Collections - Beginning Java 8 Language Features - page 573

SortedMap<K, V> headMap(K toKey) : It returns a view of the SortedMap whose entries will
have keys less than the specified toKey . If you add a new entry to the view, its key must be
less than the specified toKey . Otherwise, it will throw an exception. The view is backed by the
original SortedMap .

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K lastKey() : It returns the key of the last entry in the SortedMap . If the SortedMap is empty, it
throws a NoSuchElementException .

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SortedMap<K, V> subMap(K fromKey, K toKey) : It returns a view of the SortedMap whose
entries will have keys ranging from the specified fromKey (inclusive) and toKey (exclusive).
The original SortedMap backs the partial view of the SortedMap . Any changes made to either
map will be reflected in both. You can put new entries in the sub map whose keys must fall in
the range fromKey (inclusive) and toKey (Exclusive).

SortedMap<K, V> tailMap(K fromKey) : It returns a view of the SortedMap whose entries will
have keys equal to or greater than the specified fromKey . If you add a new entry to the view,
its key must be equal to or greater than the specified fromKey . Otherwise, it will throw an
exception. The original SortedMap backs the tail view.
The TreeMap class is the implementation class for the SortedMap interface. For basic operations, you work with a
SortedMap the same way as you work with a Map . Listing 12-31 demonstrates how to use a SortedMap .

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Listing 12-31. Using a SortedMap

// SortedMapTest.java
package com.jdojo.collections;

import java.util.SortedMap;
import java.util.TreeMap;

public class SortedMapTest {
public static void main(String[] args) {
SortedMap<String,String> sMap = new TreeMap<>();
sMap.put("John", "[phone]");
sMap.put("Richard", "[phone]");
sMap.put("Donna","[phone]");
sMap.put("Ken", "[phone]");
System.out.println("Sorted Map: " + sMap);
// Get a sub map from Donna (inclusive) to Ken(exclusive)
SortedMap<String,String> subMap = sMap.subMap("Donna", "Ken");
System.out.println("Sorted Submap from Donna to Ken(exclusive): " + subMap);
// Get the first and last keys
String firstKey = sMap.firstKey();
String lastKey = sMap.lastKey();
System.out.println("First Key: " + firstKey);
System.out.println("Last key: " + lastKey);
}
}