Java 8:如何编写lambda流以使用JsonArray? [英] Java 8: How to write lambda stream to work with JsonArray?
问题描述
我对Java 8 lambdas和其他东西都很陌生......我想编写一个带有JsonArray的lambda函数,遍历它的JsonObjects并创建某个字段的值列表。
例如,一个接受JsonArray的函数:[{name:John},{name:David}]并返回[John列表,大卫]。
我写了以下代码:
import org.json.simple.JSONArray;
import org.json.simple.JSONObject;
公共类Main {
public static void main(String [] args){
JSONArray jsonArray = new JSONArray();
jsonArray.add(new JSONObject()。put(name,John));
jsonArray.add(new JSONObject()。put(name,David));
列表list =(List)jsonArray.stream()。map(json - > json.toString())。collect(Collectors.toList());
System.out.println(list);
}
}
但是,我收到错误:
线程main中的异常java.lang.NullPointerException
<你知道怎么解决吗?
JSONArray 是 java.util.ArrayList
和 JSONObject 是<$ c $的子类c> java.util.HashMap 。
因此, new JSONObject()。put(name, John)
返回与键关联的前一个值( null
),而不是 JSONObject
实例。因此, null
被添加到 JSONArray
。
<另一方面,这可行:
JSONArray jsonArray = new JSONArray();
JSONObject j1 = new JSONObject();
j1.put(name,John);
JSONObject j2 = new JSONObject();
j2.put(name,David);
jsonArray.add(j1);
jsonArray.add(j2);
Stream< String> ss = jsonArray.stream()。map(json-> json.toString());
List< String> list = ss.collect(Collectors.toList());
System.out.println(list);
由于某种原因,我不得不将流管道分成两个步骤,因为否则编译器不会认识到 .collect(Collectors.toList())
返回列表
。
输出为:
[{name:John},{name: 大卫}]
I'm very new to Java 8 lambdas and stuff... I want to write a lambda function that takes a JsonArray, goes over its JsonObjects and creates a list of values of certain field.
For example, a function that takes the JsonArray: [{name: "John"}, {name: "David"}] and returns a list of ["John", "David"].
I wrote the following code:
import org.json.simple.JSONArray;
import org.json.simple.JSONObject;
public class Main {
public static void main(String[] args) {
JSONArray jsonArray = new JSONArray();
jsonArray.add(new JSONObject().put("name", "John"));
jsonArray.add(new JSONObject().put("name", "David"));
List list = (List) jsonArray.stream().map(json -> json.toString()).collect(Collectors.toList());
System.out.println(list);
}
}
However, I get an error:
Exception in thread "main" java.lang.NullPointerException
DO you know how to resolve it?
JSONArray is a sub-class of java.util.ArrayList
and JSONObject is a sub-class of java.util.HashMap
.
Therefore, new JSONObject().put("name", "John")
returns the previous value associated with the key (null
), not the JSONObject
instance. As a result, null
is added to the JSONArray
.
This, on the other hand, works:
JSONArray jsonArray = new JSONArray();
JSONObject j1 = new JSONObject();
j1.put ("name", "John");
JSONObject j2 = new JSONObject();
j2.put ("name", "David");
jsonArray.add(j1);
jsonArray.add(j2);
Stream<String> ss = jsonArray.stream().map (json->json.toString ());
List<String> list = ss.collect (Collectors.toList ());
System.out.println(list);
For some reason I had to split the stream pipeline into two steps, because otherwise the compiler doesn't recognize that .collect (Collectors.toList())
returns a List
.
The output is:
[{"name":"John"}, {"name":"David"}]
这篇关于Java 8:如何编写lambda流以使用JsonArray?的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!