Arrays Flashcards
Is Array an ordered list or unordered list?
Ordered
Can an array contain duplicates?
Yes
int[] arrays= new int[0];
Integer[] arrays = new Integer[0];
is arrays primitive?
No int is the primitive data type but all arrays are objects/
What are the valid datatype of array
a pic
* primitive data type
* interface
* abstract class
* concrete class
Which of the following will not compile?
public void a(String …name)
a(“Ala”);
a(new String[] {“Bret”});
Vargs arguement can take either an array or inidividual value
Which of the following will not compile?
public void a(String [] name)
a(“Ala”);
a(new String[] {“Bret”});
arrays only accepts array values not individual value.
Can an array datatype be null?
NO
only the wrapper can be not the primitive ones
Which of the following are legal and illegal?
String lion[] = new String[] {“lion”};
String tiger[] = new String[1]{“tiger”} ;
String bear[] = new String[] {};
String whale[] = new String[0]{};
Legal are lion and bear.
If you declare the size, you are not allowed {};
Can you do the following?
int intArray[2];
No it has to be the following :
int intArray[] = new int[4];
Can you do the following?
int array = new int[];
intarray[2] = new int;
does not compile, you need the size
Can you do the following?
intarray = new int[2.3];
won’t compile
Create an array with int datatype and of size 3.
int [] arrayname = new int [3];
Create an array and initialize with 3,4,5
````
int arrayname [] = {2,3,4}; //short cut
int [] numbers = new int [] {2,3,4}; //redundant
````
int [] helloworld = {1,2,3};
This type of approach is called what?
Anonymous array. You dont specify the size or the type.
int[] ids, types
creates two reference variable of type int[]
int ids[], type
one array and one int
List the five ways of declaring an array
int [] arrayName;
int[] arrayName;
int arrayName[];
int arrayName [];
~~~
int [] number = new int[4];
int number[];
int [] numbers, codes, scores;
int number[], codes, scores;
int number [], codes [], scores[]
````
What does it output? why?
````
String [] bugs = {“a”, “b”, “c”};
String [] copy = bugs;
System.out.println(bugs.equals(copy));
````
true. referenced to the same object.
String[] bugs = {“aa”, “mo”, “c”};
String[] z = {“aa”, “mo”, “c”};
System.out.println(bugs[0].equals(z[0]));
true
String[] birds = new String[6];
System.out.println(birds.length);
6 bc there are 6 slots
How to find the size of an array?
length.
not length();
What do you need to use Arrays?
java.util.Arrays;
What can you use to sort the Array?
import java.utils.Arrays;
Arrays.sort(arrayName);
If you have String what sorts first?
- Numbers (so 10 > 9)
- Uppercases
- lowercases
What can you use to search within a array?
Java’s inbuild Arrays.binarchSearch();
However it needs to be sorted before you use them
Binary search rules.
What happens if it is found?
What happens if it is not found?
What happens if it unsorted array?
1.index of match
2. negative values
3. result is not predictible
Write three legal ways of writing main method including the Varargs way
public static void main(String[] args)
public static void main(String args[])
public static void main(String… args)
Create a 2 multidimentional array.
Created a 2d and 3d array
int [] md [];
int [] [] md;
int md[] [];
````
int [] two[], three[] [] ;
````
int [][] args = new int[4][];
you can actually do this
but you cannot do
in[][] args = new int [] [];
must provide the first = new int[4][];
int [][] array = new int[2][];
array[0] =new int[4];
array[1] = new int[1];
array[0][0] =3;
System.out.println(array[0][0]);
valid
prints 3
What to import to use ArrayList?
import java.util.ArrayList;
Is Arraylist ordered? can it contain duplicates?
Ordered yes, can contain duplicates
int array[] = new int[21];
System.out.println(array[-10]);
will compile successfullhy but will give ArrayIndexOutOfBoundsException at runtime.
int array[] = new int[2];
System.out.println(array[1.2]);
Will not compile.
Is null a valid datatype in String array?
Yes, so in abstract, interface and objects
Provide some important properties of arrayList
- Implements the inteface List
- allows null values to be added to it
- implements all the list operations
- allows duplicate
- maintains its insertion order
- Iterator/ListIterator to iterate over the items
- supports generics
Create a arraylist.
ArrayList list = new ArrayList(3);
ArrayList lister = new ArrayList();
Can you create a arraylist with type String?
You can with generics
ArrayList<String> liste = new ArrayList<String>();</String></String>
Can you do the following?
ArrayList<String> list4 = new ArrayList<String>(); ArrayList<String> list5 = new ArrayList<>();
yes
What does an arraylist implement?
List (interface)
Can you do the following? Why not?
List<String> aa = new ArrayList<String>(); ArrayList<String> bb = new List<String>();
First yes
Second no.
List is an interface, you cannot instantiate a List
Can you do this?
ArrayList list = new ArrayList();
list.add(“a”);
list.add(3);
list.add(Boolean.TRUE);
System.out.println(list.get(0)); System.out.println(list.get(1)); System.out.println(list.get(2));
Yes bc no specification
had you
ArrayList<String> list = new ArrayList<String>();
list.add (3); it would be error</String></String>
What does this print?
ArrayList list = new ArrayList();
list.add("a");
list.add("b");
System.out.println(list.remove("a"));
System.out.println(list.remove(0));
System.out.println(list.remove("a"));
true
b
false //cause you delete it
ArrayList list = new ArrayList();
list.add(“a”);
list.remove(0);
System.out.println(list);
[]
ArrayList<String> a = new ArrayList<String>();
a.add("wow");
a.remove(0);
a.remove(0);
System.out.println(a.remove(0));
it will compile but wont run.
Using array list can you find the length?
ArrayList<String> stringArray = new ArrayList<>(); stringArray.size();
not size, not length.
ArrayList list = new ArrayList();
list.add(“a”);
list.add(“b”);
list.clear();
System.out.println(list);
[]
ArrayList list = new ArrayList();
ArrayList list2 = new ArrayList();
System.out.println(list.equals(list2));
true
An empty list is certainly the same elements in the same order
What’s the difference between add() and set()?
set() -> without changing the size of the array list
add() -> changes size
you can do this
add(0, “one”);
add(0, “two”);
What is autoboxing?
Autoboxing is the automatic conversion that the Java compiler makes between the primitive types and their corresponding object wrapper classes. For example, converting an int to an Integer, a double to a Double, and so on. If the conversion goes the other way, this is called unboxing.
int bad1 = Integer.parseInt(“a”);
Integer bad2 = Integer.valueOf(“123.45”);
has to be an int to parse.
throws numberFormatException
How to convert between array and List?
Write an ArrayList and convert to array and vice versa.
List<String> list = new ArrayList<>();
list.add("hawk");
list.add("crow");
String[] stringArray = list.toArray(new String[0]);
String[] stringArray = {"list1", "list2"};
List<String> a = Arrays.asList(stringArray);
What is the output of the following ?
Boolean b = new Boolean(null); System.out.println(b); Boolean bb = new Boolean(false); System.out.println(bb); Boolean bbb = new Boolean(true); System.out.println(bbb);
false false true
List<String> list = Arrays.asList("30", "8", "3A", "FF");
Collections.sort(list);
int x = Collections.binarySearch(list, "GF");
System.out.println("x found in " + x);
-5
so GF is at the last
since it is not found you get -1.
Since it is at last, index is -4
-1-4= -5
Sort the following String array
“30”, “2”, “8”, “40”, “3A”, “FF”
2,30,3A, 40, 8, FF
Which is more boarder? String or object?
Object
Change String to Object
String [] a = {“aaa”};
Object[] b = a;
CHange a object array to String array
String [] backToString = (String[] ) b;
againStrings[0] = new StringBuilder();
DNC because it only allows String objects and StringBuilder is not a String.
3: String[] strings = { “stringValue” };
4: Object[] objects = strings;
5: String[] againStrings = (String[]) objects;
6: againStrings[0] = new StringBuilder();
7: objects[0] = new StringBuilder();
StringBuilder object can clearly go in an Object[].
The problem is that we don’t actually have an Object[]. We have a String[] referred to from an Object[] variable.
At runtime, the code throws an ArrayStoreException.
Can you do the following?
public static void main(String … original)
{
String … copy = original;
DNC
var args only be used in methods
WAP to loop through the arrayList :
using ListIterator
How many dimensions does the following a allow?
boolean [][] bools[], a[]?
three dimesion
You are allowed to change call(String[] a) to call(String ..a)
and vice versa?
from array to varargs yes
Not from varargs to array. If you do it it will cause compile time error.
String[] a = {“a”, “b”, “c”};
Arrays.binarySearch(“a”, a);
It will give out 0 bc it is already sorted so the answer is not undefined.
Whats the output?
String[] a = {“a”, “b”, “c”};
String result = Arrays.binarySearch(“a”, a);
Sysout(result);
Does not compile. Cant store int in a String.
Can you use .parseInt or .valueOf with Character?
Nope.
