In response to: http://stackoverflow.com/questions/14676802/auto-complete-with-emacs-24-doesnt-work-with-java-c-or-c-modes/14677270#comment20521862_14677270

init.el

;; Initialize packages.

(require 'package)

(dolist (source '(("melpa" . "http://melpa.milkbox.net/packages/")
                  ("marmalade" . "http://marmalade-repo.org/packages/")))
  (add-to-list 'package-archives source))

(package-initialize)
(unless package-archive-contents
  (package-refresh-contents))

(defun require-package (p)
  (unless (package-installed-p p)
    (package-install p))
  (require p))

(require-package 'auto-complete)
(require-package 'yasnippet)

;; ------------------------------------------------------------------------------

;; Themeing
(add-to-list 'custom-theme-load-path "~/.emacs.d/themes/")
(load-theme 'Nico t)
(set-face-attribute 'default nil :font "DejaVu Sans Mono-11")
 
;; Default windows size 
(add-to-list 'default-frame-alist '(height . 24))
(add-to-list 'default-frame-alist '(width . 80))


(global-linum-mode t)
(setq-default inhibit-startup-screen t)
(setq-default initial-scratch-message nil)
(show-paren-mode 1)

(setq ring-bell-function 'ignore)

(setq-default c-basic-offset 4)

;; ido-mode
(setq ido-enable-flex-matching t)
(setq ido-everywhere t)
(ido-mode 1)

;; yes or no becomes y or n
(defalias 'yes-or-no-p 'y-or-n-p)

;;yasnippet
(yas-global-mode 1)

;; auto-complete
(require 'auto-complete-config)
(add-to-list 'ac-dictionary-directories "~/.emacs.d/elpa/auto-complete/dict")

(set-default 'ac-sources
             '(ac-source-abbrev
               ac-source-dictionary
               ac-source-yasnippet
               ac-source-words-in-buffer
               ac-source-words-in-same-mode-buffers
               ac-source-semantic))

(ac-config-default)

(dolist (m '(c-mode c++-mode java-mode))
  (add-to-list 'ac-modes m))

(global-auto-complete-mode t)

Works great!