12.5: Parameters and Arguments

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Some of the methods we have used require arguments, which are the values you provide when you invoke the method. For example, to find the sine of a number, you have to provide the number, so sin takes a double as an argument. To display a message, you have to provide the message, so println takes a String.

When you use a method, you provide the arguments. When you write a method, you name the parameters. The parameter list indicates what arguments are required. The following class shows an example:

public class PrintTwice {

    public static void printTwice(String s) {
        System.out.println(s);
        System.out.println(s);
    }

    public static void main(String[] args) {
        printTwice("Don't make me say this twice!");
    }
}

printTwice has a parameter named s with type String. When we invoke printTwice, we have to provide an argument with type String.

Before the method executes, the argument gets assigned to the parameter. In this example, the argument "Don't make me say this twice!" gets assigned to the parameter s.

This process is called parameter passing because the value gets passed from outside the method to the inside. An argument can be any kind of expression, so if you have a String variable, you can use it as an argument:

String argument = "Never say never.";
printTwice(argument);

The value you provide as an argument must have the same type as the parameter. For example, if you try:

printTwice(17);  // syntax error

You will get an error message like this:

File: Test.java  [line: 10]
Error: method printTwice in class Test cannot be applied
        to given types;
    required: java.lang.String
    found: int
    reason: actual argument int cannot be converted to
            java.lang.String by method invocation conversion

Sometimes Java can convert an argument from one type to another automatically. For example, Math.sqrt requires a double, but if you invoke Math.sqrt(25), the integer value 25 is automatically converted to the floating-point value 25.0. But in the case of printTwice, Java can’t (or won’t) convert the integer 17 to a String.

Parameters and other variables only exist inside their own methods. Inside main, there is no such thing as s. If you try to use it there, you’ll get a compiler error. Similarly, inside printTwice there is no such thing as argument. That variable belongs to main.

Because variables only exist inside the methods where they are defined, they are often called local variables.