computer science

Instance variable

Instance variable is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Instance variable rather than just read about it. In short: In class-based, object-oriented programming, an instance variable is a variable defined in a class (i.e., a member variable), for which each instantiated object of the class has a separate copy, or instance. An instance variable has similarities with a class variable, but is non-static.

Key takeaways

Instance variable belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Instance variable to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Instance variable from memory before moving on to harder problems.

Reference excerpt

In class-based, object-oriented programming, an instance variable is a variable defined in a class (i.e., a member variable), for which each instantiated object of the class has a separate copy, or instance. An instance variable has similarities with a class variable, but is non-static. In C++ or Java language, an instance variable is a variable which is declared in a class but outside of constructors, methods, or blocks. Instance variables are created when an object is instantiated, and are accessible to all the constructors, methods, or blocks in the class. Access modifiers can be given to the instance variable. An instance variable is not a class variable, although there are similarities. Both are a type of class attribute (or class property, field, or data member). While an instance variable's value may differ between instances of a class, a class variable can only have one value at any one time, shared between all instances. The same dichotomy between instance and class members applies to methods ("member functions") as well. Each instance variable lives in memory for the lifetime of the object it is owned by. Instance variables are properties of that object. All instances of a class have their own copies of instance variables, even if the value is the same from one object to another. One class instance can change values of its instance variables without affecting all other instances. A class may have both instance variables and class variables. Instance variables can be used by all instance methods of an object, but may not be used by class methods. An instance variable may also be changed directly, provided access restrictions are set.

Examples

C++

In this C++ example, the instance variable Request::number is a copy of the class variable Request::count1 where each instance constructed is assigned a sequential value of count1 before it is incremented. Since number is an instance variable, each Request object contains its own distinct value; in contrast, there is only one object Request::count1 available to all class instances with the same value.

Java

In this Java example, we can see how instance variables can be modified in one instance without affecting another.

Python In the above Python code, the instance variable is created when an argument is parsed into the instance, with the specification of the breed positional argument.

References

Worked examples

Example 1 — a first encounter with Instance variable

Start with the simplest possible case. Write down what Instance variable claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Instance variable before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Instance variable ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Instance variable

In research: Instance variable appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Instance variable in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Instance variable is common in secondary-school and first-year university syllabi. It links to neighbouring topics Object-oriented programming, Variable (computer science), so understanding it makes those chapters shorter.
In everyday life: Look for Instance variable outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Instance variable in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Instance variable means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Instance variable out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Instance variable in simple terms?

Why does Instance variable matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Instance variable?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Instance variable.