The object is reachable if it can be involved in any potential continuing computation. So if your code refers to a local variable, and nothing else refers to it, then you might cause the object to be collected by setting it to null . This would either give a null pointer exception, or change the behaviour of your program, or if it does neither you didn't need the variable in the first place. Tricks such as explicit nulling or object pooling , which were once considered sensible techniques for improving performance, are no longer necessary or helpful (and may even be harmful) as the cost of allocation and garbage collection has been reduced considerably. More on .....Garbage Collection
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