Ch5 Core Java APIs

Question 1

What interfaces does String implement?

Answer 1

Serializable, CharSequence, Comparable

Question 2

Name as many important String methods as you can (answer contains only methods relevant for OCP exam)

Answer 2

• length()
• charAt()
• indexOf()
• substring()
• toLowerCase() & toUpperCase()
• equals() & equalsIgnoreCase()
• startsWith() & endsWith()
• replace()
• contains()
• trim() & strip() & stripLeading(), and stripTailing()
• intern()

Question 3

What are the options to construct a StringBuilder?

Answer 3

Stringbuilder b1 = new StringBuilder();
Stringbuilder b2 = new StringBuilder(“animal”);
Stringbuilder b3 = new StringBuilder(10);

Question 4

What is an easy way to reverse a String?

Answer 4

Convert it to a StringBuilder and call the reverse() method.

Question 5

How do we convert a StringBuilder object to a String object?

Answer 5

By calling the toString() method.

Question 6

Does StringBuilder implement the method equals()?

Answer 6

No.

If you call equals() on two StringBuilder instances, it will check for reference equality. To check for equality, convert it to String first.

Question 7

Why does the following code not compile?

String string = “a”;
StringBuilder builder = new StringBuilder(“a”);
System.out.println(string == builder);

Answer 7

The compiler is smart enough to know that two references to two different type of objects can never return true.

Question 8

What will happen when this code is executed?

String x = “Hello world”;
String y = “ Hello world”.trim();
System.out.println(x == y);

Answer 8

It prints false.

Even though x and y evaluate to the same string, it is computed at run-time. Since it isn’t the same at compile-time, a new String object is created.

Question 9

What is the following type of array called?

int[] numbers = {42, 55, 99};

Answer 9

Anonymous array

Question 10

How do we print the contents of an array in a human-readable way?

Answer 10

Arrays.toString(arrayHere);

Question 11

Arrays is a class provided by Java that requires an import. What package does it reside in?

Answer 11

java.util

Question 12

How can we sort an array? Class & method

Answer 12

Using the Arrays.sort() method.

Question 13

How can we search an array?

Answer 13

Using Arrays.binarySearch().

Requires the array to be sorted.

Question 14

What does Arrays.binarySearch() return when an element is not found?

Answer 14

It returns the negative value showing one smaller than the negative of the index where a match needs to be inserted to preserve sorted order.

Question 15

What does Arrays.binarySearch() return when the argument array is not sorted?

Answer 15

The answer will be unpredictable.

Question 16

How can we compare an array?

Answer 16

Using the Arrays.equals(), Arrays.compare() or the Arrays.mismatch() method.

Question 17

What are the return value rules for Arrays.compare()?

Answer 17

A negative number means the first array is smaller than the second.
A zero means the arrays are equal.
A positive number means the first array is larger than the second.

Question 18

Does this compile?

Arrays.compare(new int[] {1}, new String[] {“a”});

Answer 18

No. They must be of the same type.

Question 19

What does Arrays.mismatch() do?

Answer 19

Arrays.mismatch() is familiar to Arrays.compare(). If the arrays are equal, mismatch() returns -1. Otherwise it returns the first index where they differ.

Question 20

What does the following code return?

Arrays.mismatch(new int[] {1, 2}, new int[] {1});

Answer 20

It returns 1.

The element at index 1 of the second array is not equal the first array.

Question 21

What are three ways to instantiate an ArrayList?

Answer 21

new ArrayList();
new ArrayList(10);
new ArrayList(otherList); //shallow copy other list

Question 22

Why does

var list = new ArrayList<>();

compile? and what type is it?

Answer 22

The type of the var is ArrayList. Since there isn’t a type specified for the generic, Java assumed the ultimate superclass (Object).

Question 23

What is the advantage of calling the Wrapper class method valueOf() over using the constructor?

ex: Long.valueOf(1)

Answer 23

the valueOf() method allows object caching (like the String pool)

Question 24

Given the following String “123”

Which Wrapper class method can we use to make it into an int and which method can we use to make it into an Integer?

Answer 24

Integer.parseInt(“123”) returns a primitive

Integer.valueOf(“123”) returns a Wrapper object

Question 25

How do we convert a list to an array? (2 options)

Answer 25

list. toArray(); //returns Object[] array

list. toArray(new String[0]); //returns String[] array

Question 26

How do we convert an array to a list?

Answer 26

Arrays.asList(array); // returns fixed sized list

List.of(array); //returns immutable list

Question 27

What happens when you want to change a value in an immutable list?

Answer 27

It throws a UnsupportedOperationException

Question 28

How do we create a list using varargs? (two options)

Answer 28

Arrays.asList(“one”, “two”);

List.of(“one”, “two”);

Question 29

What is the difference between Arrays.asList() and List.of()?

Answer 29

With Arrays.asList() it is allowed to change values in the created object.

With Arrays.asList() changing values in the created object affects the original or vice versa.

Question 30

What happens when trying to add an item to a set that already contains that item?

Answer 30

It returns false.

Question 31

What are two common implementations of Set?

Answer 31

HashSet and TreeSet

Question 32

What are common Math methods?

Answer 32

• min() and max()
• round()
• pow()
• random()

Question 33

What is the difference between the equals method and the == operator?

Answer 33

== checks object equality.

equals() depends on the implementation of the object it is being called on. For Strings, equals() checks the characters inside of it.

Question 34

What is the result of the following?

List hex = Arrays.asList(“30”, “8”, “3A”, “FF”);
Collections.sort(hex);

int x = Collections.binarySearch(hex, “8”);
int y = Collections.binarySearch(hex, “3A”);
int z = Collections.binarySearch(hex, “4F”);

System.out.println(x + “ “ + y + “ “ + z);

A. 0 1 -2
B. 0 1 -3
C. 2 1 -2
D. 2 1 -3
E. None of the above.
F. The code doesn’t compile.

Answer 34

D.

After sorting, hex contains [30, 3A, 8, FF].
Remember that numbers sort before letters and strings sort alphabetically.
This makes 30 come before 8.
A binary search correctly finds 8 at index 2 and 3A at index 1.
It cannot find 4F but notices it should be at index 2.
The rule when an item isn’t found is to negate that index and subtract 1.
Therefore, we get -2-1, which is -3.

Question 35

What is the result of the following code?

12: int count = 0;
13: String s1 = “java”;
14: String s2 = ”java”;
15: StringBuilder s3 = new StringBuilder(“java”);
16: if (s1 == s2) count++;
17: if (s1.equals(s2)) count++;
18: if (s1 == s3) count++;
19: if (s1.equals(s3)) count++;
20: System.out.println(count);

A. 0
B. 1
C. 2
D. 3
E. 4
F. An exception is thrown
G. The code does not compile

Answer 35

G.

The question is trying to distract you into paying attention to logical equality versus object reference equality. The exam creators are hoping you will miss the fact that line 18 does not compile. Java does not allow you to compare String and StringBuilder using ==.

Question 36

Which of the following return the number 5 when run independently? (Choose all that apply.)

var string = “12345”;
var builder = new StringBuilder(“12345”);
A. builder.charAt(4)
B. builder.replace(2, 4, “6”).charAt(3)
C. builder.replace(2, 5, “6”).charAt(2)
D. string.charAt(5)
E. string.length
F. string.replace(“123”, “1”).charAt(2)
G. None of the above

Answer 36

A, B, F.

Remember that indexes are zero-based, which means that index 4 corresponds to 5 and option A is correct. For option B, the replace () method starts the replacement at index 2 and ends before index 4. This means two characters are replaced, and charAt (3) is called on the intermediate value of 1265. The character at index 3 is 5, making option B correct. Option C is similar, making the intermediate value 126 and returning 6. Option D results in an exception since there is no character at index 5. Option E is incorrect. It does not compile because the parentheses for the length () method are missing. Finally, option F’s replace results in the intermediate value 145. The character at index 2 is 5, so option F is correct.

Question 37

Which of the following method calls can be applied to a String object?

repeat(int )
equalsIgnoreCase(String)
prune()
append()
intern()
compareTo(String )

Answer 37

• repeat (Returns a string whose value is the concatenation of this string repeated count times.)
• equalsIgnoreCase(String)
• intern()
• compareTo(String )