Java

ลำดับตอนที่ #8 : Java III - Method

Using Methods

          A method is a section of code to which we assign a specific task. From here, we will be splitting up the main method into smaller methods. When Java files are executed, the main method is automatically executed. When the program is divided up, the main method instructs each of the others to execute their individual tasks.

Code with Multiple Methods

          The code on the left is disorganized and hard to understand because everything is written in the main method. On the other hand, the code on the right is organized and easy to understand since it's split into smaller methods. By splitting our program into smaller methods, we can give each of them separate and specific roles.

Calling Methods

          You can think of a method as a collection of statements that are grouped together to perform a specific operation. In the left figure, we defined a method called hello, and called the hello method from the main method. It’s ok if you don’t quite understand this process yet. Check the flow on the right and try using methods in the next exercise.

Method Definitions

          First, let’s take a look at method definitions. For now, remember public static void as a fixed phrase. Later, we will learn about the () at the end of each method name. We write the code for each method inside the block, the part between { and }. We should give each method a name that makes it easy to tell what it does.

          We define methods inside a class. As shown below, the hello method is defined in the block ({}) of the Main class. If you define a method outside of a class, there will be an error, so be careful.

Calling Methods

          To call a method, just write methodName(). Don't forget to put the () when you call the method. We'll learn more about this later.

Using Arguments

Arguments

          An argument is like an additional piece of data that you give to the method. When you call a method, you can pass an argument so that you can use that value inside the method.

Receiving Arguments

          In Java, you have to specify in the definition of a method that it can handle arguments. First, we need to specify a variable (parameter) for handling an argument. You can set a parameter inside the () at the end of public static void methodName(). Remember to include the data type for each parameter just like when declaring a variable.

Passing Arguments

          To pass an argument to a method, call the method like this: methodName(argument). An argument passed to a method is assigned to the variable specified in the definition as its parameter, and that variable can be used within the method.

Multiple Arguments

Methods with Multiple Arguments

          Methods can receive multiple arguments. In the definition, to set multiple parameters, separate them with commas ( , ) . Parameters can be in any order and of any data type. When calling a method, arguments should be in order from left to right as follows: firstArgument, secondArgument, …

Passing Multiple Arguments

          Let's see how to pass multiple arguments to a method. Take a look at the example below.

The Order of Passing Arguments

          Though the order of the parameters can be arbitrary, when calling a method, you have to pass the arguments in the same order as the parameters. Like in the example below, if you try to use an integer argument where a String type parameter is defined, it causes an error (the order of passing arguments is reversed).

Method Return Values

Return Values

          A return value allows us to use the result of a method where it was called. Think of a method like a factory. The factory gets materials (arguments) from the client (main method), does the predetermined manufacturing (operations), and returns the finished product to the client. It's the return value that corresponds to the finished product.

Method Return Values

          Methods can send a return value back to the caller with return. When there is a return value, you need to set its return type (the data type of the return value). To do this, change the void part of public static void. As shown below, the return value of the add method is an integer, so we set the data type like public static int.

Return Value Example

          After a method is executed, the return value (if it exists) will replace the caller of the method. This way, we can assign the caller of the method to a variable (see the example below). This can be very useful.

Note: "Caller" refers to the operation that called the method.

Void

          So what is void from the fixed phrase public static void? The "void" means no return value. As shown in the example below, the hello method doesn't have a return value. We use void to specify that a method has no return value.

Method Overloading

Identically Named Methods

          Defining more than one method with the same name is not allowed in Java. This is because if the computer recognizes multiple methods with the same name, it cannot determine which method to call.

Overloading

          However, if the arguments of two identically named methods differ in number or data type, they can have the same name. Even if there are multiple methods with the same name, the computer can tell which one to call when argument lists are unique. Defining multiple methods with the same name is called overloading.

Combining Methods

Calling a Method from Another

          We can call a method not only from the main method but from other methods as well. As components of the larger program, methods help each other to make up one whole program.

Combining Methods

          Let's call one method from another. To calculate (a + b) / 2, we can use the add method inside the average method, which returns the average of two numbers. Note that (double) is a data type conversion we learned in the Java I lesson.

Boolean Return Values

          Finally, let's create a method that returns a boolean value. As shown in the example below, we first define the method as a boolean data type. The isEven method checks if the value of the argument is an even number and returns true if it is even, and false otherwise.

Example: Method Returns Boolean Value

          Let's try using the isEven method, which returns a boolean value. Take a look at the example below. Especially the output!