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Java Cheat Sheet Mosh Hamedani Code with Mosh (codewithmosh.com) 1st Edition About this Cheat Sheet This cheat sheet includes the materials I’ve covered in my Java tutorial for Beginners on my YouTube channel: https://www.youtube.com/user/programmingwithmosh Both the YouTube tutorial and this c...

Java Cheat Sheet Mosh Hamedani Code with Mosh (codewithmosh.com) 1st Edition About this Cheat Sheet This cheat sheet includes the materials I’ve covered in my Java tutorial for Beginners on my YouTube channel: https://www.youtube.com/user/programmingwithmosh Both the YouTube tutorial and this cheat cover the core language constructs and they are not complete by any means. If you want to learn everything Java has to offer and become a Java expert, check out my Ultimate Java Mastery Series. https://codewithmosh.com/courses/the-ultimate-java-mastery-series About the Author Hi! My name is Mosh Hamedani. I’m a software engineer with two decades of experience and I’ve taught over three million people how to code or how to become a professional software engineer. It’s my mission to make software engineering simple and accessible to everyone. https://codewithmosh.com https://youtube.com/user/programmingwithmosh https://twitter.com/moshhamedani https://facebook.com/programmingwithmosh/ Basics...................................................................6 Java Development Kit..............................................................................6 Java Editions............................................................................................6 How Java Code Gets Executed................................................................ 6 Architecture of Java Applications..........................................................6 5 Interesting Facts about Java................................................................ 7 Types....................................................................8 Variables................................................................................................... 8 Primitive Types.......................................................................................8 Declaring Variables.................................................................................8 Comments.................................................................................................9 Reference Types........................................................................................ 9 Strings.......................................................................................................9 Useful String Methods.............................................................................9 Escape Sequences................................................................................... 10 Arrays.....................................................................................................10 The Array Class.......................................................................................11 Multi-dimensional Arrays......................................................................11 Constants................................................................................................. 11 Arithmetic Expressions........................................................................... 11 Order of Operations............................................................................... 12 Casting.................................................................................................... 12 Formatting Numbers.............................................................................. 13 Reading Input........................................................................................13 Control Flow...................................................... 14 Comparison Operators........................................................................... 14 Logical Operators...................................................................................14 If Statements...........................................................................................14 The Ternary Operator............................................................................15 Switch Statements...................................................................................15 For Loops................................................................................................ 16 While Loops.............................................................................................16 Do..While Loops......................................................................................16 For-each Loops.......................................................................................16 Want to Become a Java Expert?...........................................................18 Basics Java is one of the most popular programming languages in the world. With Java you can build various types of applications such as desktop, web, mobile apps and distributed systems. Java Development Kit We use Java Development Kit (JDK) to build Java applications. JDK contains a compiler, the Java Runtime Environment (JRE) and a library of classes that we use to build applications. Java Editions We have four editions of Java, each used for building a different type of application: Java Standard Edition (SE): the core Java platform. It contains all of the libraries that every Java developer must learn. Java Enterprise Edition (EE): used for building very large scale, distributed systems. It’s built on top of Java SE and provides additional libraries for building fault-tolerant, distributed, multi-tier software. Java Micro Edition (ME): a subset of Java SE, designed for mobile devices. It also has libraries specific to mobile devices. Java Card: used in smart cards. How Java Code Gets Executed The Java compiler takes Java code and compiles it down to Java Bytecode which is a cross-platform format. When we run Java applications, Java Virtual Machine (JVM) gets loaded in the memory. It takes our bytecode as the input and translates it to the native code for the underlying operating system. There are various implementations of Java Virtual Machine for almost all operating systems. Architecture of Java Applications The smallest building blocks in Java programs are methods (also called functions in other programming languages). We combine related methods in classes, and related classes in packages. This modularity in Java allows us to break down large programs into smaller building blocks that are easier to understand and re-use. 5 Interesting Facts about Java 1. Java was developed by James Gosling in 1995 at Sun Microsystems (later acquired by Oracle). 2. It was initially called Oak. Later it was renamed to Green and was finally renamed to Java inspired by Java coffee. 3. Java has close to 9 million developers worldwide. 4. About 3 billion mobile phones run Java, as well as 125 million TV sets and every Blu-Ray player. 5. According to indeed.com, the average salary of a Java developer is just over $100,000 per year in the US. Types Variables We use variables to temporarily store data in computer’s memory. In Java, the type of a variable should be specified at the time of declaration. In Java, we have two categories of types: Primitives: for storing simple values like numbers, strings and booleans. Reference Types: for storing complex objects like email messages. Primitive Types Type Bytes Range byte 1 [-128, 127] short 2 [-32K, 32K] int 4 [-2B, 2B] long 8 float 4 double 8 char 2 A, B, C, … boolean 1 true / false Declaring Variables byte age = 30; long viewsCount = 3_123_456L; float price = 10.99F; char letter = ‘A’; boolean isEligible = true; In Java, we terminate statements with a semicolon.  We enclose characters with single quotes and strings (series of characters) with double quotes. The default integer type in Java is int. To represent a long value, we should add L to it as a postfix. The default floating-point type in Java is double. To represent a float, we should append F to it as a postfix. Comments We use comments to add notes to our code. // This is a comment and it won’t get executed. Reference Types In Java we have 8 primitive types. All the other types are reference types. These types don’t store the actual objects in memory. They store the reference (or the address of) an object in memory. To use reference types, we need to allocate memory using the new operator. The memory gets automatically released when no longer used. Date now = new Date(); Strings Strings are reference types but we don’t need to use the new operator to allocate memory to them. We can declare string variables like the primitives since we use them a lot. String name = “Mosh”; Useful String Methods The String class in Java provides a number of useful methods: startsWith(“a”) endsWith(“a”) length()  indexOf(“a”) replace(“a”, “b”) toUpperCase() toLowerCase() Strings are immutable, which means once we initialize them, their value cannot be changed. All methods that modify a string (like toUpperCase) return a new string object. The original string remains unaffected. Escape Sequences If you need to use a backslash or a double quotation mark in a string, you need to prefix it with a backslash. This is called escaping. Common escape sequences: \\ \” \n (new line) \t (tab) Arrays We use arrays to store a list of objects. We can store any type of object in an array (primitive or reference type). All items (also called elements) in an array have the same type. // Creating and and initializing an array of 5 elements int[] numbers = new int; numbers = 10; numbers = 20; numbers = 30; // Shortcut int[] numbers = { 10, 20, 30 }; Java arrays have a fixed length (size). You cannot add or remove new items once you instantiate an array. If you need to add new items or remove existing items, you need to use one of the collection classes. The Array Class The Array class provides a few useful methods for working with arrays. int[] numbers = { 4, 2, 7 }; Arrays.sort(numbers); String result = Arrays.toString(numbers); System.out.println(result); Multi-dimensional Arrays // Creating a 2x3 array (two rows, three columns) int matrix = new int; matrix = 10; // Shortcut int matrix = { { 1, 2, 3 }, { 4, 5, 6 } }; Constants Constants (also called final variables) have a fixed value. Once we set them, we cannot change them. final float INTEREST_RATE = 0.04; By convention, we use CAPITAL LETTERS to name constants. Multiple words can be separated using an underscore. Arithmetic Expressions int x = 10 + 3; int x = 10 - 3; int x = 10 * 3; int x = 10 / 3; // returns an int float x = (float)10 / (float)3; // returns a float int x = 10 % 3; // modulus (remainder of division) Increment and Decrement Operators int x = 1; x++; // Equivalent to x = x + 1 x--; // Equivalent to x = x - 1 Augmented Assignment Operator int x = 1; x += 5; // Equivalent to x = x + 5 Order of Operations Multiplication and division operators have a higher order than addition and subtraction. They get applied first. We can always change the order using parentheses. int x = 10 + 3 * 2; // 16 int x = (10 + 3) * 2; // 26 Casting In Java, we have two types of casting: Implicit: happens automatically when we store a value in a larger or more precise data type. Explicit: we do it manually. // Implicit casting happens because we try to store a short // value (2 bytes) in an int (4 bytes). short x = 1; int y = x; // Explicit casting int x = 1; short y = (short) x; To convert a string to a number, we use one of the following methods: Byte.parseByte(“1”) Short.parseShort(“1”) Integer.parseInt(“1”) Long.parseLong(“1”) Float.parseFloat(“1.1”) Double.parseDouble(“1.1”) Formatting Numbers NumberFormat currency = NumberFormat.getCurrencyInstance(); String result = currency.format(“123456”); // $123,456 NumberFormat percent = NumberFormat.getPercentInstance(); String result = percent(“0.04”); // 4% Reading Input Scanner scanner = new Scanner(system.in); double number = scanner.nextDouble(); byte number = scanner.nextByte(); String name = scanner.next(); String line = scanner.nextLine(); Control Flow Comparison Operators We use comparison operators to compare values. x == y // equality operator x != y. // in-equality operator x > y x >= y x < y x 100_000) ? “First” : “Economy”; This is a shorthand to write the following code: String className; if (income > 100_000) className = “First”; else className = “Economy”; Switch Statements We use switch statements to execute different parts of the code depending on the value of a variable. switch (x) { case 1: … break; case 2: … break; default: … } After each case clause, we use the break statements to jump out of the switch block. For Loops For loops are useful when we know ahead of time how many times we want to repeat something. We declare a loop variable (or loop counter) and in each iteration we increment it until we reach the number of times we want to execute some code. for (int i = 0; i < 5; i++) statement While Loops While loops are useful when we don’t know ahead of time how many times we want to repeat something. This may be dependent on the values at run-time (eg what the user enters). while (someCondition) { … if (someCondition) break; } We use the break statement to jump out of a loop. Do..While Loops Do..While loops are very similar to while loops but they executed at least once. In contrast, a while loop may never get executed if the condition is initially false. do { … } while (someCondition); For-each Loops For-each loops are useful for iterating over an array or a collection. int[] numbers = {1, 2, 3, 4}; for (int number : numbers) … Want to Become a Java Expert? If you’re serious about learning Java and getting a job as a Java developer, I highly encourage you to enroll in my Ultimate Java Mastery Series. Don’t waste your time following disconnected, outdated tutorials. My Ultimate Java Mastery Series has everything you need in one place: More than 10 hours of HD video Unlimited access - watch it as many times as you want Self-paced learning - take your time if you prefer Watch it online or download and watch offline Certificate of completion - add it to your resume to stand out 30-day money-back guarantee - no questions asked Click here to get started: https://codewithmosh.com/courses/the-ultimate-java-mastery-series