What kind of tree is used in Java's TreeSet and TreeMap?

Question

Are they AVL trees, red-black trees, or something else?

Answer 1

Red-black trees as described in the first line of the javadoc.

• Tree Map
• Tree Set

Answer 2

The first sentence of the TreeMap Javadoc states:

A Red-Black tree based NavigableMap implementation.

Answer 3

From the java.util.TreeMap<K,V> documentation:

A Red-Black tree based NavigableMap implementation.

For questions like these, you should always first consult the documentation. The API shouldn't describe ALL of the inner-workings of a class, but elementary informations such as general data structures and algorithms used are usually documented.

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Other Java Collections Framework trivias

These are all little trivias that are also clearly documented:

• TreeSet is implemented with a TreeMap
• HashSet is implemented with a HashMap
• Collections.sort uses modified mergesort
• Map<K,V> is not a Collection<?>
• ArrayList doesn't specify exact growth policy (unlike, say, Vector)

Related questions

• Why does java.util.Arrays.sort(Object[]) use 2 kinds of sorting algorithms?
• Why does the Java Collections Framework offer two different ways to sort?
• Why doesn't Java Map extends Collection?

Answer 4

TreeSet is based on TreeMap. And they uses red-black tree, red-black tree is a kind of AVL.