Java Instance Variables vs Local Variables

Question

I\'m in my first programming class in high school. We\'re doing our end of the first semester project. This project only involves one class, but many methods. My question is

Answer 1

Use instance variables when it's a core concept of your class. If you're iterating, recursing or doing some processing, then use local variables.

When you need to use two (or more) variables in the same places, it's time to create a new class with those attributes (and appropriate means to set them). This will make your code cleaner and help you think about problems (each class is a new term in your vocabulary).

One variable may be made a class when it is a core concept. For example real-world identifiers: these could be represented as Strings, but often, if you encapsulate them into their own object they suddenly start "attracting" functionality (validation, association to other objects etc)

Also (not entirely related) is object consistency - an object is able to ensure that it's state makes sense. Setting one property may alter another. It also makes it far easier to alter your program to be thread-safe later (if required).

Answer 2

Try not to return more than one value from your methods in first place. If you can't, and in some cases you really can't, then I would recommend encapsulating that in a class. Just in last case I would recommend changing another variable inside your class (an instance variable). The problem with the instance variables approach is that it increases side effects - for example, you call method A in your program and it modifies some instance(s) variable(s). Over time, that leads to increased complexity in your code and maintenance becomes harder and harder.

When I have to use instance variables, I try to make then final and initialize then in the class constructors, so side effects are minimized. This programming style (minimizing the state changes in your application) should lead to better code that is easier to maintain.

Answer 3

Use instance variable when

1. If 2 functions in the class need the same value then make it instance variable OR
2. If the state is not expected to change make it instance variable. eg: immutable object, DTO, LinkedList, those with final variables OR
3. If it is an underlying data on whom actions are performed. eg: final in arr[] in PriorityQueue.java source code OR
4. Even if it is used only once && state is expected to change, make it instance if it is used only once by a function whose parameter list should be empty. eg: HTTPCookie.java Line: 860 hashcode() function uses 'path variable'.

Similarly Use local variable when none of these conditions match, specifically role of the variable would end after stack is popped off. eg: Comparator.compare(o1, o2);

Answer 4

Try to think about your problem in terms of objects. Each class represents a different type of object. Instance variables are the pieces of data that a class needs to remember in order to work, either with itself or with other objects. Local variables should just be used intermediate calculations, data that you don't need to save once you leave the method.