question: private List < File > findDuplicateFiles ( List < File > files ) { HashSet < File > sourceFileSet = new HashSet < > ( ) ; List < File > duplicateFiles = new ArrayList < > ( ) ; for ( File file : files ) { if ( ! sourceFileSet . contains ( file ) ) { sourceFileSet . add ( file ) ; } else { duplicateFiles . add ( file ) ; } } return duplicateFiles ; }
target:  Returns a list of all duplicate files found in the specified list of files.