Question

I am confused with the memory allocation of references in Java when you pass them to the methods in Java. As a Test I did an example as below,

class Store{
public int[] buffer = new int[5];
 }

class ConsumeStore{
Store store;
public ConsumeStore(Store store){
    this.store = store; 
}

public int useStore(){
    return store.buffer[0];
}
  }

class ProduceStore{
Store store;
public ProduceStore(Store store){
    this.store = store;
}

public void putStore(){
    store.buffer[0] = 100;
}
 }

public class CheckRef {
  public static void main(String args[]){
  Store store = new Store();
  ConsumeStore consStore = new ConsumeStore(store);
  ProduceStore prodStore = new ProduceStore(store);
  prodStore.putStore();
  System.out.println(consStore.useStore());   
   }
 }

Well the output is 100.

Here I am passing the Store reference to ProducerStore. Inside ProducerStore I am assigning it to the class member of it. I am doing the same for ConsumerStore as well.

Could someone explain me how the memory allocation is done for Store referece and how it shared among the ProducerStore and ConsumerStore?

Answer 1

Both your ProducerStore and ConsumerStore have a reference to the same object store, that you passed to each one.

So when you modify store internal state (its buffer member), either externally or in any of the Producer/Consumer, all the classes that have reference to it are “updated” because they don’t hold a distinct value, they only hold a reference to the unique value store.

Something different would happen if you reallocated the object entirely. For example:

public void putStore(){
    store = new Store()
    store.buffer[0] = 100
}

In this case, the Consumer and the Producer would have a different store: Consumer would still have a reference to the original store, while Producer would have the new one defined within the Put method.

You can think of it as: everytime you do a new, a new object is created in memory. Then when you pass it around to classes, everyone only has reference to that unique object.