Question 210931 in Json Question posted in
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How to read several Json arrays from input and convert them to Java array or List

I’m receiving an input of one or more Json arrays like

[{"operation":"buy", "unit-cost":10.00, "quantity": 10000},
{"operation":"sell", "unit-cost":20.00, "quantity": 5000}]
[{"operation":"buy", "unit-cost":20.00, "quantity": 10000},
{"operation":"sell", "unit-cost":10.00, "quantity": 5000}]

I tried to read the array using JSON-Java in this way:

JSONArray array = new JSONArray(json)

But this only reads the first array and ignores the second one.

How can I read multiple arrays from a Json String in Java?

I already tried with JSON-java and Jackson but I haven’t found the way to get the expected result.

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2

Answers


  1. 0

    "I’m receiving an input of one or more Json arrays …
    … How can I read multiple arrays from a Json String in Java? …"

    You could evaluate the characters to determine when a complete array has been reached.

    In this example, I’m ignoring the fact that a the brackets may occur within a text value.
    This doesn’t appear to be the case with your data, so I’ve left out the check.

    String string =
    "[{"operation":"buy", "unit-cost":10.00, "quantity": 10000},n" +
    "{"operation":"sell", "unit-cost":20.00, "quantity": 5000}]n" +
    "[{"operation":"buy", "unit-cost":20.00, "quantity": 10000},n" +
    "{"operation":"sell", "unit-cost":10.00, "quantity": 5000}]";
List<String> list = new ArrayList<>();
int index = 0, offset = 0, count = 0;
for (char character : string.toCharArray()) {
    switch (character) {
        case '[' -> count++;
        case ']' -> {
            count--;
            if (count == 0) {
                list.add(string.substring(offset, index + 1).strip());
                offset = index + 1;
            }
        }
    }
    index++;
}

    Output, for list.

    0 = '[{"operation":"buy", "unit-cost":10.00, "quantity": 10000},
{"operation":"sell", "unit-cost":20.00, "quantity": 5000}]'
1 = '[{"operation":"buy", "unit-cost":20.00, "quantity": 10000},
{"operation":"sell", "unit-cost":10.00, "quantity": 5000}]'

    I then used Gson to parse the data.
    I’m using the SerializedName annotation here to conform the hyphenated name.
    Additionally, I added a toString override to debug the output.

    class Data {
    String operation;
    @SerializedName("unit-cost") float unitCost;
    float quantity;

    @Override
    public String toString() {
        return "{operation= '%s', ".formatted(operation)
            + "unitCost= %s, ".formatted(unitCost)
            + "quantity= %s}".formatted(quantity);
    }
}

    Here is a basic parse and output.

    Gson gson = new Gson();
List<Data[]> data = new ArrayList<>();
for (String value : list)
    data.add(gson.fromJson(value, Data[].class));

for (Data[] datum : data)
    System.out.println(Arrays.toString(datum));

    [{operation= 'buy', unitCost= 10.0, quantity= 10000.0}, {operation= 'sell', unitCost= 20.0, quantity= 5000.0}]
[{operation= 'buy', unitCost= 20.0, quantity= 10000.0}, {operation= 'sell', unitCost= 10.0, quantity= 5000.0}]
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  2. 0

    There is no automatic way to read it since the object is an appended array of objects. You’ll have to parse it yourself programmatically. Here is a possible way:

    import java.util.ArrayList;
import org.json.*;

public class JsonTest {

    public static final void main(String... args){
        String str = "[{"operation":"buy", "unit-cost":10.00, "quantity": 10000},n" +
                        "{"operation":"sell", "unit-cost":20.00, "quantity": 5000}]n" +
                        "[{"operation":"buy", "unit-cost":20.00, "quantity": 10000},n" +
                        "{"operation":"sell", "unit-cost":10.00, "quantity": 5000}]";

        // store all the arrays in a list
        ArrayList<JSONArray> jsonArrays = new ArrayList<>();
        
        int last, beg = last = 0;
        while(true){
            // substring up to ] and treat it as a json array
            last = str.indexOf("]", beg);
            if(last == -1) break;
            JSONArray arr = new JSONArray(str.substring(beg, last+1));
            jsonArrays.add(arr);
            beg = last+1;
        }
        
        // print all the json objects in the array list
        for(JSONArray arr : jsonArrays){
            for(int i = 0; i < arr.length(); i++){
                JSONObject obj = arr.getJSONObject(i);
                System.out.println(obj.toString());
            }
        }

    }
}
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