Is Java a 100% pure object oriented language ?

Although people widely consider it pure OO Language, Java isn't 100% pure OO language.

That is because in order to say pure object oriented means that EVERYTHING in the language is an object, treated as an object, and calculations/changes are made through methods. 

Java has primitive types that are the predefined types which are NOT stored in the heap. I'm not sure why the designers of the language made the primitive types like that.

Nope, Many of the most widely used programming languages are multi-paradigm programming languages that support object-oriented programming to a greater or lesser degree, typically in combination with imperative, procedural programming. Significant object-oriented languages include Java, C++, C#, Python, PHP, Ruby, Perl, Delphi, Objective-C, Swift, Common Lisp, and Smalltalk.

Languages called "pure" OO languages, because everything in them is treated consistently as an object, from primitives such as characters and punctuation, all the way up to whole classes, prototypes, blocks, modules, etc. They were designed specifically to facilitate, even enforce, OO methods. Examples: Ruby, Scala, Smalltalk, Eiffel, Emerald, JADE, Self.

Languages designed mainly for OO programming, but with some procedural elements. Examples: Java, C++, C#, Delphi/Object Pascal, VB.NET.

Languages that are historically procedural languages, but have been extended with some OO features. Examples: PHP, Perl, Visual Basic (derived from BASIC), MATLAB, COBOL 2002, Fortran 2003, ABAP, Ada 95, Pascal.

Languages with most of the features of objects (classes, methods, inheritance), but in a distinctly original form. Examples: Oberon (Oberon-1 or Oberon-2).

Languages with abstract data type support which may be used to resemble OO programming, but without all features of object-orientation. This includes object-based and prototype-based languages. Examples: JavaScript, Lua, Modula-2, CLU.

Chameleon languages that support multiple paradigms, including OO. Tcl stands out among these for TclOO, a hybrid object system that supports both prototype-based programming and class-based OO.

It is not 100% pure Object Oriented Language, but we can say that Java support O.O.