Object-Orientation - Methods (in Java)

# Methods

> "Take a method and try it. If it fails, admit it frankly, and try another. But by all means, try something."
>
> -[Franklin D. Roosevelt](https://en.wikipedia.org/wiki/Franklin_D._Roosevelt)

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# Agenda

1. [Overview](#concept)
1. [Simple Methods](#simple)
1. [Parameters & Arguments](#parameters)
1. [Return Values](#return)
1. [Overloading](#overloading)
1. [Conclusions](#conclusions)

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# Overview

## Concept

Methods are modular reusable blocks of code.

- The control flow of a program can easily switch to code within a method from anywhere else in the code.

- Switching to code within a method is termed 'calling' or 'invoking' that method.

- Calling a method is a form of [unconditional branching](../branching) - disrupting the 'usual' flow of a program.

- Once the control flow reaches the end of an invoked method, the control flow returns to the line of code from which it originally branched.

---

# Simple Methods

## Super simple

At their simplest, methods can have no parameters and no return value.

For example, a method named 'doSomething1':

```java
public static void doSomething1() {
    System.out.println("Running  doSomething1");
    // imagine some useful stuff happens in the middle here
    System.out.println("Exiting  doSomething1");
}
```

---

## Methods calling methods

Of course, methods can call other methods, as our `doSomething1` already displayed - it called the `System.out.println` method several times.

Note the order in which the print statements are executed.

```java
public static void doSomething1() {
    System.out.println("Begin doSomething1");
    // imagine some useful stuff happens in the middle here
    System.out.println("End doSomething1");
}

public static void doSomething2() {
    System.out.println("Begin doSommething2");
    doSomething1(); // call the method
    System.out.println("End doSomething2");
}
```

---

## Call stack

The order is determined by the control flow of the program.

- Each method invocation creates a new 'stack frame' - an area of memory dedicated to the newly invoked method.

- The Java interpreter is only ever looking at the method invocation at the top of the stack - the most recently invoked method.

- Each stack frame has its own variable namespace, so you can have two variables named `x` in two different method invocations, and they will be different variables in two different areas of memory with potentially different values.

- Once a method that has been invoked completes, its stack frame is popped (deleted) from the call stack and its memory is wiped clean.

---

# Parameters

## Concept

Methods can accept 'arguments' - values sent into the method.

- These arguments are stored in 'parameters' - local variables within the namespace of the method invocation's stack frame.

- Once the method invocation completes, any local variables, including parameters, are wiped out of memory.

```java
public static void doSomething1(int x) {
    x++; // increment x
    System.out.println("Begin doSomething1, x=" + x);
    System.out.println("End doSomething1, x=" + x);
}
```

---

# Return values

## Concept

Methods can return a single value in Java.

- The method signature line must state the data type of the returned value

- This value is returned to the part of the code from which the method was originally invoked.

- Think of it as if the method invocation is replaced by that method's return value.

```java
public static int doSomething1(int x) {
    x++; // increment x
    System.out.println("Begin doSomething1, x=" + x);
    System.out.println("End doSomething1, x=" + x);
    return x;
}
```

---

# Overloading

## Concept

Java allows multiple methods with the same name but different parameter sets. These are called overloaded methods.

- Which version of the method is invoked depends upon the arguments in the method call.

```java
public static void foo() {
    ...
}

public static void foo(String bar) {
    ...
}

public static void foo(String bar, boolean baz) {
    ...
}
```

---

# Conclusions

You now have a basic understanding of methods in Java.

- Thank you. Bye.