freeCodeCamp/guide/english/java/methods/index.md

title
Methods

Methods

The most recognizable method in Java is probably public static void main(String[]args) where public means that users have access to the method, static means that the method is based on a "class" rather than an "instance," void means that nothing will be returned from the method to another (higher level) method, and main which is the name of this particular method.

getName() and getManufacturerName() are two "Getter" methods we have used here. Generally, methods in Java consist of these parts -

• Access Modifer (Optional) - public, private, or protected. Defaults to package private if omitted
• Return Type - This is required, it denotes what value the method returns, or void if nothing is returned
• Method Name - follows camelCase convention
• Parameter List - List of parameters with their name and type, empty if no parameters are accepted
• Method body surrounded by { }

Methods can also optionally have the static keyword, meaning it is associated with the class itself, rather than an instance of the class, ex - public static void main().

Notice, unlike JavaScript, we have to define the return type of any method we write, otherwise it will fail at compile time. If you do not want a method to return anything, use void return type.

Each method has a signature, which is the combination of the data type, the name, and the number of arguments the method takes. In public static void main the method does not have a specified data type and instead uses void to declare that no data is returned. In a method named public static double ave(double val, double val) the data type is "double" (0.0), the name is "ave" (average) and the method takes 2 arguments. Each method must have a unique signature.

public class Car {
    private String name;
    private String manufacturersName;

    public void changeName() {
        name = "Tesla";
    }
    
    public String getName(){
        return name;
    }
    
    public String getManufacurername(){
        return manufacturersName;
    }
    
}

Parameters can be passed into methods. Parameters are declared just after the name of the method, inside brackets. Syntax for parameter declaration is [Data Type] [Name].

public class Car {
    private String name;

    public void changeName(String newName) {
        name = newName;
    }
}

If we not sure with the distinct number of parameters passed in a method then use 'Variable Arguments (Varargs) Method'

Initialize parameter with three dots and will have same dataType. Passed arguments will store in an array and also accessible as array

public class Main
{
	public static void main(String[] args) {
	   
       Main p=new Main();
		
        System.out.println(p.sum(5,5,5,5,5,5,5,5,5)); //can pass any number of argument.
	}
	
int sum(int ...a) //decleration of variable argument method.
	{int sum=0;
	    for(int i=0;i<a.length;i++)
	    sum=sum+a[i]; //getting the passed arguments in form of array
	    return sum;}
	
}

As with any other language, methods (or functions, if you are here from JS world) are used often for their modularity and reusability.

Methods often serve many purposes such as updating information in an object or providing data back to the caller. Here are some examples.

public class Car {
    private int numberOfWheels;
    
    public void setNumberOfWheels(int newNumberOfWheels) {
        numberOfWheels = newNumberOfWheels;
    }
    
    public int getNumberOfWheels() {
        return numberOfWheels;
    }
}

In the case of getNumberOfWheels() the return type is int which is a whole number. The keyword return tells java to pass back the value of the instance variable numberOfWheels. setNumberOfWheels(int newNumberOfWheels) however has no return type as it is a setter method as previously seen. In this case though it takes in an argument of type int and makes the instance varible numberOfWheels equal to newNumberOfWheels.

There is also a special method called a constructor that allows for data to be set or operations to be performed when the class is instantiated. This constructor has no return type.

public class Car {
    private String model;
    private int numberOfWheels;
    
    public Car(String model, int numberOfWheels) {
        this.model = model;
        this.numberOfWheels = numberOfWheels;
    }
}

The Car class and the Car(String model, int numberOfWheels) method have to have the same name in order for java to know that it is the constructor. Now anytime you instantiate a new Car instance with the new keyword you will need to call this constructor and pass in the needed data.