如何从 JsonNode 中提取数组？

Question

I have the following input:

I want to extract lat and long. I tried the following implementation, but I received null pointer exception for positionNode.get(i + 1).asDouble()

private List<CoordinateBE> getCoordinate(final JsonNode positionNode) {
        
        final List<CoordinateBE> listOfEntrances = new ArrayList<>();
        for (int i = 0; i < positionNode.size(); i = i + 2) {
            final CoordinateBE coordinateBE = new CoordinateBE();
            coordinateBE.setLatitude(positionNode.get(i).asDouble());
            coordinateBE.setLongitude(positionNode.get(i + 1).asDouble());  <--- Null Pointer Exception !!
            listOfEntrances.add(coordinateBE);
        }
        return listOfEntrances;
    }

How can I fix the above implementation?

Answer 1

If you are using com.fasterxml.jackson.databind.JsonNode, you can get the expected field by name, instead of using the position

• positionNode.get("lat").asDouble() for the lat
• positionNode.get("lng").asDouble() for the lng

Here an example in Java

    @Test
    public void jsonNodeTest() throws Exception{
        JsonNode positionNode =  new ObjectMapper().readTree("{\"lat\":35.85, \"lng\":139.85}");
        System.out.println("Read simple object " + positionNode.get("lat").asDouble());
        System.out.println("Read simple object " +positionNode.get("lng").asDouble());

        ArrayNode positionNodeArray = (ArrayNode) new ObjectMapper().readTree("[" +
                "{\"lat\":35.85, \"lng\":139.85} , " +
                "{\"lat\":36.85, \"lng\":140.85}" +
                "]");

        // With Stream API
        positionNodeArray.elements().forEachRemaining(jsonNode -> {
            System.out.println("Read in array " + jsonNode.get("lat").asDouble());
            System.out.println("Read in array " +jsonNode.get("lng").asDouble());
        });
        
        // Without Stream API
        Iterator<JsonNode> iter = positionNodeArray.elements();
        while(iter.hasNext()) {
            JsonNode positionNodeInArray = iter.next();
            System.out.println("Read in array with iterator " + positionNodeInArray.get("lat").asDouble());
            System.out.println("Read in array with iterator " +positionNodeInArray.get("lng").asDouble());
        }
    }

Answer 2

Your input "[{"lat":35.65, "lng":139.61}]" is an array of one element. The loop you're using goes through every other element, because of i = i + 2

The code inside your setLatitude gets the element at position 0 in your array, which is {"lat":35.65, "lng":139.61} , and converts it into a Double.

The code inside your setLongitude tries to retrieve the element at position 1, which is null. The method asDouble on a null object causes the NullPointerException.

Here's how you can fix it:

    private List<CoordinateBE> getCoordinate(final JsonNode positionNodes) {
        
        final List<CoordinateBE> listOfEntrances = new ArrayList<>();
        for (JsonNode positionNode : positionNodes) {
            final CoordinateBE coordinateBE = new CoordinateBE();
            coordinateBE.setLatitude(positionNode.get("lat").asDouble());
            coordinateBE.setLongitude(positionNode.get("lng").asDouble());
            listOfEntrances.add(coordinateBE);
        }
        return listOfEntrances;
    }

Notice that the for loop iterates through every object in positionNodes, and lat and lng are extracted using their name rather than position.