Java Primitive Data Types

In Java, there are two categories of data types:

1. Primitive types for storing simple values like numbers, strings and booleans.
2. Reference types for storing complex objects. We'll get to these later in the course.

Learning Objectives

• Explain what primitive data types are
• Build a simple Java program using primitive data types
• Build a simple Java program using strings

Primitive Types

Data Type	Examples	Purpose
boolean	true, false	Answer yes or no questions
byte	-128, 0, 20, 127	Store whole numbers from -128 to 127
short	-32768, 0, 20, 32767	Store whole numbers from -32768 to 32767
int	-2Billion, 0, 20, 2Billion	Store whole numbers from -2147483648 to 2147483647
long	-2L, 0L, 20L, 2L	Store whole numbers from -9223372036854775808 to 9223372036854775808
float	-5.91f, 0.29f, 2.19f, 5.67f	Store fractional numbers from 3.4e−038 to 3.4e+038
double	-19.99d, 0.29d, 2.19d, 5.67d	Store fractional numbers from 1.7e−308 to 1.7e+308. They have double the precision of floats
char	'A', 'B', 'C'	Store a single character

boolean myBool = true;
byte myByte = 23;
short myShort = 5000;
int myInt = 100000;
long myLong = 15000000000L; // Longs must have an L at the end of the number
float myFloat = 5.12f; // Floats must have an f at the end
double myDouble = 19.99d; // Doubles must end with a d
char myChar = 'F'; // Chars must use single quotes

As well as the above primitive types, there is also a reference type that we'll need to get comfortable with early on: String. Don't worry about what a reference type is right now, we can use strings in the same way we use the primitive types above but the first letter of the data type is capitalised. E.g.

String myString = "Hello, world!"; // Strings must use double quotes
// Strings are used to store a collection of characters

There are a number of useful functions we use often with strings, for example:

String name = "Nathan";
int length = name.length(); // 6
String upper = name.toUpperCase(); // "NATHAN"

Here's some useful reference guides:

• https://www.w3schools.com/java/java_ref_string.asp
• https://docs.oracle.com/en/java/javase/18/docs/api/java.base/java/lang/String.html#method-summary

Set up instructions

• Fork this repository and clone the forked version to your machine
• Open the root directory of the project in IntelliJ
• Implement the requirements listed in comments in the ./src/main/java/com.booleanuk/core/Exercise.java file
• When ready to test your solution, open the ./src/test/java/com.booleanuk/core/ExerciseTest.java file and click a "Run Test" button. You can either run the entire test suite via figure 1 in the screenshot below, or run a specific test via figure 2.

Note: Some of the requirements in this exercise may require you to use documentation to figure out the correct solution. When that is the case, links have been provided. Get used to this! A big part of our process is reading documentation to try and find useful information.

Extensions

Create some Java classes that could represent any real world object. Start with the sample list below which gets increasingly more difficult, and then think of some of your own.

1. Book (example)

public class Book {
    public String title = "A Game of Thrones";
    public int publishYear = 1996;
    public float price = 11.99f;
    public String isbn = "0-00-224584-1";
    public String author = "George R. R. Martin";
}

2. Car

   • Make, model, transmission type, number of doors, engine litres, engine type, colour, registration

3. Movie

4. Computer

5. Social media post

6. User

7. E-commerce product

8. E-commerce order

Test Output

When you run a test, it's either going to pass or fail. When it fails, you'll be presented with a big red stream of text. This is called a stack trace and, though intimidating, does contain some useful information.

One of the core skills of a developer is debugging stack traces like this. The stack trace details in which classes & files the failure happened, and gives you a line number at the end. Most of the lines in the stack trace are irrelevant most of the time, you want to try and identify the files that you're actually working with.

In the sample screenshot below, we've tried to complete the first step of the exercise but provided an invalid value. Then we run the test associated with it and we see a big red stack trace, a test failure.

At the top, we see expected: <32> but was: <33>. This means the test expected the value to be 32, but the value the student provided was 33. We can see this in the code snippets at the top of the screenshot.

In the stack trace itself, we see this line: at app//com.booleanuk.core.ExerciseTest.shouldBeAged32(ExerciseTest.java:20). This is helpful! This tells us the exact line in the ExerciseTest.java file (line 20) where the failure happened, as well as the method name (shouldBeAged32), helping us to identify where the issue began. This is the kind of thing you need to look for; a relevant file name, method name, class name and line number to give you a good starting point for debugging.