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Method Invocation 

                         

Without methods, an object cannot do anything. When an object calls a method, it is actually requesting to perform some action like to set a value, return a value, write to a file or to provide some functionality required. 

There are two types of method invocation in java: 
i) Class method 
ii) Instance method

i.) Class method : Method invocation on a class
A method that is invoked on a class instead of on an actual object. Methods are class methods if they contain the static keyword in their declaration.

Java supports static methods as well as static variables. Static methods have the static modifier in their declarations. They should be invoked with the class name, and they do not need an instance of a class to be created, as in
ClassName.methodName(args)

Note: An object reference can also be used to refer to the static methods like

instanceName.methodName(args)

Commonly it is used for static methods to access static fields e.g. adding a static method to the Car class to access its color static field:

public static int getColor () {
    return color ;
}

Class methods don't need any instance for invoking and it uses the static binding. In the class method it select the reference of the object at compile time.

ii.) Instance method: Method invocation on a class instance

In object-oriented programming language objects play a crucial role. Objects need methods to do something. And to invoke these instance method it need an instance of a class to be created. The actual logic behind the method invoking begins when an object calls a method requesting to perform some action.
Data that are passed to a method are known as arguments, the required arguments for a method are defined by a method's constraint list.

Note : Not all combinations of instance and class variables and methods are allowed:

  • Instance methods can access instance variables and instance methods directly.
  • Instance methods can access class variables and class methods directly.
  • Class methods can access class variables and class methods directly.
  • Class methods cannot access instance variables or instance methods directly—they must use an object reference. Also, class methods cannot use the this keyword as there is no instance for this to refer to.

                         

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