Java With Us

What Is an Instance in Java?

Updated on 2026-04-21 Reviewed by the Java With Us editorial team

"Instance" is a synonym for object: a concrete realisation of a class in memory. "Create an instance of ArrayList" means call new ArrayList<>() and get back a real, working list object. The term is especially common when distinguishing instance (per-object) members from static (per-class) members.

Instance vs class-level (static)

public class Lamp {
    // Instance field — each Lamp object has its own:
    private boolean on = false;

    // Static field — ONE copy shared by ALL Lamp objects:
    private static int totalLamps = 0;

    public Lamp() { totalLamps++; }

    // Instance method — operates on THIS lamp:
    public void toggle() { on = !on; }

    // Static method — operates on the class:
    public static int getTotalLamps() { return totalLamps; }
}

Lamp l1 = new Lamp();
Lamp l2 = new Lamp();
l1.toggle();

System.out.println(l1.on);              // true
System.out.println(l2.on);              // false (independent)
System.out.println(Lamp.getTotalLamps()); // 2 (shared)

instanceof operator

Test whether an object is an instance of a specific class at runtime:

Object obj = "Hello";
if (obj instanceof String s) {     // pattern matching (Java 16+)
    System.out.println(s.length()); // 5
}

"Instance variable" and "instance method"

  • Instance variable (aka instance field) — a field declared without static. Each object gets its own copy.
  • Instance method — a method declared without static. It can access this and the object's instance fields.

Lifecycle of an instance

  1. Memory allocated on the heap when new is called.
  2. Fields initialised to defaults (0, false, null).
  3. Constructor runs, setting fields to their intended starting values.
  4. The object lives until no references point to it.
  5. Garbage collector reclaims the memory.

Common phrasing

  • "Create an instance" = call new ClassName()
  • "An instance of Map" = any object whose type implements the Map interface (HashMap, TreeMap, etc.)
  • "Instance fields" = non-static fields, unique per object
  • "Singleton" = a class designed to have exactly one instance

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