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I was using Netbeans and had no problem. I migrated to Eclipse a short time ago and now I went to make a code and Eclipse doesn’t even recognize the main Java Classes. System, Scanner, etc. What might it be? Any incompatibility with JDK? Any settings in Eclipse that I did wrong? Follow screenshot. 
Already checked if the JDK is configured correctly in the eclipse?
– Filipe L. Constante
Most likely is the configuration of JDK, look and return to us as there ta.
– Isaías de Lima Coelho
Right-click the project > Properties > Tab Libraries; Select JRE System Library and click Edit. Select the Workspace default JRE (Latest version of JRE). Click Finish and then OK. Let us know if it works.
– Ronaldo Araújo Alves
@Ronaldoaraújoalves Apparently the problem was in JRE itself, I entered the following right menu in the project > Properties > Java Build Path > Libraries and JRE was unlinked(Unbound) from the project, I changed and saved the modification. Is working normally.
– LaSu