Arrays

Question 1

Is Array an ordered list or unordered list?

Answer 1

Ordered

Question 2

Can an array contain duplicates?

Answer 2

Yes

Question 3

int[] arrays= new int[0];
Integer[] arrays = new Integer[0];
is arrays primitive?

Answer 3

No int is the primitive data type but all arrays are objects/

Question 4

What are the valid datatype of array

Answer 4

a pic
* primitive data type
* interface
* abstract class
* concrete class

Question 5

Which of the following will not compile?
public void a(String …name)
a(“Ala”);
a(new String[] {“Bret”});

Answer 5

Vargs arguement can take either an array or inidividual value

Question 6

Which of the following will not compile?
public void a(String [] name)
a(“Ala”);
a(new String[] {“Bret”});

Answer 6

arrays only accepts array values not individual value.

Question 7

Can an array datatype be null?

Answer 7

NO

only the wrapper can be not the primitive ones

Question 8

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]{};

Answer 8

Legal are lion and bear.
If you declare the size, you are not allowed {};

Question 9

Can you do the following?
int intArray[2];

Answer 9

No it has to be the following :
int intArray[] = new int[4];

Question 10

Can you do the following?
int array = new int[];
intarray[2] = new int;

Answer 10

does not compile, you need the size

Question 11

Can you do the following?
intarray = new int[2.3];

Answer 11

won’t compile

Question 12

Create an array with int datatype and of size 3.

Answer 12

int [] arrayname = new int [3];

Question 13

Create an array and initialize with 3,4,5

Answer 13

````
int arrayname [] = {2,3,4}; //short cut

int [] numbers = new int [] {2,3,4}; //redundant

````

Question 14

int [] helloworld = {1,2,3};
This type of approach is called what?

Answer 14

Anonymous array. You dont specify the size or the type.

Question 15

int[] ids, types

Answer 15

creates two reference variable of type int[]

Question 16

int ids[], type

Answer 16

one array and one int

Question 17

List the five ways of declaring an array

Answer 17

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[] 

`

Question 18

What does it output? why?

````
String [] bugs = {“a”, “b”, “c”};
String [] copy = bugs;

System.out.println(bugs.equals(copy));
~~~
`

Answer 18

true. referenced to the same object.

Question 19

String[] bugs = {“aa”, “mo”, “c”};
String[] z = {“aa”, “mo”, “c”};
System.out.println(bugs[0].equals(z[0]));

Answer 19

true

Question 20

String[] birds = new String[6];
System.out.println(birds.length);

Answer 20

6 bc there are 6 slots

Question 21

How to find the size of an array?

Answer 21

length.

not length();

Question 22

What do you need to use Arrays?

Answer 22

java.util.Arrays;

Question 23

What can you use to sort the Array?

Answer 23

import java.utils.Arrays;

Arrays.sort(arrayName);

Question 24

If you have String what sorts first?

Answer 24

1. Numbers (so 10 > 9)
2. Uppercases
3. lowercases

Question 25

What can you use to search within a array?

Answer 25

Java's inbuild Arrays.binarchSearch(); However it needs to be sorted before you use them

Question 26

Binary search rules. What happens if it is found? What happens if it is not found? What happens if it unsorted array?

Answer 26

1.index of match 2. negative values 3. result is not predictible

Question 27

Write three legal ways of writing main method including the Varargs way

Answer 27

public static void main(String[] args) public static void main(String args[]) public static void main(String... args)

Question 28

Create a 2 multidimentional array. Created a 2d and 3d array

Answer 28

int [] md []; int [] [] md; int md[] []; ```` int [] two[], three[] [] ; ````

Question 29

int [][] args = new int[4][];

Answer 29

you can actually do this but you cannot do in[][] args = new int [] []; must provide the first = new int[4][];

Question 30

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]);

Answer 30

valid prints 3

Question 31

What to import to use ArrayList?

Answer 31

import java.util.ArrayList;

Question 32

Is Arraylist ordered? can it contain duplicates?

Answer 32

Ordered yes, can contain duplicates

Question 33

int array[] = new int[21]; System.out.println(array[-10]);

Answer 33

will compile successfullhy but will give ArrayIndexOutOfBoundsException at runtime.

Question 34

int array[] = new int[2]; System.out.println(array[1.2]);

Answer 34

Will not compile.

Question 35

Is null a valid datatype in String array?

Answer 35

Yes, so in abstract, interface and objects

Question 36

Provide some important properties of arrayList

Answer 36

* 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

Question 37

Create a arraylist.

Answer 37

ArrayList list = new ArrayList(3); ArrayList lister = new ArrayList();

Question 38

Can you create a arraylist with type String?

Answer 38

You can with generics ArrayList liste = new ArrayList();

Question 39

Can you do the following? ``` ArrayList list4 = new ArrayList(); ArrayList list5 = new ArrayList<>(); ```

Answer 39

yes

Question 40

What does an arraylist implement?

Answer 40

List (interface)

Question 41

Can you do the following? Why not? ``` List aa = new ArrayList(); ArrayList bb = new List(); ```

Answer 41

First yes Second no. List is an interface, you cannot instantiate a List

Question 42

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));

Answer 42

Yes bc no specification had you ArrayList list = new ArrayList(); list.add (3); it would be error

Question 43

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")); ```

Answer 43

true b false //cause you delete it

Question 44

ArrayList list = new ArrayList(); list.add("a"); list.remove(0); System.out.println(list);

Answer 44

[]

Question 45

``` ArrayList a = new ArrayList(); a.add("wow"); a.remove(0); a.remove(0); System.out.println(a.remove(0)); ```

Answer 45

it will compile but wont run.

Question 46

Using array list can you find the length?

Answer 46

``` ArrayList stringArray = new ArrayList<>(); stringArray.size(); ``` not size, not length.

Question 47

ArrayList list = new ArrayList(); list.add("a"); list.add("b"); list.clear(); System.out.println(list);

Answer 47

[]

Question 48

ArrayList list = new ArrayList(); ArrayList list2 = new ArrayList(); System.out.println(list.equals(list2));

Answer 48

true An empty list is certainly the same elements in the same order

Question 49

What's the difference between add() and set()?

Answer 49

set() -> without changing the size of the array list add() -> changes size you can do this add(0, "one"); add(0, "two");

Question 50

What is autoboxing?

Answer 50

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.

Question 51

int bad1 = Integer.parseInt("a"); Integer bad2 = Integer.valueOf("123.45");

Answer 51

has to be an int to parse. throws numberFormatException

Question 52

How to convert between array and List? Write an ArrayList and convert to array and vice versa.

Answer 52

``` List list = new ArrayList<>(); list.add("hawk"); list.add("crow"); String[] stringArray = list.toArray(new String[0]); String[] stringArray = {"list1", "list2"}; List a = Arrays.asList(stringArray); ```

Question 53

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); ```

Answer 53

``` false false true ```

Question 54

``` List list = Arrays.asList("30", "8", "3A", "FF"); Collections.sort(list); int x = Collections.binarySearch(list, "GF"); System.out.println("x found in " + x); ```

Answer 54

-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

Question 55

Sort the following String array "30", "2", "8", "40", "3A", "FF"

Answer 55

2,30,3A, 40, 8, FF

Question 56

Which is more boarder? String or object?

Answer 56

Object

Question 57

Change String to Object

Answer 57

String [] a = {"aaa"}; Object[] b = a;

Question 58

CHange a object array to String array

Answer 58

String [] backToString = (String[] ) b;

Question 59

againStrings[0] = new StringBuilder();

Answer 59

DNC because it only allows String objects and StringBuilder is not a String.

Question 60

3: String[] strings = { "stringValue" }; 4: Object[] objects = strings; 5: String[] againStrings = (String[]) objects; 6: againStrings[0] = new StringBuilder(); 7: objects[0] = new StringBuilder();

Answer 60

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.

Question 61

Can you do the following? public static void main(String ... original) { String ... copy = original;

Answer 61

DNC var args only be used in methods

Question 62

WAP to loop through the arrayList : using ListIterator

Question 63

How many dimensions does the following a allow? boolean [][] bools[], a[]?

Answer 63

three dimesion

Question 64

You are allowed to change call(String[] a) to call(String ..a) and vice versa?

Answer 64

from array to varargs yes Not from varargs to array. If you do it it will cause compile time error.

Question 65

String[] a = {"a", "b", "c"}; Arrays.binarySearch("a", a);

Answer 65

It will give out 0 bc it is already sorted so the answer is not undefined.

Question 66

Whats the output? String[] a = {"a", "b", "c"}; String result = Arrays.binarySearch("a", a); Sysout(result);

Answer 66

Does not compile. Cant store int in a String.

Question 67

Can you use .parseInt or .valueOf with Character?

Answer 67

Nope.

Question 68

What will this return? new Boolean(true)

Answer 68

true

Question 69

What will this return? new Boolean(false)

Answer 69

false

Question 70

What will this return? new Boolean("yes")

Answer 70

false

Question 71

What will this return? new Boolean(null)

Answer 71

false

Question 72

What will this return? new Boolean(True)

Answer 72

Cannot find symbol error

Question 73

What will this return? new Boolean(False)

Answer 73

Cannot find symbol error

Question 74

Create a wrapper class for Byte

Answer 74

Byte a = new Byte((byte)1);