The 5 Branches of Philosophy
Life As the Moral Standard
Reason Is Man's Means of Survival
Harmony Of Interests
Justice is the virtue of treating people in accordance with their actions. It is applying the law of identity to people. All people are not equal. Some are good and some are evil and they must be treated accordingly.
Acting justly requires the willingness to judge. You must identify people and their actions as being good or evil and act on it. Justice is judging and acting on your judgment. But judging requires a standard of judgment, and that standard should be the same as your moral standard, man's life.
Injustice is turning the other cheek or showing mercy to wrongdoers. Those actions are evasions of reality because you are acting as if a person hasn't acted the way they have. Injustice is also not recognizing achievement and greatness in others -- specifically treating an achiever as a non-achiever or worse. Judging people by methods other than their actions (such as racism) is also an injustice.
Acting justly is in your self interest because good people around you will feel recognized and continue their good behavior. Bad people around you will be encouraged to stop their bad behavior because of the consequences. Also, the people around you will tend to treat you justly, which is a good thing assuming that you live a normal, productive life.