Set field value with reflection

Question

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<>()); 

Answer 1

Hope this is something 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 

Answer 2

It's worth reading Oracle Java Tutorial - Getting and Setting Field Values

Field#set(Object object, Object value) sets the field represented by this Field object on the specified object argument to the specified new value.

It should be like this

f.set(objectOfTheClass, new ConcurrentHashMap<>()); 

You can't set any value in null Object If tried then it will result in NullPointerException

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Note: Setting a field's value via reflection has a certain amount of performance overhead because various operations must occur such as validating access permissions. From the runtime's point of view, the effects are the same, and the operation is as atomic as if the value was changed in the class code directly.