Great news, Rubyists and JavaScripters, if you know ES6 and have read up on object-oriented Ruby, it's not terribly difficult to learn Java. I know because I've been learning it over the past few months, and I've survived! Here are some basics I've picked up along the way that may help you get started.

static typing

This means that the type of the variable (boolean, int, string, etc.) is known at compile time. Java is a statically typed language, which means that it'll catch incompatible type errors when it tries to compiles your code into lower level machine code.

types

The eight main primitive data types in Java are boolean, char, byte, short, int, long, float, and double. The ones I've most commonly used are boolean, int, long, float, and double. Booleans are either true or false. Ints are numbers that occupy up to 32 bits. Longs are like bigger numbers that can occupy up to 64 bits. Float are decimal values up to 32 bits, and doubles are decimal values up to 64 bits. Strings aren't primitive types but actually reference the String class in Java. I do end up using that type frequently.

static

The static keyword means that a variable or method belongs to the class rather than an instance of a class. You've probably also seen this in ES6 syntax to denote that something belongs to the "class" rather than the instance.

Here's a trivial example.

public class IceCream {
  public static String getMessage() {
      return "I love ice cream!";
  }

  public static void main(String[] args) {
      System.out.print(IceCream.getMessage());
  }
}

The main method in a Java class pretty much means run the code in here. So above, I'm saying print out the result of this static method call. It prints out "I love ice cream!"

In Ruby, you'd have to do something like this to achieve the same thing:

class IceCream
  def self.get_message
    "I love ice cream!" 
  end

  puts self.get_message
end

Above, you'll see a class method as well as an invocation of the class method when the class is loaded. Yes, you can call methods in the class definition on load, but it's a little weird.

This is all in contrast to instance variables and methods.

public class IceCream {
  public static String getMessage() {
      return "I love ice cream!";
  }

  public String getOtherMessage() {
    return "I the instance love ice cream too!";
  }

  public static void main(String[] args) {
      // This is how you instantiate a new instance of a class
      IceCream iceCream = new IceCream();
      System.out.print(iceCream.getOtherMessage());
  }
}

In the main method, I'm instantiating a new instance of the IceCream class and storing it in a variable called iceCream. You'll also notice that I've declared that variable's type as IceCream. Then, in the line that prints out to the console, I'm calling the instance method getOtherMessage on the instance of the IceCream class.

final

The final keyword is like setting a constant. When you use the final keyword, you cannot reset the value of the thing you've declared final. In JavaScript ES6, you'd probably use the const keyword to denote the same thing. In Ruby, you'd set your constant in all caps at the top of your class.

public class IceCream {
  public static void main(String[] args) {
      final int version = 1;
      version = IceCream.resetVersion(version);
  }
}

Here, you wouldn't be able to build your project and would get the error "cannot assign a value to final variable version."

interface

Interfaces are sort of like classes, but they can't be instantiated. They provide a contract that ensures that a class implements certain methods. As this handy tutorial explains, interfaces only include method signatures, but not the implementation of methods. To ensure that a class implements an interface, you'd write something like this:

public class IceCream implements Eatable {

}

where Eatable is my interface.

The Eatable interface might look something like this:

public interface Eatable {
    public String delicious = "delicious";
    public String eat();
    public String digest();
}

Now, the IceCream class would have to implement the eat and digest methods. It would have access to the delicious variable as well. That wouldn't need to be defined.

Interfaces are useful for polymorphism, that is, if you want multiple classes to respond to the same method in a slightly different way.

public, private, protected

These mean nearly the same thing as in Ruby. Public methods can be accessed from outside the class. Private methods can only be accessed within the same class. Protected methods can be accessed by subclasses as well as classes in the same package.

As you probably already know, there is no concept of "public versus private" in JavaScript, but a lot of teams develop their own conventions to indicate to other developers on their team. Also, since the introduction of JavaScript modules, the distinction between public and private has become a little more explicit.

constructors

You've seen constructors in Ruby and JavaScript. The major difference is that a class in Java can have multiple constructors, and the most appropriate one based on the number of arguments passed in is selected.

The other thing that was strange to me at first was that constructors have a method signature equivalent to the name of a class. So a constructor for the ice cream class would look like this:

public class IceCream {
    public IceCream() {
    }
}

Here it is with some arguments:

public class IceCream {
   // These are instance variables
    int parlorId;
    String flavor;

    public IceCream(int parlorId, String flavor) {
      // Here we're setting the instance variables equal to the arguments
      //  passed in
      // `this` refers to the instance itself
      this.parlorId = parlorId;
      this.flavor = flavor;
    }
}

inheritance

Inheritance functions mostly the same, but the syntax is of course a little different from Ruby. It looks a lot like inheritance in ES6 though.

public class IceCream extends Dessert {

}

Looks a lot like the same behavior in JS, especially as used in React.

class IceCream extends React.Component {
  constructor(props) {
    super(props)
      this.state = {
        parlorId: props.parlorId,
        flavor: props.flavor
      }
  }
  ...
}

void

Void tripped me up for a while. When you're declaring the type of the return value of a function, you'd use void to indicate that there is no return value. If a function doesn't return anything, but just takes some type of action, you'd write something like this:

public static void doAThing() {
    // body in here would take action but not have an explicit return value
}

So that's all for now, all the super basic Java things I've picked up over the past month and a half. Hopefully that'll clear up some confusion for anyone trying to read Java code and eventually learn to write it themselves.

Resources

• Java types
• Default, public, protected, private