Scope of objects
Java objects do not have the same lifetimes as primitives. When you create a Java object using new, it hangs around past the end of the scope. Thus if you use:
String s = new String("a string");
} // End of scope
the reference s vanishes at the end of the scope. However, the String object that s was pointing to is still occupying memory. In this bit of code, there is no way to access the object, because the only reference to it is out of scope. In later chapters you’ll see how the reference to the object can be passed around and duplicated during the course of a program.
It turns out that because objects created with new stay around for as long as you want them, a whole slew of C++ programming problems simply vanish in Java. The hardest problems seem to occur in C++ because you don’t get any help from the language in making sure that the objects are available when they’re needed. And more important, in C++ you must make sure that you destroy the objects when you’re done with them.
That brings up an interesting question. If Java leaves the objects lying around, what keeps them from filling up memory and halting your program? This is exactly the kind of problem that would occur in C++. This is where a bit of magic happens. Java has a garbage collector, which looks at all the objects that were created with new and figures out which ones are not being referenced anymore. Then it releases the memory for those objects, so the memory can be used for new objects. This means that you never need to worry about reclaiming memory yourself. You simply create objects, and when you no longer need them, they will go away by themselves. This eliminates a certain class of programming problem: the so-called “memory leak,” in which a programmer forgets to release memory.