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A method is a block of code which only runs when it is called.
You can pass data, known as parameters, into a method.
Methods are used to perform certain actions, and they are also known as functions.
Why use methods? To reuse code: define the code once, and use it many times.
A method must be declared within a class. It is defined with the name of the method, followed by parentheses (). Java provides some pre-defined methods, such as System.out.println(), but you can also create your own methods to perform certain actions:
Example
Create a method inside Main:
public class Main {
static void myMethod() {
// code to be executed
}
}Example Explained
myMethod()is the name of the methodstaticmeans that the method belongs to the Main class and not an object of the Main class. You will learn more about objects and how to access methods through objects later in this tutorial.voidmeans that this method does not have a return value. You will learn more about return values later in this chapter
To call a method in Java, write the method's name followed by two parentheses () and a semicolon;
In the following example, myMethod() is used to print a text (the action), when it is called
Example
Inside main, call the myMethod() method:
public class Main {
static void myMethod() {
System.out.println("I just got executed!");
}
public static void main(String[] args) {
myMethod();
}
}
// Outputs "I just got executed!"A method can also be called multiple times:
Example
public class Main {
static void myMethod() {
System.out.println("I just got executed!");
}
public static void main(String[] args) {
myMethod();
myMethod();
myMethod();
}
}
// I just got executed!
// I just got executed!
// I just got executed!Information can be passed to methods as parameter. Parameters act as variables inside the method.
Parameters are specified after the method name, inside the parentheses. You can add as many parameters as you want, just separate them with a comma.
The following example has a method that takes a String called fname as parameter. When the method is called, we pass along a first name, which is used inside the method to print the full name:
Example
public class Main {
static void myMethod(String fname) {
System.out.println(fname + " Refsnes");
}
public static void main(String[] args) {
myMethod("Liam");
myMethod("Jenny");
myMethod("Anja");
}
}
// Liam Refsnes
// Jenny Refsnes
// Anja RefsnesYou can have as many parameters as you like:
Example
public class Main {
static void myMethod(String fname, int age) {
System.out.println(fname + " is " + age);
}
public static void main(String[] args) {
myMethod("Liam", 5);
myMethod("Jenny", 8);
myMethod("Anja", 31);
}
}
// Liam is 5
// Jenny is 8
// Anja is 31The void keyword, used in the examples above, indicates that the method should not return a value. If you want the method to return a value, you can use a primitive data type (such as int, char, etc.) instead of void, and use the return keyword inside the method:
Example
public class Main {
static int myMethod(int x) {
return 5 + x;
}
public static void main(String[] args) {
System.out.println(myMethod(3));
}
}
// Outputs 8 (5 + 3)This example returns the sum of a method's two parameters:
Example
public class Main {
static int myMethod(int x, int y) {
return x + y;
}
public static void main(String[] args) {
System.out.println(myMethod(5, 3));
}
}
// Outputs 8 (5 + 3)It is common to use if...else statements inside methods:
Example
public class Main {
// Create a checkAge() method with an integer variable called age
static void checkAge(int age) {
// If age is less than 18, print "access denied"
if (age < 18) {
System.out.println("Access denied - You are not old enough!");
// If age is greater than, or equal to, 18, print "access granted"
} else {
System.out.println("Access granted - You are old enough!");
}
}
public static void main(String[] args) {
checkAge(20); // Call the checkAge method and pass along an age of 20
}
}
// Outputs "Access granted - You are old enough!"Test Yourself With Exercises
Exercise:
Add a fname parameter of type String to myMethod, and output "John Doe":
static void myMethod(
) {
System.out.println(
+ " Doe");
}
public static void main(String[] args) {
myMethod("John");
}With method overloading, multiple methods can have the same name with different parameters:
Example
int myMethod(int x)
float myMethod(float x)
double myMethod(double x, double y)
Consider the following example, which has two methods that add numbers of different type:
ExampleGet your own Java Server
static int plusMethodInt(int x, int y) {
return x + y;
}
static double plusMethodDouble(double x, double y) {
return x + y;
}
public static void main(String[] args) {
int myNum1 = plusMethodInt(8, 5);
double myNum2 = plusMethodDouble(4.3, 6.26);
System.out.println("int: " + myNum1);
System.out.println("double: " + myNum2);
}Instead of defining two methods that should do the same thing, it is better to overload one.
In the example below, we overload the plusMethod method to work for both int and double:
Example
static int plusMethod(int x, int y) {
return x + y;
}
static double plusMethod(double x, double y) {
return x + y;
}
public static void main(String[] args) {
int myNum1 = plusMethod(8, 5);
double myNum2 = plusMethod(4.3, 6.26);
System.out.println("int: " + myNum1);
System.out.println("double: " + myNum2);
}Recursion is the technique of making a function call itself. This technique provides a way to break complicated problems down into simple problems which are easier to solve.
Recursion may be a bit difficult to understand. The best way to figure out how it works is to experiment with it.
Recursion Example Adding two numbers together is easy to do, but adding a range of numbers is more complicated. In the following example, recursion is used to add a range of numbers together by breaking it down into the simple task of adding two numbers:
Example
Use recursion to add all of the numbers up to 10.
public class Main {
public static void main(String[] args) {
int result = sum(10);
System.out.println(result);
}
public static int sum(int k) {
if (k > 0) {
return k + sum(k - 1);
} else {
return 0;
}
}
}Example Explained:
When the sum() function is called, it adds parameter k to the sum of all numbers smaller than k and returns the result. When k becomes 0, the function just returns 0. When running, the program follows these steps:
10 + sum(9)
10 + ( 9 + sum(8) )
10 + ( 9 + ( 8 + sum(7) ) )
...
10 + 9 + 8 + 7 + 6 + 5 + 4 + 3 + 2 + 1 + sum(0)
10 + 9 + 8 + 7 + 6 + 5 + 4 + 3 + 2 + 1 + 0
Since the function does not call itself when k is 0, the program stops there and returns the result.
Just as loops can run into the problem of infinite looping, recursive functions can run into the problem of infinite recursion. Infinite recursion is when the function never stops calling itself. Every recursive function should have a halting condition, which is the condition where the function stops calling itself. In the previous example, the halting condition is when the parameter k becomes 0.
It is helpful to see a variety of different examples to better understand the concept. In this example, the function adds a range of numbers between a start and an end. The halting condition for this recursive function is when end is not greater than start:
Example
Use recursion to add all of the numbers between 5 to 10.
public class Main {
public static void main(String[] args) {
int result = sum(5, 10);
System.out.println(result);
}
public static int sum(int start, int end) {
if (end > start) {
return end + sum(start, end - 1);
} else {
return end;
}
}
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