Code storage from some of the multiples programming challenges that exists:

• programame: http://www.programa-me.com/
• project-euler: http://www.projecteuler.net
• tuenti challenges: https://contest.tuenti.net
• codingbat: http://codingbat.com/java
• leetcode: http://leetcode.com
• codewars: http://www.codewar.com