Chapter 3: Core Java APIs

Question 1

What are the features of an ArrayList

Answer 1

1) its an ordered list 2) Dynamic in size 3) Allows duplicates

Question 2

What must you do before using the ArrayList class?

Answer 2

you must import the ArrayList class using either : import java.util.*; (imports entire package) OR import java.util.ArrayList; (imports just the class)

Question 3

How many ways can you create an ArrayList for the purposes of the exam?List them

Answer 3

You can use 3 different constructors ArrayList list = new ArrayList( ); // sets elements to their default values ArrayList list2 = new ArrayList(10); // set the initial capacity ArrayList list3 = new ArrayList(list2); // creating a copy of another list. This constructor takes a collection as a param.

Question 4

What interface does List extend? How would you create a new type of this interface

Answer 4

Interface extends Collection. Collection collection = new ArrayList( ) OR Collection collection2 = new ArrayList<>( );

Question 5

Name all the methods for the ArrayList (only the ones you need to know for the exam).

Answer 5

add( ) , set( ) remove ( ) , contains ( ) equals ( ) , clear ( ) IsEmpty ( ) , size ( )

Question 6

Will this compile? Explain.

ArrayList list = new ArrayList <>( );

list.add(Boolean.TRUE);

Answer 6

No because we are using generics to specify that this list only accepts type String which Boolean.TRUE isn’t.

Question 7

Will this compile? Explain.

ArrayList list = new ArrayList( );

list.add(Boolean.TRUE); list.add(“hawk”);

Answer 7

Yes, as we haven’t specified a type for this list, we can add any object.

Question 8

What does this code do?

List <string>birds = new ArrayList&lt;&gt;( );</string>

birds. add(“hawk”);
birds. add(1, “robin”);

Answer 8

it adds “hawk” to the end of the birds list and then adds “robin” to index 1 of the list which is after “hawk”

Question 9

What will be the output of this code? List<> list1 = new ArrayList( ); list1. add(“hello”)

Answer 9

It won’t compile as the diamond operator on the RHS is empty. it should have a type declared inside

Question 10

What are the method signatures for remove( ) method of an arrayList?

Answer 10

boolean remove(Object) // returns a boolean when an element is passed in. AND E remove(int index) // removes an element at the set index

Question 11

In arrayList, what is the method signature for set( )? what does this method do?

Answer 11

E set (index, newElement). set( ) replaces an element at the index provided with the new element passed in

Question 12

What do the IsEmpty( ) and size( ) methods of ArrayList return when called independently?

Answer 12

A boolean and an int

Question 13

What does ArrayLists clear( ) method do?

Answer 13

clear () empties all the contents of the list

Question 14

what does the ArrayList contains( ) do?

Answer 14

it checks to see whether the list has the specified value and returns a boolean

Question 15

what is the output of this code?

List<string> one = new ArrayList&lt;&gt;( );</string>

List<string> two = new ArrayList&lt;&gt;( );</string>

one. add(“a”); one.add(“b”);
two. add(“a”);
two. add(0, “b”);

System.out.println(one.equals(two));

Answer 15

false. because for the equals to return true, the size and the order of the list has to be the same

Question 16

What does the equals() method do?

Answer 16

it checks whether the two lists contain the same elements and in the same order.

Question 17

What is the method for converting a String into an int primitive?

Answer 17

parseInt( ) e.g : int primitive = Integer.parseInt(“123”)

Question 18

What is the method for converting a String into an Integer wrapper class?

Answer 18

valueOf( ) e.g: Integer wrapper = Integer.valueOf(“123”)

Question 19

How do you convert a String into a boolean primitive and a float primitive?

Answer 19

Boolean.parseBoolean(“true”); Float.parseFloat(“2.2”);

Question 20

How do you convert a String into a Boolean and Float wrapper class?

Answer 20

Boolean.valueOf(“true”); Float.valueOf(“2.2”);

Question 21

How do you convert a String into a Character wrapper class

Answer 21

No such method exists, you just call charAt normally.

Question 22

What is Autoboxing?

Answer 22

This is when Java automatically converts a primitive into its relevant wrapper class. This is great as ArrayLists aren’t allowed to contain primitives

Question 23

What is unboxing?

Answer 23

When you convert a wrapper into a primitive

Question 24

What happens on line 1 & 2 of this code? List heights = new ArrayList<>( ); heights. add(null); // 1 int h = heights.get(0); // 2

Answer 24

// 1 compiles fine as you can assign null to any variable reference. // 2 will result in a NullPointerEx as you are calling a method on null

Question 25

What is the output of this code? List = new ArrayList<>( ); numbers. add(1); numbers.add(2); System.out.println(numbers.remove(1));

Answer 25

1 is the output! As you recall the remove method also takes the index as a param thus this is invoked instead of autoboxing.

Question 26

How do you convert an ArrayList into an array?

Answer 26

ArrayList has a method called “toArray” that you can call on a list. You can then only assign that value to an object array unless you explicitly specify the type of array.

Question 27

Example of converting a list into an array and specifying the type of the array.

Answer 27

String[] array = list.toArray( new String [0]); Even though the code says to create an array of size 0, java will figure it out and create a new array to fit the actual size of the array.

Question 28

How do you convert an Array into a List?

Answer 28

you can use the Arrays.asList( ) method that takes the original array as a paramater.

Question 29

How do you sort an ArrayList?

Answer 29

Using Collections.sort( list )

Question 30

What happens when you convert an Array into a list?

Answer 30

1)The list remains fixed meaning you can’t change its size 2)Any changes you make to the list using set() will also update the original array as both refs are pointing to the same data

Question 31

What interface does ArrayList implement? How do you create a new type of this interface?

Answer 31

ArrayList implements List. List list = new ArrayList( ) OR List list2 = new ArrayList<>( );

Question 32

Post Java 5, how else can you create an ArrayList? Give two variations of this.

Answer 32

Using generics which allow you to specify the “type” you want to store in the list. It uses the diamond operator e.g :

1) ArrayList list1 = new ArrayList( );
2) ArrayList<boolean> list2 = new ArrayList&lt; &gt;( );</boolean>

Question 33

What are the string concatenation rules?

Answer 33

1. When used with numbers it is used for addition.
2. If used with Strings/booleans, + means concatenation, The expression is evaluated left to right.
   3) When used with += it is added on the end of a string

Question 34

What would be the output of this code? int one = 1, two = 2, four = 4; String three = “3”; System.out.println( one + two + three + four);

Answer 34

output is 334. this expression is evaluated from left to right, 1 & 2 are added together to get 3 then “3” & 4 are added to the end of 3.

Question 35

1)System.out.println(1 + 2); 2)System.out.println(“a” + “b”); 3)System.out.println(“a” + “b” + 3); 4)System.out.println(1 + 2 + “c”);

Answer 35

1) 2 2) ab 3) ab3 4) 2c

Question 36

What is the signature for the charAt method?

Answer 36

char charAt(int index) - allows you to find out what character is at the specific index in a string. Note: if you call an index that is beyond the string, you get a StringIndexOutOfBoundsException

Question 37

What is the signature for the indexOf method?

Answer 37

int indexOf() returns the index of a string and can take various parameters ; it can work with indv characters or the whole string

Question 38

What does the indexOf method do and what is the signature?

Answer 38

indexOf looks at the characters in the string and finds the first index that matches the desired value. IndexOf can work with an individual character or a whole String as input. It can also start from a requested position. The method signatures are as follows: int indexOf(char ch) int indexOf(char ch, index fromIndex) int indexOf(String str) int indexOf(String str, index fromIndex) String string = “animals”; System.out.println(string.indexOf(‘a’)); // 0 System.out.println(string.indexOf(“al”)); // 4 System.out.println(string.indexOf(‘a’, 4)); // 4 System.out.println(string.indexOf(“al”, 5)); // -1

Question 39

What is the method signature of String’s substring and what does it do?

Answer 39

subString(int beginIndex) Or

subString(int beginIndex, int endIndex)

This method takes one or two indexes and returns part of a string. The end index is exclusive which means that it isnt included in the output and the counting stops right before that index.

Question 40

What is the output on each line of code?

String String = “Hello World!”; System.out.println(String.substring(3)); //1

System.out.println(String.substring(String.indexOf(“ W”))); //2

System.out.println(String.charAt(8)); System.out.println(String.substring(6,6)); // 3

System.out.println(String.substring(6,12)); // 4

System.out.println(String.substring(6,12)); // 5

System.out.println(String.substring(5,13)); //6

System.out.println(String.trim()); // 7

Answer 40

1) lo World
2) World// a space then world
3) nothing will be printed as 6 is exclusive
4) World
5) World!// eventhough there are only 11 characters, it is legal to access up 1 past the end of the sequence which in this case is 12.
6) StringIndexOutOfBoundsException
7) Hello World!// trim removes spaces before and after but not in the middle thus the string will remain unchanged

Question 41

What does it mean that Strings are immutable?

Answer 41

It means that once created, they can’t be changed, you can add or remove things

Question 42

What is the string pool?

Answer 42

Storage in the jvm that holds string literals e.g “hello”

Question 43

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

Answer 43

== checks for the equality of references so it will only return true if the references are pointing to the same object

equals returns true if the contents inside the string are the same. The two references don’t have to be pointing to the same object.

Question 44

True or False:

String x1 = “a”;

String x2 = new String(“a”);

x1.equals(x2). // 1

x1 == x2 // 2

Answer 44

1) True because even though x1 and x2 are pointing to different objects, the content of the objects are same
2) False as the references are pointing to different objects

Question 45

Convert the string “1” to a long primitive

Answer 45

long num = Long.parseLong(“1);

Question 46

What will the output of this code be?

List list4 = new ArrayList();

list4. add(“hawk”);
list4. add(“robin”);

Object[] objectArray = list4.toArray(); // 1

System.out.println(objectArray.length); // 2

String[] stringArray = list4.toArray(new String[0]); // 3

System.out.println(stringArray.length); // 4

Answer 46

Line 4 will not compile because an object/ array will not fit into a String array reference without explicit cast.list. toArray() returns a type of Object[]

If the cast (String[] ) was present on line 3, the answer would be 2 & 2 which shows us that the arrays are basically copies

Additionally on line 3, the initialisation (new String [0]) iniside the toArray method is improtant as you are letting the jvm know explicitly that we require a string arrary. without this, a runtime error would occur

Question 47

What is the output of the following code?
StringBuilder a = new StringBuilder("abc");
StringBuilder b = a.append("de");
b = b.append("f").append("g");
System.out.println("a=" + a);
System.out.println("b=" + b);

Answer 47

a=abcdefg
b=abcdefg

There is only one StringBuilder object as StringBuilder was called just the once. Thus both references are pointing to the same object, changes made on one variable will reflect on the other

Question 48

What are the 3 ways you can create a StringBuilder?

Answer 48

StringBuilder sb1 = new StringBuilder();

StringBuilder sb2 = new StringBuilder(“animal’);

StringBuilder sb3 = new StringBuilder(10); // asking to reseve a certain no of slots for the eventual value aka capacity

Question 49

StringBuilder sb = new StringBuilder(“start”);

sb.append(“+middle”);

Stringbuilder same =sb.append(“+end”);

What is the output of sb and same?

Answer 49

Both variables output: start+middle+end

StringBuilder changes its own state and returns a reference to itself. Here sb and same point to the exact same object and as a result print the same value.

Question 50

Will line 1 of this code compile?

StringBuilder sb = new StringBuilder(“animal”);

sb.append(“+middle”);

StringBuilder same = sb.append(“+end”);

System.out.println( sb + same); //1

Answer 50

No, the concatenation operator can’t be used with StringBuilder

append is the StringBuilders version of +

Question 51

What are some of the methods of StringBuilder?

Answer 51

append()

Insert() // adds chars to the SB

delete( ), deleteCharAt( )

reverse( )

toString ( )

The following are also similar to Strings methods:

charAt() , indexOf, length(), substring()

Question 52

True or false?

StringBuilder one = new StringBuilder( );

StringBuilder two = new StringBuilder( );

StringBuilder three = one.append(“a”);

System.out.println(one == two); // 1

System.out.println(one == three); // 2

Answer 52

1) false, one and two point to completely seperate objects
2) true, as references are pointing to the same obj

Question 53

True or False?

String x = “Hello World”;

String z = “ Hello World”.trim( );

System.out.println(x == z);

Answer 53

false // eventhough we have two of the same string literal, a new object was created for string z at run time when trim() was called.

Question 54

What is the output of the following code?

LocalDate date = LocalDate.of(2020, Month.JANUARY, 20);
LocalTime time = LocalTime.of(5, 15);
LocalDateTime dateTime = LocalDateTime.of(date, time);
System.out.println(dateTime);
datetime = dateTime.minusDays(1);
System.out.println(dateTime);
dateTime = dateTime.minusHours(10);
System.out.println(dateTime);
dateTime = dateTime.minusSeconds(30);
System.out.println(dateTime);

Answer 54

2020-01-20T05:15
2020-01-19T05:15
2020-01-18T19:15
2020-01-18T19:14:30

Question 55

What is the output of this code?

String result = “AniMal “.trim().toLowerCase().replace(‘a’, ‘A’);

System.out.println(result);

Answer 55

AnimAl

Question 56

What does the following code output?

StringBuilder sb = new StringBuilder(“abcdef”);

sb. delete(1,3); // 1
sb. deleteCharAt(5); // 2

Answer 56

1) adef// like the substring it takes 2 params, the begin and end index; the end index is exclusive.
2) throws a StringIndexOutOfBoundsEx

Question 57

What is the method signature for the insert method of the StringBuilder?

Answer 57

StringBuilder insert(int offset, String str)

offset is the index where we want to insert the requested param ( can be a string, boolean, int, char, char array , object etc. theres a no of constructors)

e.g

StringBuilder sb = new StringBuilder(“animals”);

sb.insert(7, “-“); // sb = animals-

Question 58

What is the output of the following code?

StringBuilder puzzle = new StringBuilder("Stephen");
System.out.println(puzzle);
puzzle.substring(1);
System.out.println(puzzle);

Answer 58

Stephen

Stephen

the substring method prints tephen but returns a String which is why puzzle is unchanged

Question 59

What would the output of the following code be?

public class ArrayType {

public static void main(String args[]) {
String [] bugs = {“cricket”, “beetle”, “ladybug”};
String [] alias = bugs;
System.out.println(bugs.equals(alias));
System.out.println(bugs.toString());
}
}

Answer 59

true // we can call equals cos an array is an object
[Ljava.lang.String;@160bc7c0 // L is the type of array , String is the reference type and the no is the hashcode

Question 60

What would the output of the following code be?

int[] newVals = {};
newVals[0] = 1;
newVals[1] = 2;
if (newVals == null) {
System.out.println(“newVals is null”);

} else {
System.out.println(“newVals is not null”);
}

Answer 60

Index out of bounds exception on this line:
newVals[0] = 1; // this is due to the fact that arrays can’t change their size once they have been created.

Question 61

What is an Array?

Answer 61

An array is an ordered list that can contain duplicates.

Question 62

How would you instantiate an anonymous array?

Answer 62

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

Question 63

What is the output of this code?

String [] stringsArray = new String[3];

stringsArray[0] = “”;

for (String str : stringsArray) System.out.println(str);

Answer 63

blank space

null

null

Note: remember you can ommit the parenthesis in the for loop for a single statement

Question 64

What is the syntax for sorting an array?

Answer 64

int numbers [] = {6,9,1};

Arrays.sort(numbers);

Question 65

What will this print?

int numbers [] = {6,9,1};

Arrays.sort(numbers);

System.out.println(numbers)

Answer 65

A hascode of I@2bd9c3e7.

You have to loop through an array in order to print the values

Question 66

What must you do before using an array’s methods??

Answer 66

you must either import the class or package like so:

import java.util.*

import java.util.Arrays

or

use java.util.Arrays.someArrayMethod( )

Question 67

What is the output of this code?

int [] a = {1,2,3,4}

int [] b = {2,3,1,0}

System.out.println( a [(a=b)[3] ] )

Answer 67

It prints 1. // statement is evaluated from left to right

1) a on the LHS is assigned the value of the a array
2) inside the paranthesis, array b is assigned to a
3) it now looks like this a[b[3] ]; b[3] = 0 // acceses the 4th index of the b array
4) finally a[0] accesses index 0 of array a which is 1.

Question 68

What are the 3 ways of creating arrays?

Answer 68

int numbers = new int [5] // an empty array

OR

int [] numbers = new [] {1,2,3,4,5} // an initialised array

&

int [] numbers = {1,2,3,4,5} // an anonymous array

Question 69

What will be the result of trying to compile and execute the following program? public class TestClass{

public static void main(String args[] ){

int i = 0 ;

int[] iA = {10, 20} ;

iA[i] = i = 30 ;

System.out.println(“”+ iA[0] + “ “ + iA[1] + “ “+i) ; } }

Answer 69

It will print 30 20 30

The statement iA[i] = i = 30 ; will be processed as follows:
iA[i] = i = 30; => iA[0] = i = 30 ; => i = 30; iA[0] = i ; => iA[0] = 30 ;

Here is what JLS says on this:
1 Evaluate Left-Hand Operand First
2 Evaluate Operands before Operation
3 Evaluation Respects Parentheses and Precedence
4 Argument Lists are Evaluated Left-to-Right

For Arrays: First, the dimension expressions are evaluated, left-to-right. If any of the expression evaluations completes abruptly, the expressions to the right of it are not evaluated.

Question 70

The following class will print ‘index = 2’ when compiled and run?

class Test {

public static int[] getArray() { return null; }

public static void main(String[] args){

int index = 1;

try{

getArray()[index=2]++;

}

catch (Exception e){ } //empty catch

System.out.println(“index = “ + index);

}

}

Answer 70

True

If the array reference expression produces null instead of a reference to an array, then a NullPointerException is thrown at runtime, but only after all parts of the array reference expression have been evaluated and only if these evaluations completed normally.

This means, first index = 2 will be executed, which assigns 2 to index. After that null[2] is executed, which throws a NullPointerException. But this exception is caught by the catch block, which prints nothing. So it seems like NullPointerException is not thrown but it actually is.

In other words, the embedded assignment of 2 to index occurs before the check for array reference produced by getArray().

In an array access, the expression to the left of the brackets appears to be fully evaluated before any part of the expression within the brackets is evaluated. Note that if evaluation of the expression to the left of the brackets completes abruptly, no part of the expression within the brackets will appear to have been evaluated.

Question 71

What will happen when the following code is compiled and run?

class AX{

static int[] x = new int[0];

static{

x[0] = 10;

}

public static void main(String[] args){

AX ax = new AX();

}

}

Answer 71

It will throw ExceptionInInitializerError at runtime.

The following is the output when the program is run:
java.lang.ExceptionInInitializerError
Caused by: java.lang.ArrayIndexOutOfBoundsException: 0
at AX.(SM.java:6)
Exception in thread “main”
Java Result: 1

Note that the program ends with ExceptionInInitializerError because any exception thrown in a static block is wrapped into ExceptionInInitializerError and then that ExceptionInInitializerError is thrown. Remember that a static or instance initializer can only throw a RuntimeException.

If you try to throw a checked exception from a static block to the outside, the code will not compile.

An instance block is allowed to throw a checked exception but only if that exception is declared in the throws clause of all the constructors of the class.

Even though the line x[0] = 10; will throw java.lang.ArrayIndexOutOfBoundsException, JVM will wrap it and rethrow java.lang.ExceptionInInitializerError.

Question 72

What will the following code snippet print?

int index = 1;

String[] strArr = new String[5];

String myStr = strArr[index];

System.out.println(myStr);

Answer 72

null

Question 73

what will this output?

String[] strings = {“10”, “9”, “100”};

Arrays.sort(strings);

for (String string : numbers)

System.out.println(string + “ “);

Answer 73

10 100 9 // String sorts in alphabetical order 1 in 10 & 100 comes before 9

Question 74

What will the following class print when run?
public class Sample{
 public static void main(String[] args) {
 String s1 = new String("java");
 StringBuilder s2 = new StringBuilder("java");
 replaceString(s1);
 replaceStringBuilder(s2);
 System.out.println(s1 + s2);
 }
 static void replaceString(String s) {
 s = s.replace('j', 'l');
 }
 static void replaceStringBuilder(StringBuilder s) {
 s.append("c");
 }
}

Answer 74

javajavac

String is immutable while StringBuilder is not. So no matter what you do in replaceString() method, the original String that was passed to it will not change.

On the other hand, StringBuilder methods, such as replace or append, change the StringBuilder itself. So, ‘c’ is appended to java in replaceStringBuilder() method.

Question 75

What will the following code print?

System.out.println(“12345”.charAt(6));

Answer 75

It will throw an IndexOutOfBoundsException

Since indexing starts with 0, the maximum value that you can pass to charAt is its length-1 so in this case 4

charAt throws IndexOutOfBoundsException if you pass an invalid value (that is, if the index argument is negative or not less than the length of this string).

Both - ArrayIndexOutOfBoundsException and StringIndexOutOfBoundsException, extend IndexOutOfBoundsException and although in practice, the charAt method throws StringIndexOutOfBoundsException, it is not a valid option because the implementation is free to throw some other exception as long as it is an IndexOutOfBoundsException.

Question 76

What will the following program print?

public class TestClass{

static String str = “Hello World”;

public static void changeIt(String s){

s = “Good bye world”;

}

public static void main(String[] args){

changeIt(str);

System.out.println(str); } }

Answer 76

“Hello World”

Java supports pass by value for primitives and objects.

In the method changeIt(…) you are giving a new value to the local variable but the original reference remains the same.

An example:
 Object o1 = new Object(); //Let us say, the object is stored at memory location 15000.
 //Since o1 actually stores the address of the memory location where the object is stored, it contains 15000.

Now, when you call someMethod(o1); the value 15000 is passed to the method.

Inside the method someMethod():

someMethod( Object localVar) {
/*localVar now contains 15000, which means it also points to the same memory location where the object is stored.
Therefore, when you call a method on localVar, it will be executed on the same object.
However, when you change the value of localVar itself, for example if you do localVar=null,
it then starts pointing to a different memory location. But the original variable o1 still
contains 15000 so it still points to the same object. */

Question 77

Which of the following classes should you use to represent just a date without any time or zone information?

1) java.util.Date
2) java.sql.Date
3) java.time.Date
4) java.time.LocalDate

Answer 77

4) java.time.LocalDate
java. util.Date is the old package

Question 78

What classes are included in the java.time Package?

What are some of the features of these classes?

Answer 78

LocalDate, LocalTime, LocalDateTime, Instant, Period, Duration

These classes are immutable and thread Safe

Question 79

In what package can java.time.format.DateTimeFormatter be found?

Answer 79

java.time.format package

Question 80

Given the following line of code:

LocalDateTime dt = LocalDateTime.parse(“2015-01-02T17:13:50”);

Which of the following lines will return the date string in ISO 8601 format?

1) dt.format(java.time.format.DateTimeFormatter.DATE_TIME);
2) dt.format(java.time.format.DateTimeFormatter.ISO_DATE_TIME);
3) dt.format(java.time.format.DateTimeFormatter.LOCAL_DATE_TIME);
4) dt.toString();

Answer 80

2 & 4

LocalDateTime’s toString method generates the String in ISO 8601 format.

Question 81

what is the output of this code?
public String getDateString(LocalDateTime ldt){

return DateTimeFormatter.ISO_ZONED_DATE_TIME.format(ldt);

}

Answer 81

The code will compile but will always throw a DateTimeException (or its subclass) at run time.

Note that LocalDateTime class does not contain Zone information but **ISO\_ZONED\_DATE\_TIME** requires it. Thus, it will throw the following exception:
**Exception in thread "main" java.time.temporal.UnsupportedTemporalTypeException: Unsupported field: OffsetSeconds**

UnsupportedTemporalTypeException extends DateTimeException.

Question 82

Name 2 features of the Date-Time API?

Answer 82

It uses the calendar system defined in ISO-8601 as the default calendar.

Most of the actual date related classes in the Date-Time API such as LocalDate, LocalTime, and LocalDateTime are immutable.

These classes do not have any setters. Once created you cannot change their contents. Even their constructors are private.

Question 83

What will the following code print?

String abc = “”;

abc. concat(“abc”);
abc. concat(“def”);

System.out.print(abc);

Answer 83

It will print empty string (or in other words, nothing).

Strings are immutable so doing abc.concat(“abc”) will create a new string “abc” but will not affect the original string “”.

Question 84

How can you initialize a StringBuilder to have a capacity of at least 100 characters?

Answer 84

StringBuilder sb = new StringBuilder(100);

Or

StringBuilder sb = new StringBuilder();
sb.ensureCapacity(100); // Ensures that the capacity is at least equal to the specified minimum

Observe that the question says “at least 100 characters”. In the exam, you may get a question that says “100 characters”, in that case, ensureCapacity() may not be a valid option.

Question 85

Given:

abstract class Vehicle{ }

interface Drivable{ }

class Car extends Vehicle implements Drivable{ }

class SUV extends Car { }

Which of the following options will compile?

1) ArrayList<vehicle> al1 = new ArrayList&lt;&gt;();<br></br>SUV s = al1.get(0)</vehicle>
2) ArrayList<drivable> al2 = new ArrayList&lt;&gt;();<br></br>Car c1 = al2.get(0)</drivable>
3) ArrayList<suv> al3 = new ArrayList&lt;&gt;();<br></br>Drivable d1 = al3.get(0);</suv>
4) ArrayList<suv> al4 = new ArrayList&lt;&gt;();<br></br>Car c2 = al4.get(0);</suv>
5) ArrayList<vehicle> al5 = new ArrayList&lt;&gt;();<br></br>Drivable d2 = al5.get(0);</vehicle>

Answer 85

3 and 4 will compile // the rest will need to be cast as they are more general types being assigned to more specific types

Question 86

What would the output of the following code be?

int[] numbers = {2, 4, 6, 8};
System.out.println(Arrays.binarySearch(numbers, 2));
System.out.println(Arrays.binarySearch(numbers, 4));
System.out.println(Arrays.binarySearch(numbers, 1));
System.out.println(Arrays.binarySearch(numbers, 3));
System.out.println(Arrays.binarySearch(numbers, 9));

Answer 86

0
1
-1
-2
-5

Question 87

true/false, the following lines of code will compile:

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

Answer 87

true

Question 88

What kind of results would you get when trying to do a binary search in the following array?

int[] numbers = {3, 2, 1, 5};

Answer 88

Unpredictable as the array hasnt been sorted first before the search

Question 89

When using Arrays.binarySearch, if no match is found, what happens?

Answer 89

it negates the position where the element would need to be inserted and subtracts 1.

Question 90

Arrays.binarySearch() searches a sorted array and returns what? What does it take as its parameters?

Answer 90

It takes the arrayList and an element in the list and returns the index of a match

Question 91

What calendar does the Date Time API work on?

Answer 91

The ISO-8601 calendar aka the gregorian calendar

Question 92

What import do you need inorder to use the date time package?

Answer 92

import java.time.*;

Question 93

If today is the 27th of October 2020 and the current time is 7.30 pm, print out the current date, time and dateTime .

Answer 93

you use the static now method

LocalDate.now(); // 2020-10-27

LocalTime.now(); // 19:30:43.875 hour:min:seconds:nanoseconds

LocalDateTime.now(); // 2020-10-27T19:30:43:875

Question 94

Given:

LocalDate d1 = LocalDate.parse(“2015-02-05”, DateTimeFormatter.ISO_DATE); LocalDate d2 = LocalDate.of(2015, 2, 5);

LocalDate d3 = LocalDate.now();

System.out.println(d1);

System.out.println(d2);

System.out.println(d3);

Assuming that the current date on the system is 5th Feb, 2015, which of the following will be a part of the output?

Answer 94

None of the mentioned.

All these prints will produce dates in this format : 2015-02-05

Question 95

What will this print?

Period period = Period.of(3,5,12);

System.out.println(period);

Answer 95

It will print P3Y5M12D.

inorder to get a date format, you have to use period with either the LocalDate or LocalDateTime objects

E.g - dateNow.minus(period); // 2017-05-15

or

dateTimeNow.minus(period); // 2017-05-15T21:15:56.280

If you use the time object, you will get a runtime exception

Question 96

What is the output of the following code?

DateTimeFormatter f = DateTimeFormatter.ofPattern(“dd MM yyyy”); // 1

LocalDate date = LocalDate.parse(“06 12 1992”, f ); //2

LocalTime time = LocalTime.parse(“11:22”);

System.out.println( “The date is: “ + date + "n” + “ The time is: “ + time);

Answer 96

The date is: 1992-12-06
The time is: 11:22

On line 1, we create a DTF object with pattern on dd MM yyyy.

On line 2, we use the parse method of LocalDate that takes a char sequence/ text and the preveiously DTF object. the text inside the parse params should match the pattern of the DTF obj otherwise, we get a runtime exception

Question 97

What is the output of the following code?
LocalDate date = LocalDate.of(2020, Month.JANUARY, 20);
date.plusDays(10);
System.out.println(date);

Answer 97

2020-01-20

As LocalDate is immutable it can’t be changed after its been created

Question 98

What method in the StringBuilder class does not modify the value of the object?

Answer 98

substring() // substring() returns a string and not a reference to the SB object thus calling this method will have no effect on the reference

Question 99

What is the output of the following code?

LocalDate d = LocalDate.of(2020, Month.JANUARY, 32);

Answer 99

DateTimeException which is a runtime exception

Question 100

What is the output of the following code?

LocalDate date = LocalDate.of(2020, Month.JANUARY, 20);
LocalTime time = LocalTime.of(5, 15);
LocalDateTime dateTime = LocalDateTime.of(date, time);
System.out.println(dateTime);
datetime = dateTime.minusDays(1);
System.out.println(dateTime);
dateTime = dateTime.minusHours(10);
System.out.println(dateTime);
dateTime = dateTime.minusSeconds(30);
System.out.println(dateTime);

Answer 100

2020-01-20T05:15
2020-01-19T05:15
2020-01-18T19:15
2020-01-18T19:14:30

Question 101

What is the output of the following code?

LocalDate date = LocalDate.of(2020, Month.JANUARY, 20);
LocalTime time = LocalTime.of(11, 12, 34);
LocalDateTime dateTime = LocalDateTime.of(date, time);
DateTimeFormatter shortF = DateTimeFormatter.ofLocalizedDateTime(FormatStyle.SHORT);
DateTimeFormatter mediumF = DateTimeFormatter.ofLocalizedDateTime(FormatStyle.MEDIUM);
System.out.println(shortF.format(dateTime));
System.out.println(mediumF.format(dateTime));

Answer 101

1/20/20 11:12 AM
Jan 20, 2020 11:12:34 AM

Question 102

What does this code print?

StringBuilder sb1 = new StringBuilder();

StringBuilder sb2 = sb1;

sb1.append(“girl,”); sb2.append(“woman,”); sb2.append(“other”);

System.out.println(sb1.equals(sb2));

System.out.println(sb1 == sb2);

System.out.println(sb1);

Answer 102

true
true
girl,woman,other. // sb1 & sb2 are pointing to the same object thus will have the same contents

equals method checks to see whether the references are pointing to the same object rather than looking at the values inside.

Question 104

What does this code print?

Period p = Period.ofYears(5).ofDays(7).ofMonths(6);

Answer 104

P6M

Chaining methods doesnt work on Periods “of” method and will only evaluate the last method.

Question 105

Period period = Period.of(3,5,12);

period = period.minusMonths(3).plusDays(15).plusYears(4);

System.out.println(period)

Answer 105

P7Y2M27D

Question 106

What are some of the features of the java.time package?

Answer 106

Classes are immutable

The constructors are private which forces us to use their static methods

Question 107

Which class is concat method a member of?

Answer 107

String class

Question 108

What will be the output of this code?

public class Whiz{

public static void main(String [] args) {

chars[] chars = {‘1’,’Z’, ‘0’,’-‘,’8’,’1’};

StringBuilder sb = new StringBuilder();

sb. append(chars,0,chars.length-1);
sb. append(‘0’);
sb. append(‘8’);

System.out.println(sb); }}

1. 1Z0-808
2. 1Z0-8108
3. 1Z0-810
4. Compilation fails due to an error on line 6
5. Compilation fails due to an error on line 7

Answer 108

1) 1Z0-808 is correct

We are using one of the overloaded versions of SB’s append method that takes a char sequence, starting index and length of the seq.

So this method will add the sub char array starting from index 0 up to 4 {1Z0-8} to sb then we continue to add 0 then 8 to make 1Z0-808.

Question 109

if “x.equals(y)” returns true, which of the folowing is true?

I. “y.hashCode()” must be equal to “x.hashCode()”

II. Both “x” and “y” objects should have the same field status

III. “y.hashCode()” must be equal to “x.hashCode()”

Answer 109

I is correct.

if “x.equals(y)” returns true, then the hashcodes of both objects must be equal

Question 110

Consider below code of Test.java file, What is the result?

package com.udayankhattry.oca;

import java.time.LocalDate;

public class Test {

public static void main(String [] args) {

LocalDate date = LocalDate.parse(“1983-06-30”);

System.out.println(date.plusMonths(8));

}

}

Answer 110

1984-02-29

Explanation:

plusMonths(long) method of LocalDate class returns a copy of this LocalDate with the specified number of months added.

This method adds the specified amount to the months field in three steps:

Add the input months to the month-of-year field

Check if the resulting date would be invalid

Adjust the day-of-month to the last valid day if necessary

For the given code,

1983-06-30 plus 8 months would result in the invalid date 1984-02-30. Instead of returning an invalid result, the last valid day of the month, 1984-02-29, is returned.

Please note, 1984 is leap year and hence last day of February is 29 and not 28.

Question 111

Given code of Test.java file:

public class Test {

public static void main(String[] args) {

args[1] = “Day!”;

System.out.println(args[0] + “ “ + args[1]);

}

}

What is the result?

1. Compilation Error
2. Good
3. An exception is thrown at Runtime
4. Good Day!

Answer 111

3.An exception is thrown at Runtime

When command java Test Goo is run, Good is passed into args. Now the args array is of sixe 1 with Good at index 0.

args[1] = “Day!” is trying to access index 1 which doesnt exist and thus will cause an ArrayIndexOutOfBoundsException

Question 112

Consider below code of Test.java file, what will be printed?

public class Test {

public static void main(String[] args) {

List<string> list = new ArrayList&lt;&gt;();</string>

list. add(“P”);
list. add(“O”);
list. add(“T”);

List<string> subList = list.subList(1, 2); //Line 1</string>

subList.set(0, “E”); //Line 2

System.out.println(list); //Line 3

}

}

Answer 112

[P,E,T] will be printed

1. On line 1, we are creating a new list called sublist using List’s subList method. Similar to String’s subString method, it returns the element at the begining index inclusive and end index exclusive.
2. So sublist = [O]
3. On line 2 we replace the element at index 0 with E so sublist = [E]
4. On line 3 list is printed: as we used list to create sublist, the two are now linked so whatever changes made to sublist are reflected in list.