What is the shortcut key to back to the previous tab?

Question

For example, after clicking open declaration, is there a shortkey which let me back to the previous tab?

Answer 1

By next tab, you probably mean the "Previous File".

By default switching between open files (called "Editors" in eclipse parlance) can be done by Ctrl+F6 and Ctrl+Shift+F6

This is configurable, so if you find this combination inconvenient (like me) you can change it in the Preferences: General -> Keys. Look for Next Editor and Previous Editor

(I have mapped them to Ctrl+Tab and Ctrl+Shift+Tab.)