15: COLLECTIONS AND MAPS in Java

Drawing QR Code in Java 15: COLLECTIONS AND MAPS
CHAPTER 15: COLLECTIONS AND MAPS
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Adding, removing, and finding entries in a LinkedHashMap can be slightly slower than in a HashMap, as an ordered doubly-linked list has to be maintained Traversal of a map is through one of its collection-views For an underlying LinkedHashMap, the traversal time is proportional to the size of the map regardless of its capacity However, for an underlying HashMap, it is proportional to the capacity of the map The concrete map implementations override the toString() method The standard textual representation generated by the toString() method for a map is
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{key1=value1,
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key2=value2,
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,
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keyn=valuen}
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where each keyi and each valuei is the textual representation generated by the toString() method of the individual key and value objects in the map, respectively As was the case with collections, implementation classes provide a standard constructor that creates a new empty map, and a constructor that creates a new map based on an existing one Additional constructors create empty maps with given initial capacities and load factors The HashMap class provides the following constructors:
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HashMap() HashMap(int initialCapacity) HashMap(int initialCapacity, float loadFactor)
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Constructs a new, empty HashMap, using either specified or default initial capacity and load factor
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HashMap(Map< extends K, extends V> otherMap)
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Constructs a new map containing the elements in the specified map The LinkedHashMap and Hashtable classes have constructors analogous to the four constructors for the HashMap class In addition, the LinkedHashMap class provides a constructor where the ordering mode can also be specified:
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LinkedHashMap(int initialCapacity, float loadFactor, boolean accessOrder)
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Constructs a new, empty LinkedHashMap with the specified initial capacity, the specified load factor, and the specified ordering mode The ordering mode is true for access order and false for key insertion order Example 1521 prints a textual histogram for the frequency of weight measurements in a weight group, where a weight group is defined as an interval of five units The weight measurements are supplied as program arguments The example illustrates the use of maps, the creation of key views, and the use of a for(:) loop to traverse a map The program proceeds as follows: An empty HashMap<Integer, Integer> is created at (1), where the key is the weight group and the value is the frequency
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159: MAP IMPLEMENTATIONS
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A for(:) loop is used at (2) to read the weights specified as program arguments, converting each weight to its corresponding weight group and updating the frequency of the weight group: The weight group is determined at (3) The count is incremented at (4), if necessary, and a new entry is registered at (5) Since keys are unique in a map, any previous entry is overwritten Generic types guarantee that the keys and the values in the map are of the correct type, and autoboxing/unboxing of primitive values guarantees the correct type of an operand in an expression:
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Integer frequency = groupFreqMapget(weightGroup); frequency = (frequency == null) 1 : frequency+1; groupFreqMapput(weightGroup, frequency); // (4) // (5) // (6)
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The program creates a sorted set of keys (which are weight groups) from the groupFreqMap at (7) The keySet() method returns a set view of keys, which is passed as argument to a TreeSet
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SortedSet<Integer> groupSet = new TreeSet<Integer>(groupFreqMapkeySet()); // (7)
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The histogram is printed by traversing the sorted key set in a for(:) loop at (8), looking up the frequency in the groupFreqMap A map can only be traversed through one of its views For each key, the corresponding value (ie, the frequency) is retrieved at (9):
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int frequency = groupFreqMapget(group); // (9)
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A bar (char[]) for each frequency is created using the Arraysfill() method at (10), which is converted to string and printed
Example 1521 Using Maps import import import import import javautilArrays; javautilHashMap; javautilMap; javautilSortedSet; javautilTreeSet;
public class WeightGroups { public static void main(String[] args) { // Create a map to store the frequency for each group Map<Integer, Integer> groupFreqMap = new HashMap<Integer, Integer>(); // Determine the frequencies: for (String argument : args) { // Get the value from an argument and group into intervals of 5 double weight = DoubleparseDouble(argument); int weightGroup = (int) Mathround(weight/5)*5; Integer frequency = groupFreqMapget(weightGroup); // Increment frequency if necessary frequency = (frequency == null) 1 : frequency+1;
// (1)
// (2)
// (3) // (4) // (5)
CHAPTER 15: COLLECTIONS AND MAPS groupFreqMapput(weightGroup, frequency); } // Print the histogram: // Create a sorted set of groups (keys) SortedSet<Integer> groupSet = new TreeSet<Integer>(groupFreqMapkeySet()); // Traverse the keys, looking up the frequency from the map for (int group : groupSet) { int frequency = groupFreqMapget(group); /* Use the Arraysfill() method to fill a char array with equivalent * number of * as the frequency value * Convert the char array to string in order to print */ char[] bar = new char[frequency]; Arraysfill(bar, * ); Systemoutprintln(group + ":\t" + new String(bar)); } } } // (6)
// (7) // (8) // (9)
// (10)
Running the program with the following arguments: