what's more efficient? to empty an object or create a new one?

Question

how expensive is 'new'? I mean, should I aim at reusing the same object or if the object is 'out of scope' it's the same as emptying it?

example, say a method creates a list:

List<Integer> list = new ArrayList<Integer>(); 

at the end of the method the list is no longer in use - does it mean that there's no memory allocated to it anymore or does it mean that there's a null pointer to it (since it was 'created').

Alternately, I can send a 'list' to the method and empty it at the end of the method with: list.removeAll(list); will that make any difference from memory point of view?

Thanks!

Answer 1

its an array list, so creating a new object means allocating a slab of memory and zeroing it, plus any bookkeeping overhead. Clearing the list means zeroing the memory. This view would lead you to believe that clearing an existing object is faster. But, it's likely that the JVM is optimized to make memory allocations fast, so probably none of this matters. So just write clear, readable code, and don't worry about it. This is java after all, not c.

Answer 2

at the end of the method the list is no longer in use - does it mean that there's no memory allocated to it anymore or does it mean that there's a null pointer to it (since it was 'created').

Means there are no references to it and object is eligible for GC.

Alternately, I can send a 'list' to the method and empty it at the end of the method with: list.removeAll(list); will that make any difference from memory point of view?

It's tradeoff between time/space. Removing elements from list is time consuming, even though you don't need to create new objects.

With the latest JVMs GC collection capabilities, it is ok to create new object WHEN REQUIRED (but avoiding object creation in loop is best). Longer references to an object sometimes make that object NOT eligible for GC and may cause memory leak if not handled properly.