Although Java is object-oriented to a great extent, it is not a pure object-oriented language. One of the reasons Java is not purely object-oriented is that not everything in it is an object. For example, Java allows you to declare variables of primitive types (int, float, boolean, etc.) that aren't objects. And Java has static fields and methods, which are independent and separate from objects. This article gives advice on how to use static fields and methods in a Java program, while maintaining an object-oriented focus in your designs.
The lifetime of a class in a Java virtual machine (JVM) has many similarities to the lifetime of an object. Just as an object can have state, represented by the values of its instance variables, a class can have state, represented by the values of its class variables. Just as the JVM sets instance variables to default initial values before executing initialization code, the JVM sets class variables to default initial values before executing initialization code. And like objects, classes can be garbage collected if they are no longer referenced by the running application.
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