4.1.1.1 - Data types and references (Java)

Built-in types and representation

A type determines the values a piece of data can hold and the operations that make sense on it.

An integer count can be added; a Boolean decision can be combined with another condition. The digits in a telephone number are usually a string because leading zeroes matter and arithmetic does not.

Java is statically typed: declarations specify a type, and the compiler checks compatible operations before execution. int seats = 12; declares and initialises a variable. seats = 8; later replaces its value. A local variable must be given a value before it is read. Java identifiers and type names are case-sensitive, so String and string are different.

import java.time.LocalDateTime;

public class Main {
    public static void main(String[] args) {
        int seats = 12;
        double temperature = 18.75;
        boolean open = true;
        char zone = 'B';
        String organiser = "Amira";
        LocalDateTime meeting = LocalDateTime.of(2027, 3, 5, 14, 30);
        System.out.println(organiser + " " + seats);
    }
}

main is the program's entry point; statements inside its braces execute in order. System.out.println displays a value and then a newline. The example displays Amira 12. Single quotes denote a character; double quotes denote a string. LocalDateTime represents a local date and time without a time zone. A structured date type permits date operations rather than fragile character slicing.

int, double, boolean and char are primitive types. Floating-point values approximate many real numbers: a double is useful for measurements but does not represent every decimal fraction exactly. A type also has limits: an int is a fixed-size signed whole number, not an unlimited mathematical integer. These limits should inform a choice of type.

References and dynamic objects

A reference variable identifies an object rather than containing all of that object's data.

An explicit pointer in some languages stores a memory address and may permit operations on addresses. Java gives controlled references instead. new creates an object dynamically during execution. Arrays and class instances are objects; a variable referring to one can hold null, meaning no object. Reading an element through null is an error.

int[] original = {4, 7};
int[] alias = original;
alias[0] = 9;
System.out.println(original[0]);
alias = new int[] {1, 2};
System.out.println(original[0]);
System.out.println(alias[0]);