1.x++ is a method , the return value is x. (post increment)
++x is also a method , the return value is x+1. (pre increment)
2.muti-dim Array
int[][] matrix = new int[2][3];
matrix[0][1] = 5;
3.Boxing : Creating an object around a particular primitive
Unboxing : getting the value out of the box
4.array and ArrayList
Pros of ArrayList : dynamic resizing , contains other operations
Cons of ArrayList : less efficient , syntax bulky ( error prone ) , pre java 5.0
If have fixed size , use array.
5.Map is a interface in java, it's to add Key and value pairs.
Dictionary is a map.
6.Iterator : list through set of values. Hashmap has no ordering , so we use Iterator get the order. ( ex. it.next() )