Java · Theory

Java TreeMap

Theory

208/270

Java TreeMap

TreeMap belongs to the Collections story. List = ordered, duplicates OK (ArrayList). Set = unique (HashSet). Map = key → value (HashMap). Pick by need, not by what you revised last.

For TreeMap: ArrayList is the daily List: get(i) is fast, add at end is fast. HashMap lookup by key is average O(1). HashSet is unique values, no duplicates.

Use TreeMap with generics: List<String>, Map<String, Integer>. Iterate with for-each or iterator. Don’t use a raw array when the size must grow.

Let's take this on the board with one tiny Main class — no extra files. Java TreeMap — treeMap prints keys in sorted order. Start from main, go line by line, and stop at each print. That output is the proof for TreeMap.

Exam tip

Say which Map keeps sorted keys (TreeMap).

Example

import java.util.TreeMap;
public class Main {
  public static void main(String[] args) {
    TreeMap<String, Integer> m = new TreeMap<>();
    m.put("b", 2);
    m.put("a", 1);
    System.out.println(m); // {a=1, b=2}
  }
}

Java TreeMap — treeMap prints keys in sorted order.

Short notes

DefTreeMap — List = order + duplicates OK. Set = unique. Map = key → value.
RuleTreeMap — pick it only when that structure matches the problem.
RememberTreeMap — generics List<String>. ArrayList for daily lists. HashMap for lookup.
TrapTreeMap — using arrays for everything that must grow, or mixing List/Set/Map.

import java.util.TreeMap; public class Main { public static void main(String[] args) { TreeMap<String, Integer> m = new TreeMap<>(); m.put("b", 2); m.put("a", 1); System.out.println(m); // {a=1, b=2} } }

Java TreeMap

Example

Short notes

Questions

Java TreeMap

Example

Short notes

Questions