Which type of method parameter receives the address of the variable passed in

There are different ways in which parameter data can be passed into and out of methods and functions. Let us assume that a function B() is called from another function A(). In this case A is called the “caller function” and B is called the “called function or callee function”. Also, the arguments which A sends to B are called actual arguments and the parameters of B are called formal arguments.

Terminology

• Formal Parameter : A variable and its type as they appear in the prototype of the function or method.
• Actual Parameter : The variable or expression corresponding to a formal parameter that appears in the function or method call in the calling environment.
• Modes:
  • IN: Passes info from caller to callee.
  • OUT: Callee writes values in caller.
  • IN/OUT: Caller tells callee value of variable, which may be updated by callee.

Important methods of Parameter Passing

1. Pass By Value: This method uses in-mode semantics. Changes made to formal parameter do not get transmitted back to the caller. Any modifications to the formal parameter variable inside the called function or method affect only the separate storage location and will not be reflected in the actual parameter in the calling environment. This method is also called as call by value.

   printf("In func, a = %d b = %d\n", a, b);     printf("In main, x = %d y = %d\n", x, y);

   cout <<"In func, a = " << a << " b = "<< b << endl;     cout << "In main, x = " << x << " y = " << y;

   Output:

   In func, a = 12 b = 7 In main, x = 5 y = 7

   Languages like C, C++, Java support this type of parameter passing. Java in fact is strictly call by value.
   Shortcomings:

   • Inefficiency in storage allocation
   • For objects and arrays, the copy semantics are costly
2. Pass by reference(aliasing): This technique uses in/out-mode semantics. Changes made to formal parameter do get transmitted back to the caller through parameter passing. Any changes to the formal parameter are reflected in the actual parameter in the calling environment as formal parameter receives a reference (or pointer) to the actual data. This method is also called as call by reference. This method is efficient in both time and space.

   void swapnum(int* i, int* j)     printf("a is %d and b is %d\n", a, b);

   void swapnum(int* i, int* j)     cout <<"a is " << a <<" and b is "<< b;

   Output:

   a is 20 and b is 10

   C and C++ both support call by value as well as call by reference whereas Java doesn’t support call by reference.
   Shortcomings:

   • Many potential scenarios can occur
   • Programs are difficult to understand sometimes

Other methods of Parameter Passing

These techniques are older and were used in earlier programming languages like Pascal, Algol and Fortran. These techniques are not applicable in high level languages.

1. Pass by Result:This method uses out-mode semantics. Just before control is transferred back to the caller, the value of the formal parameter is transmitted back to the actual parameter.T his method is sometimes called call by result. In general, pass by result technique is implemented by copy.
2. Pass by Value-Result: This method uses in/out-mode semantics. It is a combination of Pass-by-Value and Pass-by-result. Just before the control is transferred back to the caller, the value of the formal parameter is transmitted back to the actual parameter. This method is sometimes called as call by value-result
3. Pass by name : This technique is used in programming language such as Algol. In this technique, symbolic “name” of a variable is passed, which allows it both to be accessed and update. Example:

   To double the value of C[j], you can pass its name (not its value) into the following procedure.

   procedure double(x); real x; begin x:=x*2 end;

   In general, the effect of pass-by-name is to textually substitute the argument in a procedure call for the corresponding parameter in the body of the procedure.
   Implications of Pass-by-Name mechanism:

   • The argument expression is re-evaluated each time the formal parameter is passed.
   • The procedure can change the values of variables used in the argument expression and hence change the expression’s value.

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The declaration for a method or a constructor declares the number and the type of the arguments for that method or constructor. For example, the following is a method that computes the monthly payments for a home loan, based on the amount of the loan, the interest rate, the length of the loan (the number of periods), and the future value of the loan:

public double computePayment( double loanAmt, double rate, double futureValue, int numPeriods) { double interest = rate / 100.0; double partial1 = Math.pow((1 + interest), - numPeriods); double denominator = (1 - partial1) / interest; double answer = (-loanAmt / denominator) - ((futureValue * partial1) / denominator); return answer; }

This method has four parameters: the loan amount, the interest rate, the future value and the number of periods. The first three are double-precision floating point numbers, and the fourth is an integer. The parameters are used in the method body and at runtime will take on the values of the arguments that are passed in.

Note: Parameters refers to the list of variables in a method declaration. Arguments are the actual values that are passed in when the method is invoked. When you invoke a method, the arguments used must match the declaration's parameters in type and order.

You can use any data type for a parameter of a method or a constructor. This includes primitive data types, such as doubles, floats, and integers, as you saw in the computePayment method, and reference data types, such as objects and arrays.

Here's an example of a method that accepts an array as an argument. In this example, the method creates a new Polygon object and initializes it from an array of Point objects (assume that Point is a class that represents an x, y coordinate):

public Polygon polygonFrom(Point[] corners) { // method body goes here }

Arbitrary Number of Arguments

You can use a construct called varargs to pass an arbitrary number of values to a method. You use varargs when you don't know how many of a particular type of argument will be passed to the method. It's a shortcut to creating an array manually (the previous method could have used varargs rather than an array).

To use varargs, you follow the type of the last parameter by an ellipsis (three dots, ...), then a space, and the parameter name. The method can then be called with any number of that parameter, including none.

public Polygon polygonFrom(Point... corners) { int numberOfSides = corners.length; double squareOfSide1, lengthOfSide1; squareOfSide1 = (corners[1].x - corners[0].x) * (corners[1].x - corners[0].x) + (corners[1].y - corners[0].y) * (corners[1].y - corners[0].y); lengthOfSide1 = Math.sqrt(squareOfSide1); // more method body code follows that creates and returns a // polygon connecting the Points }

You can see that, inside the method, corners is treated like an array. The method can be called either with an array or with a sequence of arguments. The code in the method body will treat the parameter as an array in either case.

You will most commonly see varargs with the printing methods; for example, this printf method:

public PrintStream printf(String format, Object... args)

allows you to print an arbitrary number of objects. It can be called like this:

System.out.printf("%s: %d, %s%n", name, idnum, address);

or like this

System.out.printf("%s: %d, %s, %s, %s%n", name, idnum, address, phone, email);

or with yet a different number of arguments.

Parameter Names

When you declare a parameter to a method or a constructor, you provide a name for that parameter. This name is used within the method body to refer to the passed-in argument.

The name of a parameter must be unique in its scope. It cannot be the same as the name of another parameter for the same method or constructor, and it cannot be the name of a local variable within the method or constructor.

A parameter can have the same name as one of the class's fields. If this is the case, the parameter is said to shadow the field. Shadowing fields can make your code difficult to read and is conventionally used only within constructors and methods that set a particular field. For example, consider the following Circle class and its setOrigin method:

public class Circle { private int x, y, radius; public void setOrigin(int x, int y) { ... } }

The Circle class has three fields: x, y, and radius. The setOrigin method has two parameters, each of which has the same name as one of the fields. Each method parameter shadows the field that shares its name. So using the simple names x or y within the body of the method refers to the parameter, not to the field. To access the field, you must use a qualified name. This will be discussed later in this lesson in the section titled "Using the this Keyword."

Passing Primitive Data Type Arguments

Primitive arguments, such as an int or a double, are passed into methods by value. This means that any changes to the values of the parameters exist only within the scope of the method. When the method returns, the parameters are gone and any changes to them are lost. Here is an example:

public class PassPrimitiveByValue { public static void main(String[] args) { int x = 3; // invoke passMethod() with // x as argument passMethod(x); // print x to see if its // value has changed System.out.println("After invoking passMethod, x = " + x); } // change parameter in passMethod() public static void passMethod(int p) { p = 10; } }

When you run this program, the output is:

After invoking passMethod, x = 3

Passing Reference Data Type Arguments

Reference data type parameters, such as objects, are also passed into methods by value. This means that when the method returns, the passed-in reference still references the same object as before. However, the values of the object's fields can be changed in the method, if they have the proper access level.

For example, consider a method in an arbitrary class that moves Circle objects:

public void moveCircle(Circle circle, int deltaX, int deltaY) { // code to move origin of circle to x+deltaX, y+deltaY circle.setX(circle.getX() + deltaX); circle.setY(circle.getY() + deltaY); // code to assign a new reference to circle circle = new Circle(0, 0); }

Let the method be invoked with these arguments:

moveCircle(myCircle, 23, 56)

Inside the method, circle initially refers to myCircle. The method changes the x and y coordinates of the object that circle references (that is, myCircle) by 23 and 56, respectively. These changes will persist when the method returns. Then circle is assigned a reference to a new Circle object with x = y = 0. This reassignment has no permanence, however, because the reference was passed in by value and cannot change. Within the method, the object pointed to by circle has changed, but, when the method returns, myCircle still references the same Circle object as before the method was called.