Java vs Javascript

Java and Javascript are really two very distinct languages, but they do both share similar underlying C-style syntax. If you're using the Javascript exercises on this site but are actually trying to learn Java, here are a couple key differences to be aware of:
Java
Javascript
Variable Types
 Java has many different types of variables, e.g. int, double, and String. Javascript only has one generic type of variable that holds numbers, strings, objects, etc.
Declaring Variables

Variable type is specified:

int myNum;
String myText = "blah";

Variables are declared using the keyword "var":

var myNum;
var myText = "blah";
Function Headers

Function name is preceded by keywords like "public" and "static", as well as the variable type of the return value:

public static int getNum() {
    return 7;
}

Function name is preceded only by the word "function":

function getNum() {
    return 7;
}
Function Arguments

The formal arguments in a function header are prefixed by their variable type:

public static int doubleMe(int myNum) {
    return myNum * 2;
}

The formal argument names in a function header have no prefix:

function doubleMe(myNum) {
    return myNum * 2;
}

Displaying Text

 Uses libraries to print text to the console, e.g.:

System.out.println("Hello world!");		 Has a function to pop up a message box:

alert("Hello world!");

(however, some of Java's input/output functions can also be used within the Javascript exercises on this site)
Dividing Integers
 In Java, dividing two integers returns another integer, so 12/5 returns 2 (the decimal portion is cropped). Since Javascript doesn't actually have integers (all numbers are stored as 64-bit floats), 12/5 returns 2.4.
Array Size
 The size of a Java array is fixed at creation (new positions can't be added to the array later). Javascript arrays are dynamic; elements can be added or deleted later on.
Array Initialization
 int[ ] myArray = {4, 7, 16, 3, 2, 9, 4}; myArray = [4, 7, 16, 3, 2, 9, 4];

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