May 25

It is quite important for SCJP to know exactly differences and similarities of the single classes. Therefore here an overview about the characteristics of the classes:

class abilities
HashMap Connection with a unique key, null keys and multiple null values are allowed 

good performance (dependent on hashCode() implementation, but no order

HashTable Connection with an unique key 

Thread safe

otherwise analogue HashMap

TreeMap Connection with an unique key 

inserting new elements is more time consuming as eg HashMap, but therefore

permanent sorting and

fast, sorted output

LinkedHashMap Connection with an unique key 

faster at adding and deleting elements than HashMap

slower while iterating trough elements

HashSet Uniqueness of values 

fast sorting and finding of elements, but

no order

TreeSet Uniqueness of values 

adding of new elements more time consuming as eg HashSet, therfore

permanent sorting and

fast, sorted output

LinkedHashSet Uniqueness of values, 

high performance at multiple operations

ordered HashSet

ArrayList Values can appear double, 

faster in iterating through elements

Vector Values can appear double 

Thread safe

otherwise analogue ArrayList

LinkedList Values can appear double 

faster in adding and deleting elements than ArrayList

slower in iterating through elements

PriorityQueue “waiting queue” 

sorting by priority

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May 08

Vector is similar to ArrayList with one exception: Vector is Thread-safe and ArrayList not.

Responsible class is java.util.Vector.

Definition/ Initialization is similar to ArrayList:

Vector a;
a = new Vector <String>();

Vector <Integer> b = new Vector();

Vector c = new Vector(100); //The size of ArrayList/ Vector can be given but there is no need to do so.

For working with vectors we have the same methods as for ArrayList

a.add (“Hello”); // adding an object
a.size(); // returning size
a.get(0); // returning first element

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