The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
SeeDev.java for updated tutorials taking advantage of the latest releases.
SeeJava Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
SeeJDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.

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(                  doubleloanAmt,                  doublerate,                  doublefutureValue,                  intnumPeriods) {    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.

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 thecomputePayment 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 newPolygon object and initializes it from an array ofPoint objects (assume thatPoint 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 calledvarargs 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, thisprintf 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 toshadow 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 followingCircle class and itssetOrigin method:

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

TheCircle class has three fields:x,y, andradius. ThesetOrigin 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 namesx ory 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 thethis Keyword."

Passing Primitive Data Type Arguments

Primitive arguments, such as anint or adouble, are passed into methodsby 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 methodsby 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 fieldscan be changed in the method, if they have the proper access level.

For example, consider a method in an arbitrary class that movesCircle 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 tomyCircle. The method changes the x and y coordinates of the object thatcircle references (that is,myCircle) by 23 and 56, respectively. These changes will persist when the method returns. Thencircle is assigned a reference to a newCircle object withx = 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 bycircle has changed, but, when the method returns,myCircle still references the sameCircle object as before the method was called.