Java for JavaScript Developers

Java?

Java is an object-oriented programming language. It's compiled to byte code which can then run on any Java Virtual Machine (JVM).

Why Java?

1. Fast: Java is fast! To execute a program you must first compile it down to byte code. The Java Virtual Machine (JVM) then interprets the byte code much like how a JavaScript engine (like v8) interprets JavaScript. However, byte code runs faster since it is much closer to the native language of the computer.
2. Agnostic: Once compiled, it can be ran on any computer that has the JVM installed. Write once, run anywhere.
3. Strict: Java is great for large development teams since (a) the language itself is statically-typed so it's much stricter than JavaScript and (b) errors are generally caught earlier in the compilation phase.
4. Popular: It's the most widely taught programming language. Plus, Java developers are typically the highest paid.

Hello World

JavaScript:

console.log('Hello World!');

Java:

public class Hello {
  public static void main(String[] args) {
    System.out.println("Hello World!");
  }
}

Save this in a new file called Hello.java. The file name must be the same as the public class.

How do we run a Java program?

Unlike JavaScript, we can't just run the code as is. It's a two-step process:

$ javac Hello.java
$ java Hello
Hello World!

The first command compiles the source code down to the byte code, while the second executes the code.

What's happening?

Let's jump back to the code...

public class Hello {
  public static void main(String[] args) {
    System.out.println("Hello World!");
  }
}

First, we defined a public (available to anyone) class called Hello. Then we defined a public method called main:

1. public - a type of method that anyone can call
2. static - a static method is part of class, but we do not need to create a instance to call it
3. void - indicates to the compiler that this method does not return a value
4. args - name of the parameter
5. String[] - value of the parameter, an array of strings

The main method is executed when the file is ran. Let's pass in some arguments:

public class Hello {
  public static void main(String[] args) {
    System.out.println("Hello " + args[0] + "!");
  }
}

Compile then run:

$ javac Hello.java
$ java Hello Java
Hello Java!

Data Types

Since JavaScript is dynamically-typed you don't have to declare function or variable types. JavaScript engines just figure it out. Java, on the other hand, is a statically-typed language. You have to explicitly declare the types of variables, parameters, and return values. Further, you cannot change a type once it's been declared.

Java has primitive and reference types. Primitives are the most basic building blocks, while reference types are based on a class made up of primitives.

• Primitives: int, long, float, double, boolean, char, byte, short
• Reference types: String, Scanner, Random, Die, int[], String[]

Variables

Again, you must specify a type when declaring a variable:

public class Sum {
  public static void main(String[] args) {
    int num = 2;
    System.out.println(num);
  }
}

Compile then run:

$ javac Sum.java
$ java Sum
2

Variables declared inside of a method are local to that method and are called local variables. Instance variables are available anywhere in a class and can be used in any non-static method.

Classes

JavaScript utilizes prototypes to create reusable behavior, while Java utilizes classes.

public class Person {

  public String name;

  public Person(String name) {
    this.name = name;
  }

  public String name() {
    return this.name;
  }

  public static void main(String[] args) {
    Person teacher = new Person("Michael");
    System.out.println(teacher.name());
  }

}

Take note of this like - Person teacher = new Person("Michael");. Here, we invoked the constructor to create a new instance. The constructor has the same name, Person, as the class.

What else is new here? The name() method is not static (so it needs to be instantiated to use) and the appearance of this...

Methods (instance vs static)

• Static methods: Part of the class. Do not have to be instantiated to use.
• Instance methods: Must be instantiated to use.

this

In Java this is always refers back to the current instance.