hf-kotlin-griffiths

Practical part of the "Head First Kotlin" book.

Chapter 1. Getting started: A Quick Dip

Key points:

• Use fun to define a function.
• Every application needs a function named main.
• Use // to denote a single-lined comment.
• A String is a string of characters. You denote a String value by enclosing its characters in double quotes.
• Code blocks are defined by a pair of curly braces { }.
• The assignment operator is one equals sign =.
• The equals operator uses two equals signs ==.
• Use var to define a variable whose value may change.
• Use val to define a value whose value will stay the same.
• A while loop runs everything within its block so long as the conditional test is true.
• If the conditional test is false, the while loop code block won’t run, and execution will move down to the code immediately after the loop block.
• Put a conditional test inside parentheses ( ).
• Add conditional branches to your code using if and else. The else clause is optional.
• You can use if as an expression so that it returns a value. In this case, the else clause is mandatory.

Chapter 2. Basic types and variables: Being a Variable

Key points:

• In order to create a variable, the compiler needs to know its name, its type, and whether it can be reused.
• If the variable’s type isn’t explicitly defined, the compiler infers it from its value.
• A variable holds a reference to an object.
• An object has state and behavior. Its behavior is exposed through its functions.
• Defining the variable with var means the variable’s object reference can be replaced. Defining the variable with val means the variable holds a reference to the same object forever.
• Kotlin has a number of basic types: Byte, Short, Int, Long, Float, Double, Boolean, Char and String.
• Explicitly define a variable’s type by putting a colon after the variable’s name, followed by the type: var tinyNum: Byte
• You can only assign a value to a variable that has a compatible type.
• You can convert one numeric type to another. If the value won’t fit into the new type, some precision is lost.
• Create an array using the arrayOf function: var myArray = arrayOf(1, 2, 3)
• Access an array’s items using, for example, myArray[0]. The first item in an array has an index of 0.
• Get an array’s size using myArray.size.
• The compiler infers the array’s type from its items. You can explicitly define an array’s type like this: var myArray: Array<Byte>
• If you define an array using val, you can still update the items in the array.
• String templates provide a quick and easy way of referring to a variable or evaluating an expression from inside a String.

Chapter 3. Functions: Getting Out of Main

Key points:

• Use functions to organize your code and make it more reusable.
• A function can have parameters, so you can pass more than one value to it.
• The number and type of values you pass to the function must match the order and type of the parameters declared by the function.
• A function can return a value. You must define the type of value (if any) it returns.
• A Unit return type means that the function doesn’t return anything.
• Choose for loops over while loops when you know how many times you want to repeat the loop code.
• The readLine() function reads a line of input from the standard input stream. It returns a String value, the text entered by the user.
• If the input stream has been redirected to a file and the end of the file has been reached, the readLine() function returns null. null means it has no value, or it’s missing.
• && means “and”. || means “or”. ! means “not”.