TreeMap in Java

The TreeMap class in Java implements the NavigableMap interface and the NavigableMap interface extends the SortedMap. This means, the TreeMap stores the entries in a sorted order.

Key Properties of TreeMap

We can also provide a comparator while initializing the TreeMap. If a comparator is not provided then it stores the elements in their natural ordering.

The NavigableMap interface provides several navigation methods such as lowerEntry(), floorEntry(), ceilingEntry(), and higherEntry(). It can be traversed in either ascending or descending key order.

Unlike HashMap, the TreeMap does not allow null keys.

public class _1a_TreeMap {  
    public static void main(String[] args) {  
        TreeMap<Integer, String> treeMap = new TreeMap<>();  
        treeMap.put(3, "Three");  
        treeMap.put(1, "One");  
        treeMap.put(4, "Four");  
        treeMap.put(2, "Two");  

        for (Map.Entry<Integer, String> entry : treeMap.entrySet()) {  
            System.out.println(entry.getKey() + " -> " + entry.getValue());  
        }  

        // Accessing elements  
        System.out.println("Value for key 2: " + treeMap.get(2));  

        // Getting first and last keys  
        System.out.println("First key: " + treeMap.firstKey());  
        System.out.println("Last key: " + treeMap.lastKey());  

        // Getting a subMap  
        System.out.println("SubMap from 2 to 4: " + treeMap.subMap(2, 4));  

        // Removing an element  
        treeMap.remove(3);  
        System.out.println("TreeMap after removal of key 3: " + treeMap);  
    }  
}

/*
Outputs:
1 -> One
2 -> Two
3 -> Three
4 -> Four
Value for key 2: Two
First key: 1
Last key: 4
SubMap from 2 to 4: {2=Two, 3=Three}
TreeMap after removal of key 3: {1=One, 2=Two, 4=Four}
*/

TreeMap using Custom Comparator

A custom comparator can be set to TreeMap during initialization. The above example can be rewritten using a comparator that stores entries in the reverse order of keys.

public class _1b_TreeMapCustomComparator {  
    public static void main(String[] args) {  
        Comparator<Integer> reverseOrderComparator = (key1, key2) -> key2.compareTo(key1);  
        TreeMap<Integer, String> treeMap = new TreeMap<>(reverseOrderComparator);  
        treeMap.put(3, "Three");  
        treeMap.put(1, "One");  
        treeMap.put(4, "Four");  
        treeMap.put(2, "Two");  

        for (Map.Entry<Integer, String> entry : treeMap.entrySet()) {  
            System.out.println(entry.getKey() + " -> " + entry.getValue());  
        }  

        treeMap.remove(3);  
        System.out.println("TreeMap after removal of key 3: " + treeMap);  
    }  
}
/*
Outputs:
4 -> Four
3 -> Three
2 -> Two
1 -> One
TreeMap after removal of key 3: {4=Four, 2=Two, 1=One}
*/

References

[[https://docs.oracle.com/javase/8/docs/api/java/util/NavigableMap.html]] [[https://docs.oracle.com/javase/8/docs/api/java/util/SortedMap.html]]