Java Basic things to know


|    Classes – variables – methods :       |

private :  allow access only to the current class.

public : allow access to everybody regardless of the package.

protected : allow access only to subclasses or class of the same package.

default : allow access only to class of the same package.

transient : variable is not a part of a persistent object

final : class can’t be extended, method can’t be overriden and variable can’t be reinitialised.

static : class variable and not instance of class variable (useful for class constant used with final), class method can be called without create an instance of the class,

volatile : indicates that a thread must reconcile its working copy of the field with the master copy every time it accesses the variable.


|              Classes – Interfaces :             |

Implements : a class can implements several interfaces, but can extend only one class.

Extends : a class can only extend one another class, interface can extend an another interface.

Interfaces and Abstract classes  do not need to fully implement the interfaces they respectively extend or implement


|         Collections :   Set, List & Map :             |

Set : contains unique objects and no null elements are inserted.

HashSet : (Faster) but there absolutely no order.

TreeSet : (Normal) Object inside the set must implement the Comparable interface and compareTo method.

LinkedHashSet : (Slower) The order is respected regarding the order of objects inserted.

List : could contains null elements and duplicated objects, the order how the elements has been inserted is keeped.

ArrayList : (Faster) To get indexed element.  (Slower)  To delete or insert element.

LinkedList : (Faster) To remove or insert element.  (Slower) Have to traverse the list to get an element.

Map : contains a pair of element of key/object.

HashMap : No ordering.

TreeMap : Key ordering.

LinkedHashMap : Preserves the insertion order.

HashTable : is a synchronized hashmap.


|      Interface:   Comparator & Comparable :        |

Comparator :  To implement as an utility to compare two objects and override compare() method;

Comparable :  To implement on the object to compare and override compareTo() method;


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