Difference between HashMap, LinkedHashMap and TreeMap

Question

What is the difference between HashMap, LinkedHashMap and TreeMap in Java? I don't see any difference in the output as all the three has keySet and values. What are Hashtables?

Map m1 = new HashMap();
m1.put("map", "HashMap");
m1.put("schildt", "java2");
m1.put("mathew", "Hyden");
m1.put("schildt", "java2s");
print(m1.keySet()); 
print(m1.values()); 

SortedMap sm = new TreeMap();
sm.put("map", "TreeMap");
sm.put("schildt", "java2");
sm.put("mathew", "Hyden");
sm.put("schildt", "java2s");
print(sm.keySet()); 
print(sm.values());

LinkedHashMap lm = new LinkedHashMap();
lm.put("map", "LinkedHashMap");
lm.put("schildt", "java2");
lm.put("mathew", "Hyden");
lm.put("schildt", "java2s");
print(lm.keySet()); 
print(lm.values());

Answer 1

All three classes implement the same java.util.map interface and mostly have the same functionality. But here are their functionalities-

HashMap

• HashMap is equipped with the complexity of O(1) for insertion and lookup.

• It allows only one null key but multiple null values.

• It does not maintain any order.

TreeMap

• TreeMap is equipped with complexity of O(logN).

• It doesn’t allow null key but allows multiple null values.

• It does maintain order.

LinkedHashMap

• LinkedHashMap is having complexity of O(1) for insertion and lookup.

• In contrast to others it allows one null key and multiple null values.

• It maintains order in which key-value pairs are inserted.