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Parameter variables: definition (how to recognize them), life time and scope.

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• Previously discusses: different kinds of variables
  • Java has 4 different kinds of variables (See: click here)

    Class variables Instance variables Local variables Parameter variables

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    We have just learned about the local variables

    In this webpage, we will discuss the parameter variables

    Specifically:

    Purpose of parameter variables How to identify (and define) parameter variables The life time of parameter variables The scope of parameter variables

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• Purpose of parameter variables
  • Parameter variables:

    A parameter variable is used to store information that is being passed from the location of the method call into the method that is called

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    Example:

    public class Class1 { public static void method1() { double x; // x contains // some information .... Class2.method2( x ); // Pass (give) information // stored in x to method2 !!! .... } .... }

    public class Class2 { public static void method2( double paramVar ) { statements that operate on paramVar .... } .... }

    Effect: the statement Class2.method2( x ) pass (= copy) the information stored in x into the parameter variable paramVar

    Explanation:

    Inside the method method1, we call (invoke) method2 with the statement: Class2.method2( x ); Method1 has some information (stored in the variable x inside the method method1) that it want to pass (convey) to method 2 The mechanism used to accomplish this passing of information are parameter variables

  • Terminology:

    The method that calls (invokes, runs) another method is called the caller or the calling method In the above example, method1 is the caller The method that is called is called the callee or the called method In the above example, method2 is the callee

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  • A previously used example:

    Explanation:

    Method main is calling (invoke) the method min main is the caller Method main has some information (the values 1.0 and 4.0) that it want to pass (convey) to method min min is the callee The mechanism used to accomplish this passing of information uses the parameter variables a and b

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• Defining (recognizing) parameter variables
  • How to the define (and recognize) a parameter variable:

    A parameter variable is defined inside the brackets ( ... ) of the header of a method

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    Example:

    public class MyProgram { public static void main(String[] args) { // Body of method "main" double r; // *** Local variable r = MyProgram.min( 1.0, 4.0 ); System.out.println(r); r = MyProgram.min( 3.7, -2.9 ); System.out.println(r); r = MyProgram.min( -9.9, 3.8 ); System.out.println(r); } } public class ToolBox { public static double min ( double a, double b ) { // Body of method "min" double m = 0; // *** Local variable if ( a < b ) { m = a; // a is the smaller value } else { m = b; // b is the smaller value } return(m); } }

    Comments:

    The variable args in method main is a parameter variable Its type is String[] (will be explained much later...) The variables a and b in method min are 2 parameter variable Their type is double (you should know what that means...)

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• The life time and scope of parameter variables
  • The life time of a parameter variable:

    is the entire body of the method

    Example:

    Therefore, parameter variables has the same life time as local variables that are defined at the start of the method.

    In other words, you can look at parameter variable like this:

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  • The scope of a parameter variable:

    is also the entire body of the method

    Example:

    Just like the life time, parameter variables has the same scope as local variables that are defined at the start of the method.

    In other words, you can look at parameter variable like this:

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• Simple rule to remember the life time and scope of parameter variables
  • Simple rule to go by:

    A parameter variable behaves like a local variable that is defined at the start of the method However, unlike a local variable (that cannot be accessed by the calling method): The value of the parameter variable is always initialized by the calling method using the values inside the method call

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  • Example:

    The method call   ToolBox.min(1.0, 4.0): The parameter variables are initialized with the values in the method call:

    Now you can understand completely why the ToolBox.min method will return the value 1.0 !!!

    (Because it used parameter variable with values 1.0 and 4.0 - so it will return the smaller value which is 1.0)

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• Names conflicts between parameter variables and local variables
  • The following facts are important to determine whether a name of a parameter variable will conflict with names of local variables:

    The scope of parameter variables is the entire body of the method You cannot define different variables with the same name inside the same scope Rule in Java: You cannot define different variables with the same name inside an outer scope and inside an inner scope

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  • Conclusion:

    In other words:

    In Java, you cannot define a local variable inside a method that has the same name as a parameter variable.

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• Parameter variables and local variables in disjoint scopes
  • The following fact is still true for parameter and local variables that reside in disjoint scopes:

    You can define different (parameter and/or local) variables with the same name in disjoint scopes

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  • Example:

    public class MyProgram { public static double min ( double s, double r ) { double m = 0; if ( s < r ) { m = s; // s is the smaller value } else { m = r; // r is the smaller value } return(m); // Output of the method } public static void main(String[] args) { double r; r = min( 1.0, 4.0 ); System.out.println(r); r = min( 3.7, -2.9 ); System.out.println(r); r = min( -9.9, 3.8 ); System.out.println(r); } }

    Note:

    The parameter variable r in the min method and the local variable r in the main method are 2 different variables

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac MyProgram.java To run:          java MyProgram