Java parse all values from Array in JSON to ArrayList

Question

I have a problem. I have the following JSON:

{
   "Market":"USDT",
   "Coin":"BTC",
   "Period":"1h",
   "EmergencyPerc":-25,
   "TakeProfitPerc":1.2,
   "ProtectiveOrdersEnabled":"no",
   "EMACrossMarginPerc":0.5,
   "EMABuySellPeriod":"15m",
   "EMABuySellNameLow":"EMA10",
   "EMABuySellNameHigh":"EMA50",
   "EMAUnfreezePeriod":"1h",
   "EMAUnfreezeNameLow":"EMA20",
   "EMAUnfreezeNameHigh":"EMA200",
   "SimTemplate":"t001",
   "Patterns":{
      "Buy":[
         "buy_pattern_1",
         "buy_pattern_2"
      ],
      "Sell":[
         "sell_pattern_1",
         "sell_pattern_2"
      ]
   }
}

I want to parse that JSON to the following class:

public class AgentStrategy {
    
    private String Market;
    private String Coin;
    private double EmergencyPerc;
    private double TakeProfitPerc;
    private String ProtectiveOrdersEnabled;
    public double EMACrossMarginPerc;
    public String EMABuySellPeriod;
    public String EMABuySellNameLow;
    public String EMABuySellNameHigh;
    public String EMAUnfreezePeriod;
    public String EMAUnfreezeNameLow;
    public String EMAUnfreezeNameHigh;
    private String SimTemplate;
    private ArrayList<PriceDropSell> PriceDropSells;
    private ArrayList<String> buyPatternsUsed = new ArrayList<>();
    private ArrayList<String> sellPatternsUsed = new ArrayList<>();
    
}

For that I have the following method inside that class:

public AgentStrategy parseJsonToObject(String jsonString) {
    Gson gson = new Gson();
    AgentStrategy agent = gson.fromJson(jsonString, AgentStrategy.class);
    
    // PATTERNS
    Map patternsMap = (Map) map.get("Patterns");
    Map sellMap = (Map) patternsMap.get("Sell");
    Map buyMap = (Map) patternsMap.get("Buy");
    agent.buyPatternsUsed = new ArrayList<>(sellMap.values());
    agent.buyPatternsUsed = new ArrayList<>(buyMap.values());
    
    return agent;
}

But I get an error on this line at the pattern parse part:

agent.buyPatternsUsed = new ArrayList<>(sellMap.values());

With: Exception in thread "main" java.lang.ClassCastException: class java.util.ArrayList cannot be cast to class java.util.Map (java.util.ArrayList and java.util.Map are in module java.base of loader 'bootstrap')

How can I parse all the patterns from buy and sell to a String Array (seperate)?

You can use @SerializedName to manage the difference in Java fields and json fields. This will help with java variables naming convention. — Thiyanesh
– Thiyanesh, Commented Feb 8, 2021 at 0:32

Dharman · Accepted Answer · 2021-02-08 00:21:49Z

How can I parse all the patterns from buy and sell to a String Array (seperate)?

The JSON you posted has two arrays and you're trying to parse them as maps. Parse them as arrays (lists).

List sellList = (List) patternsMap.get("Sell");
List buyList = (List) patternsMap.get("Buy");

That said, the true power of GSON (and JSON parsers in general) is that do the heavy work for you if you model your classes correctly. You could have a "Patterns" class that has two lists of strings, then the strategy class would have a "Pattern". For example:

class Patterns {
    List<String> Buy;
    List<String> Sell;
}

class Agent Strategy {
    Patterns Patterns;
}

Now, when you parse the object, because the class structure matches the json structure, the lists are parsed automatically for you:

AgentStrategy agent = gson.fromJson(jsonString, AgentStrategy.class);
// agent.Patterns.Buy and .Sell now have the lists

Then you could just define helpers to get the data you care about:

class Agent Strategy {
    Patterns Patterns;

    public getBuyPatternsUsed() { return Patterns.Buy; }
    public getSellPatternsUsed() { return Patterns.Sell; }
}

P.S. - Tip: when you see "class cast exception" use your debugger to step through up to line where the crash happens and see what type you're actually getting compared to what you expect. That will help you debug your issue.

Thiyanesh · Accepted Answer · 2021-02-08 00:30:49Z

      "Buy":[
         "buy_pattern_1",
         "buy_pattern_2"
      ],
      "Sell":[
         "sell_pattern_1",
         "sell_pattern_2"
      ]

The Buy and Sell are arrays.

So the following cast will throw exception

    Map sellMap = (Map) patternsMap.get("Sell");
    Map buyMap = (Map) patternsMap.get("Buy");

Suggestion

Try to use annotations to map the java attribute name and json field name
i have only named few fields (using @SerializedName annotation)

    static class PriceDropSell {

        @SerializedName("Buy")
        List<String> buy;

        @SerializedName("Sell")
        List<String> sell;
    }

    static class AgentStrategy {

        private String Market;
        private String Coin;
        private double EmergencyPerc;
        private double TakeProfitPerc;
        private String ProtectiveOrdersEnabled;
        public double EMACrossMarginPerc;
        public String EMABuySellPeriod;
        public String EMABuySellNameLow;
        public String EMABuySellNameHigh;
        public String EMAUnfreezePeriod;
        public String EMAUnfreezeNameLow;
        public String EMAUnfreezeNameHigh;
        private String SimTemplate;

        @SerializedName(("Patterns"))
        private PriceDropSell Patterns;
    }

    public static void main(String[] args) {
        Gson gson = new Gson();
        AgentStrategy agentStrategy = gson.fromJson(str, AgentStrategy.class);
        System.out.println(gson.toJson(agentStrategy));
    }

Collectives™ on Stack Overflow

Java parse all values from Array in JSON to ArrayList

2 Answers 2

Comments

Suggestion

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Suggestion

Comments

Your Answer

Sign up or log in

Post as a guest

Related