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Chapter 16

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Data structures that take control of organizing elements Elements not in fixed positions Advantage better performance Adding Removing Finding

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Unordered collection of distinct elements Fundamental operations Adding an element Removing an element Containment testing (is something there) Listing arbitrary order Duplicates Arent permitted Attempt to add ignored Attempt to remove non existent data - ignored

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Hash tables Trees Uses set interface > Hash SetTree Set

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Create a Hash Set of Strings Set names = new HashSet (); Adding and deleting names.add(Romeo); names.remove(Juliet); Test to see if element is in set if (names.contains(Juliet);

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Iterator iter=names.iterator(); while (iter.hasNext()) { String name = iter.next(); do something with the string }

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for (String name: names) { do something with names { Iterating through elements Does not visit in order inserted Set implementation rearranges for quick location Cannot add in iterator position Create a print method Should be usable by has or tree

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Data type Associates keys and values A function from one set(key) to another set(value) Every key has unique value A value may have multiple keys Two kinds of implementation Hash Map Tree Map Both implement Map Interface

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Map favoriteColors = new HashMap

Find all keys and values in map Set keySet = m.keySet(); For(String key: KeySet) { Color value=m.get(key); System.out.println(Key + -> + value); }

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Find elements in a data structure quickly No linear search through all elements Can implement sets and maps Utilizes hash codes Function that computes an integer value from an object Integer value = has code Object class has a hashCode method Int h = x.hashCode(); Collision 2 objects with same hash code Good has functions minimizes collision Hash Code index into hash table

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Simplest form Array index to hash table In array insert each object @ location of its hash code If anything already in array object is present

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Eve Jim Joe [70068] [74478] [74656]

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Problem with this idea Not possible to allocate an array that is large enough to hold all possible integer index positions Must reduce hash function to fit the array size int h = x.hashCode(); if (h