how to change private variables in java

getSetPrivateFieldForTesting.java

 // Using java.lang.reflect  
Field field = instanceOfAClass.getClass().getDeclaredField("nameOfThePrivateVariable");
field.setAccessible(true);
Map<UUID, Long> nameOfThePrivateVariable = (Map<UUID, Long>)field.get(instanceOfAClass);
nameOfThePrivateVariable.put(aNewKey, aNewValue);

// Using org.springframework.test.util.ReflectionTestUtils
// There may be typos in the below! 
Cache<UUID, UUID> aCache = (Cache<UUID, UUID>)ReflectionTestUtils.getField(instanceOfObjectWithCacheAsPrivateField, "aCache");
aCache.invalidateAll();