chapter01-basic_types.md

Basic Types

Java has two kinds of type,

• primitive types that are directly mapped to CPU basic types
• reference types that have the address of the object in memory

Primitive types

primitive types, written in lower case, have no method

boolean (true|false)

var result = true;
var anotherResult = false;

char (character)

var firstLetter = 'j';

int (signed 32-bit integer)

var numberOfLegs = 2;

double (64-bit floating point)

var cost = 3.78;

long and float

some more exotic types that requires a suffix (L or f) long (64-bit integers) and float (32-bit floating point numbers)

var longValue = 123L;
var floatValue = 123.5f;

byte and short

you also have byte (a signed 8-bit integer) and short (a signed 16-bit short integer) that are only useful to use less memory when defining an object

record CompactHeader(byte tag, short version) {}

when used in variables, they are promoted to a 32-bit integer. In the following code, result is a 32-bit integer (so an int)

short value = 12;
var result = value + value;

primitive conversions

You have automatic conversions if there is no loose of precision and converting to double or float is always allowed

int intValue = 13;
long longValue = intValue;

you can force conversion in the opposite direction using a cast supplementary bits will be shaved (use with reluctance)

long longValue = 1_000_000_000_000L;
int intValue = (int) longValue;
System.out.println(intValue);

Objects

All other types are objects, there are two special types, String and arrays that are object but considered as built-in by the compiler

String

A String that stores a text (a sequence of characters) is delimited by two doublequotes

var text = "hello"; 
System.out.println(text);

a String can also span several lines, it's called a text block and starts and ends with 3 double quotes

var multilineText = """
   This is
   a multilines string
   """;
System.out.println(multilineText);

The indentation is determined by the alignment compared to position of the last """ By example, to have an indentation of two spaces

var multilineText = """
     This is
     a multilines string
     indented by two spaces
   """;
System.out.println(multilineText);

Strings have a lot of methods, here is some of them length of a String

System.out.println("hello".length());

to upper/lower case Locale.ROOT here ask for a result independent of the OS language

System.out.println("hello".toUpperCase(Locale.ROOT));
System.out.println("hello".toLowerCase(Locale.ROOT));

repeat the same pattern

System.out.println("|*|".repeat(3));

char at an index (starting with index 0)

System.out.println("hello".charAt(0));

index of a character

System.out.println("hello".indexOf('l'));
System.out.println("hello".indexOf('o'));

primitive to String The fastest and easy way to convert a primitive value to a String is to use the string concatenation (see chapter 'string formatting' for more)

System.out.println("" + 3);
System.out.println("" + 7.6);

String to primitive There are a bunch of static methods in Boolean, Integer or Double (see chapter 'wrapper' for more info)

System.out.println(Integer.parseInt("3"));
System.out.println(Double.parseDouble("7.6"));

Array

an array initialized with zeros (false, 0, 0.0, etc)

var intArray = new int[2];

An array initialized with some default values Because a value like 2 or 3 can be an numeric type (an int, a long, a short, etc) you have to specify the type of the array when you create it

var intArray = new int[] {2, 3 };
var longArray = new long[] { 2, 3 };

you can use the operator [] to access or change the value of an array at a specific index

System.out.println(intArray[0]);
intArray[0] = 42;
System.out.println(intArray[0]);

trying to access an array out of its bound raised an exception

intArray[-1] = 42;   // throws IndexOutOfBoundsException

and a special syntax to get the length of an array Notice that there is no parenthesis when calling length, we will see later why.

var arrayLength = intArray.length;
System.out.println(arrayLength);

arrays have methods like \toString()orequals()` but they are not implemented correctly, we will see later why

System.out.println(intArray);
System.out.println(new int[] {42}.equals(new int[] {42}));

On arrays

We don't use array much in Java, we have more powerful object like List, that we will see later

var intList = List.of(2, 3);

Static methods

Because primitive types and arrays have (almost) no method, if you want to play with them you have to use static methods. A static method is a function that is declared on a type somewhere that you can call using the syntax SomeWhere.methodName(arg0, arg1, arg2)

by example to transform a String to an int, we call the method parseInt stored in the type java.lang.Integer

var resultAsInt = java.lang.Integer.parseInt("42");
System.out.println(resultAsInt);

To transform an array to a text, there is the static method toString on the type java.util.Arrays

var text = java.util.Arrays.toString(intArray);
System.out.println(text);