Question

public class Lock{

        // ADD ATTRIBUTES HERE

        
        /** Create a lock for the specified box
         * <p>
         * The lock must be initially locked (closed) when created.
         * The lock must be given a UNIQUE ID value when created.
         *
         * @param box is a Box that will have this lock to keep is shut.
         */
        public Lock(Box box){}

        /** Retrieve the key for this lock.
         * <p>
         * This method can be called multiple times but is only allowed to
         * return its mathcing key the first time it is called. Every time
         * after the mehtod will return null.
         *
         * @return the key that matches this lock (i.e., the key that can unlock it)
         *         if it is the first time called. Returns null otherwise.
         */
        public Key getKey(){return null;}


        /** Check if this lock is locked or not.
         *
         * @return true if this lock is locked and false otherwise.
         */
        public boolean isLocked(){ return false;}

        /** Locks this lock (regardless if it was already locked or not). */
        public void lock(){}

        /** Unlocks the lock if the input key matches the lock.
         * <p>
         * If this lock is already unlocked this method will return true and
         * do nothing else. 
         * If this lock is locked and input key matches this lock, then it
         * unlocks the lock and returns true.
         * If this lock is locked and the input key does not match this lock,
         * then returns false and does nothing else.
         *
         * @param key is key.
         * @return true if the lock was already unlocked OR if the key matches the
         *         lock. Returns false otherwise.
         */
        public boolean unlock(Key key){return false;}

        /** retrieves the ID of the lock
         *
         * @return the ID of this lock.
         */
        public int getID(){return -1;}


}

Answer 1

class Lock {

        // ADD ATTRIBUTES HERE
        private Box box;
        private int id;
        private static HashSet<Integer> ids;
        private boolean locked;
        private Key key;
        private boolean keyReturned;
        
        /**
         * Create a lock for the specified box
         * <p>
         * The lock must be initially locked (closed) when created. The lock must be
         * given a UNIQUE ID value when created.
         *
         * @param box is a Box that will have this lock to keep is shut.
         */
        public Lock(Box box) {
                this.box = box;
                this.id = (int) (Math.random() * 10000);
                while (ids.contains(this.id)) {
                        this.id = (int) (Math.random() * 10000);
                }
                ids.add(this.id);
                this.locked = true;
                this.key = new Key(this);
                this.keyReturned = false;
        }

        /**
         * Retrieve the key for this lock.
         * <p>
         * This method can be called multiple times but is only allowed to return its
         * mathcing key the first time it is called. Every time after the mehtod will
         * return null.
         *
         * @return the key that matches this lock (i.e., the key that can unlock it) if
         *         it is the first time called. Returns null otherwise.
         */
        public Key getKey() {
                if(!keyReturned) {
                        keyReturned = true;
                        return key;
                }
                return null;
        }

        /**
         * Check if this lock is locked or not.
         *
         * @return true if this lock is locked and false otherwise.
         */
        public boolean isLocked() {
                return locked;
        }

        /** Locks this lock (regardless if it was already locked or not). */
        public void lock() {
                locked = true;
        }

        /**
         * Unlocks the lock if the input key matches the lock.
         * <p>
         * If this lock is already unlocked this method will return true and do nothing
         * else. If this lock is locked and input key matches this lock, then it unlocks
         * the lock and returns true. If this lock is locked and the input key does not
         * match this lock, then returns false and does nothing else.
         *
         * @param key is key.
         * @return true if the lock was already unlocked OR if the key matches the lock.
         *         Returns false otherwise.
         */
        public boolean unlock(Key key) {
                if(key.lockID() == id) {
                        locked = false;
                }
                return !locked;
        }

        /**
         * retrieves the ID of the lock
         *
         * @return the ID of this lock.
         */
        public int getID() {
                return id;
        }

}

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