Press the Tab key in Eclipse to set 8 spaces

Window --> Preferences --> Java --> Code Style --> Formatter
You must create a new Formatter. Otherwise, the system prompts that you cannot Edit the Formatter. This is the default setting. Then, select the Edit button on the right, and select Space only in the drop-down box on the right of the Tab policy on the Indentation --> General Settings page!
Then, set the Indentation size and Tab size to the number of spaces you want! Generally, there are 8 spaces !!

Note: In versions earlier than 3.6, it seems that the options on the Right of Window --> Preferences --> General --> Editors --> Text Editors are checked! Then Displayed tab width = 8! But not 3.6!