how to find file in Linux

1.Start from '/',Find log file that contain  'crash'

find / -name '*.log' | xargs grep 'crash'

2.Start from '/' ,Find every type of files which contain 'archive'

find / -type f -name "*.*" | xargs grep "archive"

3.Find all “java” files in current folder and sub-folder

find . -name "*.java"

4.Find all files that have been updated for past 15 days

find . -ctime -15

5.Find files which size is 0 and list them all

find / -type f -size 0 -exec ls -l {} \;