What are the essential building blocks of a Singleton implementation in Java, and why does each one matter?

A Singleton needs three pieces: a private static field to hold the single instance, a private constructor so no code outside the class can call new on it, and a public static method that lazily creates the instance on the first call and returns that same instance on every call after. The private constructor is what actually enforces exactly one instance — without it, any code anywhere could construct additional objects regardless of what the static method does. The static field and method have to be static specifically because there may be no instance yet when a caller first asks for one, so the method has to be callable on the class itself rather than on an object. Get any one of the three wrong — a public constructor, an instance method as the accessor, or forgetting to check for an existing instance — and the exactly-one guarantee silently breaks.