[Solved]-When deploying GAE I get "IllegalArgumentException: Class file is Java 8 but max supported is Java 7", but I'm using Java 7 everywhere-eclipse

you may try this also

in the project explorer view, right-click on the project and then select properties. select the java build path page in the properties window. go to libraries tab, click jre system library. if the jre is (jre 8 or jre 1.8), then edit select execution environment is 1.7.

press finish.

remember to press apply, in the java build path page. then ok.

try a clean build

changing the jdk compliance fixed the issue for me.
you can change the compiler compliance level by going to the java compiler page as shown in the steps below.

1. in the project explorer view, right-click on the project and then select properties.
2. select the java compiler page in the properties window.
3. in the jdk compliance section, select the desired compiler compliance level. (jdk 7)
4. click apply and then ok. see this