In Java, the main purpose of assigning an object reference to null is to release the memory space occupied by the object and allow the garbage collector to reclaim the object. When an object is no longer referenced, if no other references point to it, it becomes unreachable and can be marked as recyclable by the garbage collector.
Assigning a value of null helps remind the garbage collector that the object can be reclaimed because it is no longer used. If it is not assigned a value of null, the garbage collector may not immediately recognize that the object can be collected, thereby delaying the occurrence of garbage collection.
Additionally, assigning null to unused object references also prevents accidental object holdings. If an object reference stays the same, it will continue to occupy memory resources even if you no longer need it. By assigning a value of null, you make it clear that the object is no longer needed and allow it to be released during garbage collection.
It should be noted that an object can only be reclaimed by the garbage collector when it is no longer pointed by any references. If the object is still referenced by other references, assigning it to null does not immediately reclaim the object. Therefore, it is important to ensure that no other code is using the object before assigning it to null.