question: @ Override public Object put ( Object key , Object value ) { Entry tab [ ] = table ; int hash = 0 ; int index = 0 ; if ( key != null ) { hash = System . identityHashCode ( key ) ; index = ( hash & 0x7FFFFFFF ) % tab . length ; for ( Entry e = tab [ index ] ; e != null ; e = e . next ) { if ( ( e . hash == hash ) && key == e . key ) { Object old = e . value ; e . value = value ; return old ; } } } else { for ( Entry e = tab [ 0 ] ; e != null ; e = e . next ) { if ( e . key == null ) { Object old = e . value ; e . value = value ; return old ; } } } modCount ++ ; if ( count >= threshold ) { rehash ( ) ; tab = table ; index = ( hash & 0x7FFFFFFF ) % tab . length ; } Entry e = new Entry ( hash , key , value , tab [ index ] ) ; tab [ index ] = e ; count ++ ; return null ; }
target:  Associates the specified value with the specified key in this map. If the map previously contained a mapping for this key, the old value is replaced.