TreeMap key

In this chapter you will learn:

How to get first key in TreeMap
How to get the last key in a TreeMap
How to get key relative a give value
How to get least key value relative to a key value

First key in TreeMap

The key value pairs in the TreeMap are ordered by the key values.

K firstKey() returns the first (lowest) key currently in this map.

import java.util.TreeMap;
//  j a  v a 2s. c  o  m
public class MainClass {
  public static void main(String args[]) {
    TreeMap map = new TreeMap();
    map.put("Virginia", "Richmond");
    map.put("Massachusetts", "Boston");
    map.put("New York", "Albany");
    map.put("Maryland", "Annapolis");

    if (!map.isEmpty()) {
      Object last = map.lastKey();
      boolean first = true;
      do {
        if (!first) {
          System.out.print(", ");
        }
        System.out.print(last);
        last = map.headMap(last).lastKey();
        first = false;
      } while (last != map.firstKey());
      System.out.println();
    }
  }
}

The output:

Last key in TreeMap

K lastKey() Returns the last (highest) key currently in this sorted map.

import java.util.TreeMap;
/* j a v  a  2s .  co  m*/
public class MainClass {
  public static void main(String args[]) {
    TreeMap map = new TreeMap();
    map.put("Virginia", "Richmond");
    map.put("Massachusetts", "Boston");
    map.put("New York", "Albany");
    map.put("Maryland", "Annapolis");

    if (!map.isEmpty()) {
      Object last = map.lastKey();
      boolean first = true;
      do {
        if (!first) {
          System.out.print(", ");
        }
        System.out.print(last);
        last = map.headMap(last).lastKey();
        first = false;
      } while (last != map.firstKey());
      System.out.println();
    }
  }
}

The output:

Lower key value

Map.Entry<K,V> lowerEntry(K key) Returns the greatest key less than the given key, or null if there is no such key.

import java.util.TreeMap;
//  j a va 2s  .  c  o  m
public class Main {

  public static void main(String[] args) {
    TreeMap<Integer, Product> db = new TreeMap<Integer, Product>();
    db.put(1000, new Product("D", 350));
    db.put(1011, new Product("p", 15.75));
    db.put(1102, new Product("M", 8.50));
    db.put(2023, new Product("A", 150));
    db.put(2034, new Product("T", 9.99));

    System.out.println(db.subMap(1000, 1999) + "\n");

    System.out.println(db.tailMap(1011) + "\n");

    System.out.println(db.headMap(2023));

    System.out.println("First key higher than 2034: " + db.higherKey(2034));
    System.out.println("First key lower than 2034: " + db.lowerKey(2034));
  }
}

class Product {
  String desc;

  double price;

  Product(String desc, double price) {
    this.desc = desc;
    this.price = price;
  }

  public String toString() {
    return "Description=" + desc + ", Price=" + price;
  }
}

Higher key

K higherKey(K key) Returns the least key strictly greater than the given key.

import java.util.TreeMap;
/*from j  av  a  2s .  co  m*/
public class Main {

  public static void main(String[] args) {
    TreeMap<Integer, Product> db = new TreeMap<Integer, Product>();
    db.put(1000, new Product("D", 350));
    db.put(1011, new Product("p", 15.75));
    db.put(1102, new Product("M", 8.50));
    db.put(2023, new Product("A", 150));
    db.put(2034, new Product("T", 9.99));

    System.out.println(db.subMap(1000, 1999) + "\n");

    System.out.println(db.tailMap(1011) + "\n");

    System.out.println(db.headMap(2023));

    System.out.println("First key higher than 2034: " + db.higherKey(2034));
    System.out.println("First key lower than 2034: " + db.lowerKey(2034));
  }
}

class Product {
  String desc;

  double price;

  Product(String desc, double price) {
    this.desc = desc;
    this.price = price;
  }

  public String toString() {
    return "Description=" + desc + ", Price=" + price;
  }
}