please finish the todo parts and explian

import java.util.Map;

import java.util.HashMap;

import java.util.Collection;

$/**$

$*$ An Inventory implemented using a HashMap.

$*$ Keys are Videos; Values are Records.

$*$

$*$ @objecttype Mutable Collection of Records

$*$ @objectinvariant

$*$ Every key and value in the map is non$-$null.

$*$ @objectinvariant

$*$ Each value r is stored under key r$.$video.

$*/$

final class InventorySet $\{$

$/**$ @invariant $\_$data $!=$ null $*/$

private final Map $\_$data $=$ new HashMap$()$;

InventorySet$()\{\}$

$/**$

$*$ Return the number of Records.

$*/$

public int size$()\{$

return $0$;

$//$ TODO

$\}$

$/**$

$*$ Return a copy of the record for a given Video; if not present, return null.

$*/$

public Record get$($Video v$)\{$

return null;

$//$ TODO

$\}$

$/**$

$*$ Return a copy of the records as a collection.

$*$ Neither the underlying collection, nor the actual records are returned.

$*/$

public Collection toCollection$()\{$

$//$ TODO

return null;

$\}$

$/**$

$*$ Add or remove copies of a video from the inventory.

$*$ If a video record is not already present $($and change is

$*$ positive$),$ a record is created.

$*$ If a record is already present, numOwned is

$*$ modified using change.

$*$ If change brings the number of copies to be zero,

$*$ the record is removed from the inventory.

$*$ @param video the video to be added.

$*$ @param change the number of copies to add $($or remove if negative$).$

$*$ @throws IllegalArgumentException if video null, change is zero,

$*$ if attempting to remove more copies than are owned, or if

$*$ attempting to remove copies that are checked out.

$*$ @postcondition changes the record for the video

$*/$

public void addNumOwned$($Video video, int change$)\{$

$//$ TODO

$\}$

$/**$

$*$ Check out a video.

$*$ @param video the video to be checked out.

$*$ @throws IllegalArgumentException if video has no record or numOut

$*$ equals numOwned.

$*$ @postcondition changes the record for the video

$*/$

public void checkOut$($Video video$)\{$

$//$ TODO

$\}$

$/**$

$*$ Check in a video.

$*$ @param video the video to be checked in$.$

$*$ @throws IllegalArgumentException if video has no record or numOut

$*$ non$-$positive.

$*$ @postcondition changes the record for the video

$*/$

public void checkIn$($Video video$)\{$

$//$ TODO

$\}$

$/**$

$*$ Remove all records from the inventory.

$*$ @postcondition size$()==0$

$*/$

public void clear$()\{$

$\_$data.clear$()$;

$\}$

$/**$

$*$ Return the contents of the inventory as a string.

$*/$

public String toString$()\{$

StringBuilder buffer $=$ new StringBuilder$()$;

buffer.append$("$Database:

$")$;

for $($Record r : $\_$data.values$())\{$

buffer.append$("")$;

buffer.append$($r$)$;

buffer.append$("$

$")$;

$\}$

return buffer.toString$()$;

$\}$

$\}$

p