week6. Arrays and Array List

◆ Nested Loop

- Put loops together!

- 2중 for문/3중 for문/ .. 등을 일컫는 말

◆ Random Numbers and Simulations

- In a simulation, you repeatedly generate random numbers and use them to simulate an activity

- Random number generator

Random generator = new Random();

int n = generator.nextInt(a); // 0 < = n < a   

double x = generator.nextDouble(); // 0 <= x < 1

◆ Arrays 

- Array : Sequence of values of the same type

- Construct array

new double[10];

- Store in variable of type double[]:

double[] data = new double[10];

- When array is created, all values are initialized depending on array type ;

* Numbers : 0

* Boolean : false

* Object References : null

- Use [] to access an element

- Get array length as values.length (Not a method!-> don't length())

- Index values range from 0 to length -1

- Accessing a nonexistent element results in a bound error

- Limitation : Arrays have fixed length

◆ Array Lists (c로 비유하자면, 링크드리스트를 이용한 배열 느낌)

- ArrayList class manage a sequence of objects

- Can grow and shrink as needed

- ArrayList class supplies methods for many common tasks, such as inserting and removing elements

- ArrayList is a generic class :

ArrayList<T>

collects objects of type parameter T :

ArrayList<String> names = new ArrayList<String>();      // (사이즈 지정 X)

names.add("emily");

names.add("bob");

names.add("cindy");

- size method yields number of elements

* Type T can be String and any Object, only

* Array List >> add, set, get, remove

◆ Array List Elements

- add : To add an object to the end of the array list

names.add("Emily");                                  // names.add(element);

- get : To obtain the value an element at an index

String name = names.get(2);                      // names.get(index#);

- set : To set an element to a new value (Replace value)

names.set(2, "Carolyn");                            // names.set(index#,element);

- remove : To remove an element at an index

names.remove(1);                                     //names.remove(1);

**caution: after removing, Array list shrinks automatically

ex. [a][b][c] 

remove(1); //remove [b]

      [a][c]

◆ Wrapper Classes

* ArrayList 는 Primitive Type을 지원하지 않는다. Wrapper class를 이용하자!

  Wrapper class로 primitive type의 object를 만들고 array list에 적용하면 된다!

- For each primitive type, there is a wrapper class for storing values of that type

Double d = new double (29.95);

- Wrapper objects can be used anywhere that objects are required instead of primitive type values

* Wrappers

 

    

Primitive Type	Wrapper
byte	Byte
boolean	Boolean
char	Character
double	Double
float	Float
int	Integer
long	Long
short	Short

 

◆ Common Array Algorithm ... self study 추천