<java>: page edit; add command

Custom input and output when compiling a class with the java command

pages/common/java.md

@@ -13,3 +13,6 @@
 - Display JDK, JRE and HotSpot versions:
 
 `java -version`
+
+- Custom input and output files. Preferably .txt files
+`java {{classname}} <{{inputfile}}> outputfile`