Arrays & String Handling

Question 1

Create array of 12 integers

Answer 1

int month_days[] = new int[12];

or

int month_days[] = {31,28,25,43,23};

Question 2

Can you have multidimensional array where the sizes of second dimension are unequal?

Answer 2

yes, it’s an array of arrays, so you can define the vertical dimension per each row.

Question 3

T/F: Java implements strings as an object type String, whereas other languages implement them as character arrays?

Answer 3

True

Question 4

T/F: String objects cannot be changed.

Answer 4

True, you cannot change the characters of a String.

Each time you create an altered version, a new String object is created.

Question 5

How do you create a modifiable String?

Answer 5

use StringBuffer or StringBuilder

Question 6

Create array of characters
Create a string from 3rd letter that lasts 3 characters

Answer 6

char chars[] = {‘a’, ‘b’, ‘c’, ‘d’, ‘e’, ‘f’};
String s = new String(chars, 2,3);

Question 7

Explain how string concatenation works with other data types

int age = 34
String s = “He is “ + age + “years old”

Answer 7

int value of age is automatically converted to string representation within the String object

Question 8

Explain what happens what you do println of a newly created class

Box b = new Box(2,3,4)
System.out.println(b)

Answer 8

automatically calls b.toString() method.

However, this is seldom sufficient and you likely want to override toString() in the new class to define it how you like.

Question 9

Explain:

String s1 = “Hello”
String s2 = new String(s1)

System.out.println(s1.equals(s2))
System.out.println(s1==s2)

Output:
true
false

Answer 9

equals method compares the characters within the String object.

== checks if references the same instance

Question 10

String arr[] = {“a”, “asdf”, “as”}

Return the length of arr

Answer 10

arr.length

which is 3

Question 11

What does compareTo do to Strings? What are the output values?

String a = “AAAAAAA”
String b = “BBBBBBB”

Answer 11

Returns if before or after in the dictionary

<0 is earlier
>0 is later
0 is equal

Question 12

What does indexOf do?

Answer 12

Finds first occurrence of that character in the string

Question 13

explain String method replace()

Answer 13

replaces a character or character sequence with another

Question 14

explain String method trim()

Answer 14

removes leading and trailing whitespace

Question 15

what does valueOf() do?

Answer 15

returns the human-readable String value of that input (e.g. integer to string)

Question 16

What method would you use to concatenate strings with a delimiter between?

Answer 16

join()

String a = String.join(“, “, “alpha”, “beta”, “gamma”)

Question 17

Will StringBuffer automatically grow to fit the size object you need?

Answer 17

yes

Question 18

What does it mean an array is dynamically allocated?

Answer 18

Just declaring an array does not create it.

You need to allocate memory space by defining it’s size.

Question 19

T/F: an array is an object?

Answer 19

True

Question 20

T/F: When creating an array, you need to define the size of all dimensions, but you can leave the last dimension open and flexible

Answer 20

True

int array[][] = new int[5][]

Question 21

What does it mean that arrayA = arrayB copies the reference to arrayB and not the actual values?

Answer 21

When copying or comparing arrays, it uses references not the underlying data.

It means arrayA has pointers to arrayB, so if you update arrayB it will also update in arrayA.

Question 22

T/F: Strings also compare the references not the underlying content?

Answer 22

True. I guess this is how all objects work.

Could have one object ID = 9: “cat”
and another new String(“cat”) at ID=10

Same underlying, but not same in ==

To make it compare the underlying use .equals method:
a.equals(b)

Question 23

Write for loop that sums all elements in an array

Answer 23

for (int tempint : array)
…

Question 24

When camparing strings, explain difference between == and equals() method

Answer 24

== asks if they refer to the same object

equals() asks if contents of the strings are the same

Question 25

What are command line arguments?

Answer 25

passed to the program at run-time, enter in command line after executing a program