Instance variable
An instance variable is data associated with a particular object, rather than shared across all instances of its class or type.
The Well-Grounded Rubyist, Fourth Edition
An instance variable is a Ruby variable identified by a single-at-sign prefix and associated with an individual object.
- How it is used
- Stores per-object state so methods can retrieve or change values after initialization.
- How deep it goes
- Explains assignment, persistence across calls, ownership through `self`, and inspection with `instance_variables`.
- Examples
- Uses tickets with distinct venues and dates, a person's persistent name, and runtime inspection of accessor-backed variables.
- What you can do after
- Build it from scratch: initialize per-object state, expose or change it through methods, and identify its owner using `self`.
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Software Design for Python Programmers
An instance variable is state associated with an individual object instance.
- How it is used
- Models per-object state, class-owned relationships, and runtime state transitions in object-oriented application design.
- How deep it goes
- Explains state, visibility, dependencies, multiplicity, and defensive copying, but stops short of general language-level attribute mechanics.
- Examples
- Uses catalogue objects, Employee birthdates, and an automatic ticket machine to demonstrate storage, protection, and state changes.
- What you can do after
- Evaluate/understand how object state, ownership, visibility, and mutation paths affect class design.
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100 Java Mistakes and How to Avoid Them
- How it is used
- Prevents object-specific data from becoming accidentally shared through a static field.
- How deep it goes
- Explains per-object versus per-class storage, failure under concurrency, mutable static state, and applicable inspections.
- Examples
- Uses a Person name field and a concurrent TreeWalker queue to expose shared-state leakage.
- What you can do after
- Diagnose accidental static state by comparing field ownership with object usage and applying IDE or SonarLint inspections.
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Secrets of the JavaScript Ninja, Second Edition
A property that belongs directly to a particular object instance rather than being inherited from its prototype.
- How it is used
- Contrasts properties stored on an object instance with properties supplied through its prototype.
- How deep it goes
- Explains constructor assignment, property precedence, and per-instance copies, then flags repeated methods as a concern for many objects.
- Examples
- Uses Ninja instances with `swung` and competing `swingSword` methods to demonstrate property ownership and precedence.
- What you can do after
- Understand which same-named property is found and why constructor-created methods are duplicated across instances.
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Related concepts
- static fieldcontrasts with · A named variable whose storage is associated with a class or has static duration, rather than being allocated separately for each object instance or function call.
- scopecontrasts with · Scope is the region of a program in which names are visible and can be resolved according to the language’s lookup rules.
- stateused by · State is information or a configuration describing the condition of a system, entity, or computation at a particular time and potentially influencing subsequent behavior or decisions.
- const referenceconfigured by · A const reference is a C++ reference through which the referred-to object cannot be modified.
- accessor methodused by · An accessor method or function provides access to an object's property or state by retrieving its value, assigning a value, or defining behavior for such access.
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