$1.$Discuss the concepts of class Abstraction and Encapsulation?

$2.$What are common relationships among classes?

$3.$What is association? What is aggregation? and What is composition?

$4.$Why both aggregation and composition are together referred to as composition?

$5.$Describe primitive$-$type wrapper classes.

$6.$Explain what are BigInteger and BigDecimal classes

$7.$What is the difference between StringBuilder and StringBuffer?

Simple answer for each question please, thank you