Ensuring proper cleanup
Each object requires resources, most notably memory, in order to exist. When an object is no longer needed it must be cleaned up so that these resources are released for reuse. In simple programming situations the question of how an object is cleaned up doesnt seem too challenging: you create the object, use it for as long as its needed, and then it should be destroyed. However, its not hard to encounter situations that are more complex.
Suppose, for example, you are designing a system to manage air traffic for an airport. (The same model might also work for managing crates in a warehouse, or a video rental system, or a kennel for boarding pets.) At first it seems simple: Make a container to hold airplanes, then create a new airplane and place it in the container for each airplane that enters the air-traffic-control zone. For cleanup, simply delete the appropriate airplane object when a plane leaves the zone.
But perhaps you have some other system to record data about the planes; perhaps data that doesnt require such immediate attention as the main controller function. Maybe its a record of the flight plans of all the small planes that leave the airport. So you have a second container of small planes, and whenever you create a plane object you also put it in this second container if its a small plane. Then some background process performs operations on the objects in this container during idle moments.
Now the problem is more difficult: How can you possibly know when to destroy the objects? When youre done with the object, some other part of the system might not be. This same problem can arise in a number of other situations, and in programming systems (such as C++) in which you must explicitly delete an object when youre done with it this can become quite complex.
With Java, the garbage collector is designed to take care of the problem of releasing the memory (although this doesnt include other aspects of cleaning up an object). The garbage collector knows when an object is no longer in use, and it then automatically releases the memory for that object. This (combined with the fact that all objects are inherited from the single root class Object and that you can create objects only one wayon the heap) makes the process of programming in Java much simpler than programming in C++. You have far fewer decisions to make and hurdles to overcome.
Garbage collectors vs. efficiency and flexibility
If all this is such a good idea, why didnt they do the same thing in C++? Well of course theres a price you pay for all this programming convenience, and that price is run time overhead. As mentioned before, in C++ you can create objects on the stack, and in this case theyre automatically cleaned up (but you dont have the flexibility of creating as many as you want at run time). Creating objects on the stack is the most efficient way to allocate storage for objects and to free that storage. Creating objects on the heap can be much more expensive. Always inheriting from a base class and making all method calls polymorphic also exacts a small toll. But the garbage collector is a particular problem because you never quite know when its going to start up or how long it will take. This means that theres an inconsistency in the rate of execution of a Java program, so you cant use it in certain situations, such as when the rate of execution of a program is uniformly critical. (These are generally called real time programs, although not all real time programming problems are this stringent.)
The designers of the C++ language, trying to woo C programmers (and most successfully, at that), did not want to add any features to the language that would impact the speed or the use of C++ in any situation where programmers might otherwise choose C. This goal was realized, but at the price of greater complexity when programming in C++. Java is simpler than C++, but the trade-off is in efficiency and sometimes applicability. For a significant portion of programming problems, however, Java is the superior choice.