Master Fastjson: Create, Modify, and Query JSON Objects in Java

Create

Instantiate an empty JSON object:

JSONObject fun = new JSONObject();

Add

Insert a key‑value pair. The key must be a java.lang.String; the value can be any java.lang.Object (including wrapper types or collections):

fun.put("key", "value");

Delete

Remove an entry by its key: fun.remove("key"); Pitfall: Do not modify the JSONObject while iterating over it, otherwise a ConcurrentModificationException may be thrown.

Get

Retrieve values using the family of getter methods:

Object obj = fun.get("key");
String s = fun.getString("key");
Integer iObj = fun.getInteger("key"); // may return null
int iVal = fun.getIntValue("key");      // returns 0 if the key is absent or the value is not a number

All getters delegate to com.alibaba.fastjson.JSONObject#get. getInteger returns an Integer wrapper (null‑safe), while getIntValue returns a primitive int with a default of 0. Choose the method that matches the desired null‑handling semantics.

Common Operations

fun.size();               // number of entries
fun.isEmpty();            // true if no entries
fun.entrySet();           // Set<Map.Entry<String,Object>> – preferred for iteration
fun.keySet();             // Set<String>
fun.clear();              // remove all entries
fun.containsKey("key"); // check key existence
fun.containsValue("value"); // check value existence

Pitfall: When iterating, prefer entrySet() over keySet() to avoid concurrent‑modification issues.

Advanced Usage

// Conditional update based on current value
fun.compute("key", (k, v) -> {
    return (v == null) ? "no exists" : "exists";
});

// Create a new list if the key is absent
fun.computeIfAbsent("list", k -> new ArrayList<String>());

// Modify the value only when the key is present
fun.computeIfPresent("key", (k, v) -> {
    return (v == null) ? "yes" : "no";
});

Fastjson also allows custom iteration patterns via lambda expressions, enabling concise transformations of JSONObject contents.