Java 17 Passing Arguments
Java is different from many other languages in that all method parameters are passed by value.
In Java, method parameters can’t be passed by reference.
For value data types, primitive types, that means only a local copy of the variable is changed within the method, so the change won’t affect the original variable.
For reference data types (classes, interfaces, and arrays), it means only a copy of the memory address is passed to the method.
Therefore, if the entire object is replaced, the change won’t propagate back to the caller, but changes to the object will affect the original since the copy points to the same memory location.
public class MyApp < public static void main(String[] args) < MyApp m = new MyApp(); int x = 0; // value data type m.set(x); // value is passed System.out.println(x); // "0" int[] y = ; // reference data type m.set(y); // address is passed System.out.println(y[0]); // "10" > void set(int a) < a = 10; >void set(int[] a) < a[0] = 10; >>
This program demonstrates how methods can be used in a menu-driven program.
import java.util.Scanner; public class CircleCalculation < // w w w . d e mo 2s . c o m public static void main(String[] args) < Scanner keyboard = new Scanner(System.in); /* The variable below is local to the main method; if the value is needed by another method, it must be passed in as a parameter */ double radius = -999; // initialize with a dummy value to show that nothing has been entered char choice; // to store menu choice do < System.out.println(); System.out.println("*** CIRCLE CALCULATIONS ***"); System.out.println(); System.out.println("1. Enter the radius of the circle"); System.out.println("2. Display the area of the circle"); System.out.println("3. Display the circumference of the circle"); System.out.println("4. Quit"); System.out.println(); System.out.println("Enter a number from 1 - 4"); System.out.println(); choice = keyboard.next().charAt(0); switch(choice) < case '1' : radius = option1(); // call method option1 break; case '2' : option2(radius); // call method option2 break; case '3' : option3(radius); // call method option3 break; case '4' : break; default : System.out.println("Enter only numbers from 1 - 4"); System.out.println(); > > while(choice != '4'); > // option1 gets the user to enter the radius of the circle static double option1() < double myRadius; // local variable Scanner keyboard = new Scanner(System.in); System.out.print("Enter the radius of the circle: "); myRadius = keyboard.nextDouble(); return myRadius; > // option2 calculates and displays the area of the circle static void option2(double radiusIn) < if(radiusIn == -999) < System.out.println("Radius has not been entered"); > else < double area; // local variable area = 3.1416 * radiusIn * radiusIn; // calculate the area System.out.println("The area of the circle is: " + area); > > // option3 calculates and displays the circumference of the circle static void option3(double radiusIn) < if(radiusIn == -999) < System.out.println("Radius has not been entered"); > else < double circumference; // local variable circumference = 2 * 3.1416 * radiusIn; // calculate the circumference System.out.println("The circumference of the circle is: " + circumference); > > >
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Passing Information to a Method or a Constructor
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.
Parameter Types
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 >
Note: If you want to pass a method into a method, then use a lambda expression or a method reference.
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); 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:
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 codeblock"> 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:
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.