arrays ch6 arraylists ch14

Question 1

three ways in which [ ] are used

Answer 1

create type name
setting size - and creating new object
indexing

Question 2

array out of bounds and null pointer

Answer 2

accessing position larger than the length of the array null pointer = create an array of e.g. size 5 and you don’t create initialize it - e.g. only put 2 instances in but you want to access object 3

• When an index expression evaluates to some value other than those allowed by the array declaration, the index is said to be out of bounds

Question 3

what will happen if elements of the array arent initialized properly

Answer 3

they will automatically be initialized to the default value for their base type

Question 4

is an array of characters a string

Answer 4

no - but a string is an array of characters

Question 5

how to convert an array of characters to a string

Answer 5

E.g if array a = (a,b,c), string s = new string(a), it will join a,b,c together to form a string

Question 6

string slicing

Answer 6

String s2 = new String(sringname,start,end(not incl));

Question 7

what is each element in an object array

Answer 7

reference to the object in memory

A variable of an array type holds the address of where the array object is stored in memory

Question 8

arrays are objects

Answer 8

• collection of indexed variables

- single item whose value is a collection of values of a base type

Question 9

the process of creating a new array

Answer 9

–An array variable names the array as a single itemdouble[] a;
–A new expression creates an array object and stores the object in memory new double[10
]–An assignment statement places a reference to the memory address of an array object in the array variablea = new double[10];

Question 10

each element in a class type array must be..

Answer 10

instantiated and initialized

Question 11

using arrays as parameters

Answer 11

• array indexed variables and entire arrays can be used as arguments to methods
• When you pass an array to a method, you pass references (because an array is an object - works as class variables (pass by reference))

Question 12

how do array arguments behave

Answer 12

like objects of a class
a method can change the values stored in the indexed variables of an array arg

A method with an array parameter must specify the base type of the array onlyBaseType[]–It does not specify the length of the array

Question 13

parameter of the main method

Answer 13

String array - args

Name of parameter is args - just standard convention, you could replace args with anything else
Java programs typically executed with command line
Command line instruction can be executed with different things - input for your programs, in command line, the file that you specify will be stored in the variable (array) args - separated by white space

Question 14

for each loop

Answer 14

for (double element : a)

element = 0.0;

Question 15

how does java make allowance for variable number of parameters

Answer 15

• taking in an array as an argument

- e.g.) public static int max(int…arg)

Question 16

how to prevent privacy leaks with array instance vars

Answer 16

• return a reference to a deep copy of the private array object
• public double[ ] getArray(){
  double[ ] temp = new double[count];
  for (int i = 0; i < count; i++)
  temp[i] = a[i];
  return temp

Question 17

what to do if the array is a class type and a deep copy must be returned

Answer 17

• copies must be made of each class objecy in the array when the array is copied
• use a for loop to create a new object for each element

Question 18

what is meant by enumerated type

Answer 18

a type where all the values are given in a typically short list
- enum TypeName {VALUE_1, VALUE_2, …, VALUE_N};

Question 19

what method is typically used with enumerated types? explain what it does

Answer 19

values()
returns an array whose values are the values of the enumerated type given in the order in which the elements are listed in the definition of the enumerated type
- base type of the array is the enumerated type

Question 20

defining a multidimensional array

Answer 20

double [ ] [ ] table = new double[10][12];

Question 21

what does .length do in a 2D array?

Answer 21

number of rows in the array

Question 22

how to reference the column in the array

Answer 22

char [ ] [ ] page = new char [30][100]; page[0].length = 100

Question 23

what is a ragged array?

Answer 23

Your rows can have different numbers of columns
Not like a table
Eg. double [ ][ ] a;
a = new double [3][ ]
Number of elements in each sub array is different

Question 24

how do arrays and array lists differ?

Answer 24

arraylists can change in length while the program is running

Question 25

how is the arraylist class implemented

Answer 25

• using an array as a private instance variable
• when the hidden array is full, a new larger hidden array is created and the data is transferred to this new array
• You can create another array - add A’s elements to B and let A point to B?

Question 26

why we don’t always just use an arraylist instead of an array

Answer 26

• less efficient than an array
• doesn’t have the square bracket notation
• The base type of an ArrayListmust be a class type (or other reference type): it cannot be a primitive type–This last point is less of a problem now that Java provides automatic boxing and unboxing of primitives

Question 27

how to use the Arraylist class

Answer 27

import java.util
ArrayList aList = new ArrayList();
you can specify the initial capacity but its not compulsory

Question 28

the add method

Answer 28

list.add(“something”);

adds to the first available position

Question 29

the size method

Answer 29

how many indices already have elements in them

int howMany = list.size();

Question 30

the set and get methods

Answer 30

list.set(index,”somethingelse”);

String thing = list.get(index)