-null, or an empty stream if the value is null.

The purpose of using Stream.ofNullable is to handle the case where the value associated with the input key in the Document object is null, and to prevent any null pointer exceptions that might occur when the filter, map, or flatMap methods are applied to the stream. By using Stream.ofNullable to wrap the value in an Optional object, the stream pipeline can handle null values safely and gracefully.

Stream.ofNullable(get(key)) equal Stream<Object>?

No, Stream.ofNullable(get(key)) does not necessarily return a Stream<Object>.

The return type of the Stream.ofNullable method is a Stream<T>, where T is the type of the provided value, or Object if the value is null. So, if the value returned by the get method is not null, the return type of Stream.ofNullable(get(key)) would be a Stream of that value's type.

For example, if the get method returns a List<String> object, then the type of the Stream returned by Stream.ofNullable(get(key)) would be Stream<List<String>>. If the get method returns an Integer object, then the type of the Stream returned by Stream.ofNullable(get(key)) would be Stream<Integer>.

However, if the value returned by the get method is null, then the type of the Stream returned by Stream.ofNullable(get(key)) would be Stream<Object>.

So, the actual return type of Stream.ofNullable(get(key)) depends on the type of the value returned by the get method, and whether it is null or not.

What's Stream<Object> mean?

Stream<Object> is a Java Stream that contains elements of type Object.

The Object class is a special class in Java that serves as the root of the class hierarchy. Every class in Java is a subclass of Object, so an Object reference can refer to any Java object.

A Stream<Object> is a generic stream that can contain any type of Java object. This means that when you create a Stream<Object>, you can add elements of any type to the stream, including primitive types (such as int and double) and reference types (such as String and List).

Using a Stream<Object> can be useful in certain situations where the exact type of the elements in the stream is not known ahead of time, or when you want to create a generic method that can operate on any type of object. However, when the type of the elements in the stream is known, it is usually better to use a more specific type of stream, such as Stream<String> for a stream of String objects or Stream<Integer> for a stream of Integer objects.

What is Java Stream?

Java Stream is a sequence of elements that can be processed in parallel or sequentially. Streams were introduced in Java 8 as a new way to handle collections, replacing the need for using traditional for-loops.

Streams provide a functional programming model that allows you to operate on the elements of a collection using a set of powerful, declarative operations. The operations include transformations, filters, and reductions, among others. These operations can be chained together to create a pipeline of operations that are executed on the elements of the stream.

One of the main advantages of using streams is that they allow for more concise and expressive code, and often result in better performance than using traditional for-loops, especially when dealing with large collections. Streams also provide better support for parallel processing, which can further improve performance on multi-core systems.

Streams can be created from a variety of sources, including collections, arrays, and even I/O channels. Once created, you can apply various operations to the stream to transform, filter, and reduce the elements, and finally collect the results into a new collection or other data structure.

Overall, Java streams provide a powerful and flexible way to process collections in a functional and declarative way, with better performance and improved support for parallel processing.

You say : The filter method is called on the Optional object to discard any null values.

I have below code:

public class GFG {

    public static void main(String[] args)
    {

        // Create ArrayList containing names
        ArrayList<String> list = new ArrayList<String>();
        list.add("Aman");
        list.add(null);
        list.add("luff");

        // create a stream with ArrayList
        Stream<ArrayList<String> > value
                = Stream.ofNullable(list)
                .filter(Objects::nonNull);

        // print values
        System.out.println("Values of Stream:");
        value.forEach(System.out::println);
    }
}

Why the filter method doesn't discard null element in value ?

In the code you provided, the filter method is called on the stream of ArrayList objects, not on the Optional object. The Stream.ofNullable method returns a stream of a single element, which is either the value wrapped in an Optional if it is not null, or an empty stream if it is null. In this case, the value is not null, so Stream.ofNullable returns a stream containing a single element, which is the ArrayList object.

Therefore, the filter method is filtering the ArrayList object, not the Optional object. Since the ArrayList object is not null, the Objects::nonNull filter is not removing any elements, and the original ArrayList object is passed down the stream.

To remove null elements from the ArrayList object, you need to apply a filter on the stream of String elements that are contained in the ArrayList. Here's an example code snippet that demonstrates this:

// Create ArrayList containing names
ArrayList<String> list = new ArrayList<String>();
list.add("Aman");
list.add(null);
list.add("luff");

// create a stream with ArrayList and filter out null values
Stream<String> value = list.stream().filter(Objects::nonNull);

// print values
System.out.println("Values of Stream:");
value.forEach(System.out::println);

In this code, the list is converted to a stream of String elements using the stream() method, and the filter method is applied to remove any null values. The resulting stream contains only the non-null String values, which are then printed to the console.

You mean The Stream.ofNullable method will return an Optional if the element is not null?

Yes, that's correct. The Stream.ofNullable method is designed to wrap an element in an Optional object, and return a stream containing either the element wrapped in the Optional, or an empty stream if the element is null.

So, if you pass a non-null element to Stream.ofNullable, it will be wrapped in an Optional and returned as a stream of a single element. If you pass null, an empty stream will be returned.

In your example code, since the list variable is not null, Stream.ofNullable(list) will wrap the list in an Optional and return a stream of a single element containing the ArrayList. The filter method is then applied to this stream, but since the element is not null, it passes the Objects::nonNull test, and the stream is not modified.

Can you rewrite children method whitout stream and lambda keywords?

Sure, here's an example implementation of the children method using loops instead of streams and lambda expressions:

@Override
public <T> Stream<T> children(String key, Function<Map<String, Object>, T> constructor) {
    List<T> children = new ArrayList<>();

    // Get the value associated with the given key
    Object value = get(key);

    // If the value is a list, iterate over the list and construct objects using the constructor function
    if (value instanceof List) {
        List<Map<String, Object>> list = (List<Map<String, Object>>) value;
        for (Map<String, Object> childMap : list) {
            children.add(constructor.apply(childMap));
        }
    }

    // Return a stream containing the constructed objects
    return children.stream();
}

In this implementation, the method first retrieves the value associated with the given key using the get method. If the value is a list, it iterates over the list using a for-each loop and applies the constructor function to each element to construct objects. The constructed objects are added to a list.

Finally, the method returns a stream of the constructed objects using the stream method on the list. This implementation is functionally equivalent to the original implementation, but uses loops instead of streams and lambda expressions.

本文中关于抽象文档模式的描述来自：Abstract Document
相关代码见：https://github.com/syuez/StudyJava/tree/AbstractDocument

标签: none

评论已关闭