Set field value with reflection

I'm working with one project which is not opensource and I need to modify one or more its classes.

In one class is following collection:

private Map<Integer, TTP> ttp = new HashMap<>(); 

All what I need to do is use reflection and use concurrenthashmap here. I've tried following code but it doesnt work.

Field f = ..getClass().getDeclaredField("ttp");
f.setAccessible(true);
f.set(null, new ConcurrentHashMap<>());

Hopefully this is what you are trying to do:

import java.lang.reflect.Field;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

public class Test {
    
    private Map ttp = new HashMap(); 
    
    public  void test() {
        Field declaredField =  null;
        try {
            
            declaredField = Test.class.getDeclaredField("ttp");
            boolean accessible = declaredField.isAccessible();

            declaredField.setAccessible(true);
            
            ConcurrentHashMap<Object, Object> concHashMap = new ConcurrentHashMap<Object, Object>();
            concHashMap.put("key1", "value1");
            declaredField.set(this, concHashMap);
            Object value = ttp.get("key1");

            System.out.println(value);

            declaredField.setAccessible(accessible);
            
        } catch (NoSuchFieldException 
                | SecurityException
                | IllegalArgumentException 
                | IllegalAccessException e) {
            e.printStackTrace();
        }
        
    }

    public static void main(String... args) {
        Test test = new Test();
        test.test(); 
    }
}

It prints :

value1