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Method invocation (subroutines) in Java When we write a statement that sends a message to an object, the run-time interpreter has to work out if the message is in the object's protocol, and if so, where in the class hierarchy the appropriate implementation of the corresponding method can be found. Once found, the run-time interpreter will execute the instructions of the method, and then return execution to the line following the one where the original message was sent. Unlike selection and iteration there is no testing taking place to decide whether on not the message will be sent. Thus this kind of branching is called unconditional branching. In the example programs you have met already, you have already seen many examples of message sending, resulting in the invocation of methods. The class System and its corresponding println method is one of the (many) built-in messages/methods on the Java language. When a program sends a println message it must indicate what to print, and to which object to send the message: System.out.println ("hello") or System.out.println (2 + 2) In these examples "hello" and "2 + 2" are arguments that form part of the message, and that become inputs to the invoked println method. When this method was implemented, by the Java development team, they defined how that arguments are processed. When you define methods in your programs, you will have to do the same. A note on 'built-in' methods In fact methods such as println are not really built-in to the language itself. Technically they are built into the standard class library. However, that distinction is not important here. A note on methods and subroutines In programming languages that are not object-oriented something similar to method invocation is achieved through subroutines. A subroutine is a mechanism for the execution of a series of instructions elsewhere in a program, then returning to the original point. The whole idea of subroutines is considered rather old-fashioned these days, although some, usually much more sophisticated, equivalent can be found is the majority of modern programming languages.
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