Objects, Classes & OOP Design

Static Members

Static fields and methods belong to the class, not to any instance — shared state, factory methods, utilities, and constants. Use them deliberately: statics are procedural islands in an object-oriented sea.

One copy per class, shared by all instances (per class loader)
Static methods cannot touch instance state or this
Constants: static final, UPPER_SNAKE_CASE
Static initializer blocks run once at class initialization
Mutable static state is a concurrency hazard and a testing obstacle

Statics in action

public class IdGenerator {
    public static final int MAX_ID = 1_000_000;      // constant
    private static final AtomicLong next = new AtomicLong(1); // shared, thread-safe

    public static long nextId() {                     // class-level behavior
        return next.getAndIncrement();
    }
}

Typical legitimate uses: constants, pure utility functions (Math.max), and static factory methods (Constructors Initialization). The main method is static precisely because no object exists before the program starts.