Classes Available fields and methods and examples on how to use them Object Parent class of all Java objects.  Parent None (and itself at the same time, don't question it) Variables and Functions Name Type Info toString() String Tag collection type. equals(Object other) boolean Checks equality with another object. hashCode() int Hash code of this object. It is used to optimize maps and other things, should never be used for object equality. class Class Object's type/class. String Class of string objects, such as "abc" (and in JS 'abc' works as well)  Parent Object Variables and Functions Name Type Info empty boolean Returns if string is empty a.k.a string === '' toLowerCase() String Returns a copy of this string, but with all characters in upper case toUpperCase() String Returns a copy of this string, but with all characters in lower case equalsIgnoseCase( String other) boolean Hash code of this object. It is used to optimize maps and other things, should never be used for object equality. length() int Number of characters charAt( int index) char Single character at index Primitive Types Information Primitive types are objects that don't have a real class and don't inherit methods from Object . All primitive types Type Java class Info void Void No type byte Byte 8 bit decimal number. short Short 16 bit decimal number. int Integer 32 bit decimal number, most common decimal type. long Long 64 bit decimal number. float Float 32 bit floating point number. double Double 64 bit floating point number. char Character Single character in String such as 'a' or '-' . boolean Boolean Only true and false values. Can be checked in if function without comparing to true, as if (x) or if (!x) instead of if (x == true) or if (x == false) .