Java Collections Framework Implementations 1. Explain why it is important that e

Question

Java Collections Framework Implementations 1. Explain why it is important that elements that are stored in a Set or Map aren't modified in a way that affects the outcome of the equals() method. 2. Explain why you would choose a LinkedHashSet (or LinkedHashMap) over a Hashset or HashMap, respectively) 3. Describe the running time of the methods get (i) and set (i,x) for an ArrayList versus a LinkedList 4. Describe the running time of the method add (i,x) for an ArrayList versus a LinkedList 5. Explain why it is possible to quickly make a lot of modifications in the interior (not near either end) of a LinkedList but this is not possible in an ArrayList. 6. For each of the following methods, decide if it is fast or slow when (a) l is an ArrayList and (b) when l is a LinkedList.

Explanation / Answer

It is important that elements that are stored in a Set or Map arent't modified in a way that affects the outcome of the equals() method.

Two very common collection implementations are HashSet and HashMap which uses a hash table data structure. A hash table is a representation for mapping i.e. it is an abstract data type that maps keys to values. Keys don't have to be ordered or have a particular property except for offering "equals" or "hashCode".

If two equal objects have different hash codes, they might be places in different slots. So if you attempt to lookup a value using a key equal to the one with which it was inserted, the lookup may fail.

Object’s default hashCode() implementation is consistent with its default equals():

public class Object{

.

.

public boolean equals(Object what)

{return this == what;}

public int hashCode()

{return /* the memory address of this */;}

}

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