Method in Java

In general, a method is a way to perform some task. Similarly, the method in Java is a collection of instructions that performs a specific task. It provides the reusability of code. We can also easily modify code using methods. In this section, we will learn what is a method in Java, types of methods, method declaration, and how to call a method in Java.

What is a method in Java?

A method is a block of code or collection of statements or a set of code grouped together to perform a certain task or operation. It is used to achieve the reusability of code. We write a method once and use it many times. We do not require to write code again and again. It also provides the easy modification and readability of code, just by adding or removing a chunk of code. The method is executed only when we call or invoke it.

The most important method in Java is the main() method. 

Method Declaration

The method declaration provides information about method attributes, such as visibility, return-type, name, and arguments. It has six components that are known as method header, as we have shown in the following figure.

Method Signature: Every method has a method signature. It is a part of the method declaration. It includes the method name and parameter list.

Access Specifier: Access specifier or modifier is the access type of the method. It specifies the visibility of the method. Java provides four types of access specifier:

• Public: The method is accessible by all classes when we use public specifier in our application.
• Private: When we use a private access specifier, the method is accessible only in the classes in which it is defined.
• Protected: When we use protected access specifier, the method is accessible within the same package or subclasses in a different package.
• Default: When we do not use any access specifier in the method declaration, Java uses default access specifier by default. It is visible only from the same package only.

Return Type: Return type is a data type that the method returns. It may have a primitive data type, object, collection, void, etc. If the method does not return anything, we use void keyword.

Method Name: It is a unique name that is used to define the name of a method. It must be corresponding to the functionality of the method. Suppose, if we are creating a method for subtraction of two numbers, the method name must be subtraction(). A method is invoked by its name.

Parameter List: It is the list of parameters separated by a comma and enclosed in the pair of parentheses. It contains the data type and variable name. If the method has no parameter, left the parentheses blank.

Method Body: It is a part of the method declaration. It contains all the actions to be performed. It is enclosed within the pair of curly braces.

Naming a Method

While defining a method, remember that the method name must be a verb and start with a lowercase letter. If the method name has more than two words, the first name must be a verb followed by adjective or noun. In the multi-word method name, the first letter of each word must be in uppercase except the first word. For example:

Single-word method name: sum(), area()

Multi-word method name: areaOfCircle(), stringComparision()

It is also possible that a method has the same name as another method name in the same class, it is known as method overloading.

Types of Method

There are two types of methods in Java:

• Predefined Method
• User-defined Method

Predefined Method

In Java, predefined methods are the method that is already defined in the Java class libraries is known as predefined methods. It is also known as the standard library method or built-in method. We can directly use these methods just by calling them in the program at any point. Some pre-defined methods are length(), equals(), compareTo(), sqrt(), etc. When we call any of the predefined methods in our program, a series of codes related to the corresponding method runs in the background that is already stored in the library.

Each and every predefined method is defined inside a class. Such as print() method is defined in the java.io.PrintStream class. It prints the statement that we write inside the method. For example, print("Java"), it prints Java on the console.

Let's see an example of the predefined method.

Demo.java

1. public class Demo   
2. {  
3. public static void main(String[] args)   
4. {  
5. // using the max() method of Math class  
6. System.out.print("The maximum number is: " + Math.max(9,7));  
7. }  
8. }  

Output:

The maximum number is: 9

In the above example, we have used three predefined methods main(), print(), and max(). We have used these methods directly without declaration because they are predefined. The print() method is a method of PrintStream class that prints the result on the console. The max() method is a method of the Math class that returns the greater of two numbers.

We can also see the method signature of any predefined method by using the link https://docs.oracle.com/. When we go through the link and see the max() method signature, we find the following:

In the above method signature, we see that the method signature has access specifier public, non-access modifier static, return type int, method name max(), parameter list (int a, int b). In the above example, instead of defining the method, we have just invoked the method. This is the advantage of a predefined method. It makes programming less complicated.

Similarly, we can also see the method signature of the print() method.

User-defined Method

The method written by the user or programmer is known as a user-defined method. These methods are modified according to the requirement.

How to Create a User-defined Method

Let's create a user defined method that checks the number is even or odd. First, we will define the method.

1. //user defined method  
2. public static void findEvenOdd(int num)  
3. {  
4. //method body  
5. if(num%2==0)   
6. System.out.println(num+" is even");   
7. else   
8. System.out.println(num+" is odd");  
9. }  

We have defined the above method named findevenodd(). It has a parameter num of type int. The method does not return any value that's why we have used void. The method body contains the steps to check the number is even or odd. If the number is even, it prints the number is even, else prints the number is odd.

How to Call or Invoke a User-defined Method

Once we have defined a method, it should be called. The calling of a method in a program is simple. When we call or invoke a user-defined method, the program control transfer to the called method.

1. import java.util.Scanner;  
2. public class EvenOdd  
3. {  
4. public static void main (String args[])  
5. {  
6. //creating Scanner class object     
7. Scanner scan=new Scanner(System.in);  
8. System.out.print("Enter the number: ");  
9. //reading value from the user  
10. int num=scan.nextInt();  
11. //method calling  
12. findEvenOdd(num);  
13. }  

In the above code snippet, as soon as the compiler reaches at line findEvenOdd(num), the control transfer to the method and gives the output accordingly.

Let's combine both snippets of codes in a single program and execute it.

EvenOdd.java

1. import java.util.Scanner;  
2. public class EvenOdd  
3. {  
4. public static void main (String args[])  
5. {  
6. //creating Scanner class object     
7. Scanner scan=new Scanner(System.in);  
8. System.out.print("Enter the number: ");  
9. //reading value from user  
10. int num=scan.nextInt();  
11. //method calling  
12. findEvenOdd(num);  
13. }  
14. //user defined method  
15. public static void findEvenOdd(int num)  
16. {  
17. //method body  
18. if(num%2==0)   
19. System.out.println(num+" is even");   
20. else   
21. System.out.println(num+" is odd");  
22. }  
23. }  

Output 1:

Enter the number: 12
12 is even

Output 2:

Enter the number: 99
99 is odd

Let's see another program that return a value to the calling method.

In the following program, we have defined a method named add() that sum up the two numbers. It has two parameters n1 and n2 of integer type. The values of n1 and n2 correspond to the value of a and b, respectively. Therefore, the method adds the value of a and b and store it in the variable s and returns the sum.

Addition.java

1. public class Addition   
2. {  
3. public static void main(String[] args)   
4. {  
5. int a = 19;  
6. int b = 5;  
7. //method calling  
8. int c = add(a, b);   //a and b are actual parameters  
9. System.out.println("The sum of a and b is= " + c);  
10. }  
11. //user defined method  
12. public static int add(int n1, int n2)   //n1 and n2 are formal parameters  
13. {  
14. int s;  
15. s=n1+n2;  
16. return s; //returning the sum  
17. }  
18. }  

Output:

The sum of a and b is= 24

Static Method

A method that has static keyword is known as static method. In other words, a method that belongs to a class rather than an instance of a class is known as a static method. We can also create a static method by using the keyword static before the method name.

The main advantage of a static method is that we can call it without creating an object. It can access static data members and also change the value of it. It is used to create an instance method. It is invoked by using the class name. The best example of a static method is the main() method.

Example of static method

Display.java

1. public class Display  
2. {  
3. public static void main(String[] args)   
4. {  
5. show();  
6. }  
7. static void show()   
8. {  
9. System.out.println("It is an example of static method.");  
10. }  
11. }  

Output:

It is an example of a static method.

Instance Method

The method of the class is known as an instance method. It is a non-static method defined in the class. Before calling or invoking the instance method, it is necessary to create an object of its class. Let's see an example of an instance method.

InstanceMethodExample.java

1. public class InstanceMethodExample  
2. {  
3. public static void main(String [] args)  
4. {  
5. //Creating an object of the class  
6. InstanceMethodExample obj = new InstanceMethodExample();  
7. //invoking instance method   
8. System.out.println("The sum is: "+obj.add(12, 13));  
9. }  
10. int s;  
11. //user-defined method because we have not used static keyword  
12. public int add(int a, int b)  
13. {  
14. s = a+b;  
15. //returning the sum  
16. return s;  
17. }  
18. }  

Output:

The sum is: 25

There are two types of instance method:

• Accessor Method
• Mutator Method

Accessor Method: The method(s) that reads the instance variable(s) is known as the accessor method. We can easily identify it because the method is prefixed with the word get. It is also known as getters. It returns the value of the private field. It is used to get the value of the private field.

Example

1. public int getId()    
2. {    
3. return Id;    
4. }    

Mutator Method: The method(s) read the instance variable(s) and also modify the values. We can easily identify it because the method is prefixed with the word set. It is also known as setters or modifiers. It does not return anything. It accepts a parameter of the same data type that depends on the field. It is used to set the value of the private field.

Example

1. public void setRoll(int roll)   
2. {  
3. this.roll = roll;  
4. }  

Example of accessor and mutator method

Student.java

1. public class Student   
2. {  
3. private int roll;  
4. private String name;  
5. public int getRoll()    //accessor method  
6. {  
7. return roll;  
8. }  
9. public void setRoll(int roll) //mutator method  
10. {  
11. this.roll = roll;  
12. }  
13. public String getName()   
14. {  
15. return name;  
16. }  
17. public void setName(String name)   
18. {  
19. this.name = name;  
20. }  
21. public void display()  
22. {  
23. System.out.println("Roll no.: "+roll);  
24. System.out.println("Student name: "+name);  
25. }  
26. }  

Abstract Method

The method that does not has method body is known as abstract method. In other words, without an implementation is known as abstract method. It always declares in the abstract class. It means the class itself must be abstract if it has abstract method. To create an abstract method, we use the keyword abstract.

Syntax

1. abstract void method_name();  

Example of abstract method

Demo.java

1. abstract class Demo //abstract class  
2. {  
3. //abstract method declaration  
4. abstract void display();  
5. }  
6. public class MyClass extends Demo  
7. {  
8. //method impelmentation  
9. void display()  
10. {  
11. System.out.println("Abstract method?");  
12. }  
13. public static void main(String args[])  
14. {  
15. //creating object of abstract class  
16. Demo obj = new MyClass();  
17. //invoking abstract method  
18. obj.display();  
19. }  
20. }  

Output:

Abstract method...

Factory method

It is a method that returns an object to the class to which it belongs. All static methods are factory methods. 

For example, NumberFormat obj = NumberFormat.getNumberInstance();