15: COLLECTIONS AND MAPS in Java

Making Quick Response Code in Java 15: COLLECTIONS AND MAPS
CHAPTER 15: COLLECTIONS AND MAPS
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154 Sets
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Unlike other implementations of the Collection interface, implementations of the Set interface do not allow duplicate elements This also means that a set can contain at most one null value The Set interface does not define any new methods, and its add() and addAll() methods will not store duplicates If an element is not currently in the set, two consecutive calls to the add() method to insert the element will first return true, then false A Set models a mathematical set (see Table 153), that is, it is an unordered collection of distinct objects Multisets (also called bags) that allow duplicate elements cannot be implemented using the Set interface, since this interface requires that elements are unique in the collection
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Table 153
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Bulk Operations and Set Logic Set Methods (a and b are sets)
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acontainsAll(b) aaddAll(b) aremoveAll(b) aretainAll(b) aclear()
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Corresponding Mathematical Operations
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(subset)
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a = a
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(union) (difference) (intersection)
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a = a b a = a a = b
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(empty set)
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The HashSet<E> and LinkedHashSet<E> Classes
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The HashSet class implements the Set interface Since this implementation uses a hash table, it offers near constant-time performance for most operations A HashSet does not guarantee any ordering of the elements However, the LinkedHashSet subclass of HashSet guarantees insertion-order It is also the implementation of choice if frequent traversal over the set is necessary The sorted counterpart is TreeSet, which implements the SortedSet and the NavigableSet interfaces and has logarithmic time complexity (see Section 155, p 800) A HashSet relies on the implementation of the hashCode() and equals() methods of its elements (see Section 151, p 748) The hashCode() method is used for hashing the elements (p 760), and the equals() method is needed for comparing elements In fact, the equality and the hash codes of HashSets are defined in terms of the equality and the hash codes of their elements
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HashSet()
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Constructs a new, empty set
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154: SETS
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HashSet(Collection c)
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Constructs a new set containing the elements in the specified collection The new set will not contain any duplicates This offers a convenient way to remove duplicates from a collection
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HashSet(int initialCapacity)
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Constructs a new, empty set with the specified initial capacity
HashSet(int initialCapacity, float loadFactor)
Constructs a new, empty set with the specified initial capacity and the specified load factor As mentioned earlier, the LinkedHashSet implementation is a subclass of the HashSet class It works similarly to a HashSet except for one important detail Unlike a HashSet, a LinkedHashSet guarantees that the iterator will access the elements in insertion order, that is, in the order in which they were inserted into the LinkedHashSet The LinkedHashSet class offers constructors analogous to the ones in the HashSet class The initial capacity (ie, the number of buckets in the hash table) and its load factor (ie, the ratio of number of elements stored to its current capacity) can be tuned when the set is created The default values for these parameters will under most circumstances provide acceptable performance Example 1515 demonstrates traversing a HashSet (see (1)) and a LinkedHashSet (see (2)) Regardless of the order in which elements are inserted into a HashSet, we cannot depend on the order the for(:) loop will traverse the elements in the set, as is evident from the program output A LinkedHashSet, on the other hand, is always traversed in insertion order (ie, the last element inserted, is the first element retrieved) by the for(:) loop The program output confirms this behavior, as the meal that was inserted last into the LinkedHashSet, is served first The same behavior will be exhibited if an iterator is used explicitly
Example 1515 Traversing Over Sets import javautilHashSet; import javautilLinkedHashSet; import javautilSet; public class TraverseHashSetAndLinkedHashSet { public static void main(String[] args) { HashSet<String> set1 = new HashSet<String>(); // (1) set1add("breakfast"); set1add("lunch"); set1add("dinner"); Systemoutprintln("Serving meals from a HashSet (order can vary):"); for (String meal : set1) Systemoutprintln(meal); Set<String> set2 = new LinkedHashSet<String>(); set2add("breakfast"); set2add("lunch"); set2add("dinner"); Systemoutprintln("Serving meals from a LinkedHashSet" + " (always insertion order):"); // (2)
798 for (String meal : set2) Systemoutprintln(meal); } }