The Life Cycle of a Java Object - dummies

By Doug Lowe, Barry Burd

As you work with objects in Java, understanding how objects are born, live their lives, and die is important. This topic is called the life cycle of an object, and it goes something like this:

1. Before an object can be created from a class, the class must be loaded. To do that, the Java runtime locates the class on disk (in a .class file) and reads it into memory. Then Java looks for any static initializers that initialize static fields — fields that don’t belong to any particular instance of the class, but rather belong to the class itself and are shared by all objects created from the class.

A class is loaded the first time you create an object from the class or the first time you access a static field or method of the class. For example, when you run the main method of a class, the class is initialized because the main method is static.

2. An object is created from a class when you use the new keyword. To initialize the class, Java allocates memory for the object and sets up a reference to the object so the Java runtime can keep track of it. Then, Java calls the class constructor, which is like a method but is called only once, when the object is created. The constructor is responsible for doing any processing required to initialize the object, such as initializing variables, opening files or databases, and so on.

3. The object lives its life, providing access to its public methods and fields to whoever wants and needs them.

4. When it’s time for the object to die, the object is removed from memory and Java drops its internal reference to it. You don’t have to destroy objects yourself. A special part of the Java runtime called the garbage collector takes care of destroying all objects when they are no longer in use.