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An important property of local variables

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• Method scope
  • Notice the braces { ... } around the body of each method:

    These scopes are called method scopes

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  • Fact:

    The braces surrounding bodies of methods always define non-overlapping (disjoint) scopes I.e.: method scopes are always disjoint !!!

    Example:

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• Properties of variables defined in disjoint scopes
  • Property 1: (You have seen this property before)

    You can define different variables with the same name in disjoint scopes

    Example:

    Hence:

    You can use the same name for different variables in disjoint method scopes

    Example:

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  • Property 2:

    It is impossible for computer instructions inside one disjoint scope to access variables defined inside another disjoint scope

    The reason is as follows:

    Because the 2 scopes are disjoint (no overlap): a statement in the second disjoint scope must be outside the first disjoint scope in which the variable was defined.

    Hence:

    Statements inside one method can never access any variable defined inside another method !!!

    Example:

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• Terminology: side effect
  • Side effect (computer science):

    In computer science, a method is said to "have a side effect" if: The method modifies (= changes the value) a variable that is defined outside the body of the method

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• Local variables make finding error easier...
  • Fact in Java:

    A local variable defined inside a method cannot modified by any other method

    In other words:

    When you are writing a method, you can be 100% certain that the local variables defined inside the method will only be changed by statements inside that method Therefore: When you have an error inside a method where some variable is updated erroneously, then you do not need to suspect that the cause lies outside that method !!!

    Note:

    It is pretty difficult to find errors in complex program Having localized updates in methods does make finding errors a little bit easier....