dammytutorial: Java Interview questions and answers part3

What is the result of below code?

Given the proper import statement(s), and

PriorityQueue<String> pq = new PriorityQueue<String>();
pq.add("2");
pq.add("4");
System.out.print(pq.peek() + " ");
pq.offer("1");
pq.add("3");
pq.remove("1");
System.out.print(pq.poll() + " ");
if(pq.remove("2")) System.out.print(pq.poll() + " ");
System.out.println(pq.poll() + " " + pq.peek());

A. 2 2 3 3
B. 2 2 3 4
C. 4 3 3 4
D. 2 2 3 3 3
E. 4 3 3 3 3
F. 2 2 3 3 4
G. Compilation fails
H. An exception is thrown at runtime
Answer:
B is correct. For the sake of the exam, add() and offer() both add to (in this case),
naturally sorted queues. The calls to poll() both return and then remove the first item
from the queue, so the if test fails.

Question:
Given:

import java.util.*;
class Turtle {
int size;
public Turtle(int s) { size = s; }
public boolean equals(Object o) { return (this.size == ((Turtle)o).size); }
// insert code here
}
public class TurtleTest {
public static void main(String[] args) {
LinkedHashSet<Turtle> t = new LinkedHashSet<Turtle>();
t.add(new Turtle(1)); t.add(new Turtle(2)); t.add(new Turtle(1));
System.out.println(t.size());
}
}

And these two fragments:
I. public int hashCode() { return size/5; }
II. // no hashCode method declared
If fragment I or II is inserted, independently, at line 8, which are true? (Choose all that apply.)
A. If fragment I is inserted, the output is 2
B. If fragment I is inserted, the output is 3
C. If fragment II is inserted, the output is 2
D. If fragment II is inserted, the output is 3
E. If fragment I is inserted, compilation fails
F. If fragment II is inserted, compilation fails
Answer:
A and D are correct. While fragment II wouldn't fulfill the hashCode() contract (as you
can see by the results), it is legal Java. For the purpose of the exam, if you don’t override
hashCode(), every object will have a unique hashcode.

Which are true? (Choose all that apply.) Given the proper import statement(s), and:

TreeSet<String> s = new TreeSet<String>();
TreeSet<String> subs = new TreeSet<String>();
s.add("a"); s.add("b"); s.add("c"); s.add("d"); s.add("e");
subs = (TreeSet)s.subSet("b", true, "d", true);
s.add("g");
s.pollFirst();
s.pollFirst();
s.add("c2");
System.out.println(s.size() +" "+ subs.size());

A. The size of s is 4
B. The size of s is 5
C. The size of s is 7
D. The size of subs is 1
E. The size of subs is 2
F. The size of subs is 3
G. The size of subs is 4
H. An exception is thrown at runtime
Answer:
B and F are correct. After "g" is added, TreeSet s contains six elements and TreeSet subs
contains three (b, c, d), because "g" is out of the range of subs. The first pollFirst()
finds and removes only the "a". The second pollFirst() finds and removes the "b" from
both TreeSets (remember they are backed). The final add() is in range of both TreeSets.
The final contents are [c,c2,d,e,g] and [c,c2,d].

What is the output of below program?

class Magellan {
public static void main(String[] args) {
TreeMap<String, String> myMap = new TreeMap<String, String>();
myMap.put("a", "apple"); myMap.put("d", "date");
myMap.put("f", "fig"); myMap.put("p", "pear");
System.out.println("1st after mango: " +myMap.higherKey("f"));
System.out.println("1st after mango: " + myMap.ceilingKey("g"));
System.out.println("1st after mango: " +myMap.floorKey("g"));
SortedMap<String, String> sub = new TreeMap<String, String>();
sub = myMap.tailMap("f");
System.out.println("1st after mango: " +sub.firstKey());
System.out.println("1st after mango: " + sub.lastKey());
System.out.println(sub.size());
}
}

Answer:
1st after mango: p
1st after mango: p
1st after mango: f
1st after mango: f
1st after mango: p
2

Which, inserted independently at line 9, will compile? (Choose all that apply.)

import java.util.*;
class Business { }
class Hotel extends Business { }
class Inn extends Hotel { }
public class Travel {
ArrayList<Hotel> go() {
// insert code here
}
}

A. return new ArrayList<Inn>();
B. return new ArrayList<Hotel>();
C. return new ArrayList<Object>();
D. return new ArrayList<Business>();
Answer:
B is correct.
A is incorrect because polymorphic assignments don't apply to generic type parameters. C
and D are incorrect because they don't follow basic polymorphism rules.

What is the result?

import java.util.*;
class Dog { int size; Dog(int s) { size = s; } }
public class FirstGrade {
public static void main(String[] args) {
TreeSet<Integer> i = new TreeSet<Integer>();
TreeSet<Dog> d = new TreeSet<Dog>();

d.add(new Dog(1)); d.add(new Dog(2)); d.add(new Dog(1));
i.add(1); i.add(2); i.add(1);
System.out.println(d.size() + " " + i.size());
}
}

A. 1 2
B. 2 2
C. 2 3
D. 3 2
E. 3 3
F. Compilation fails
G. An exception is thrown at runtime
Answer:
G is correct. Class Dog needs to implement Comparable in order for a TreeSet (which
keeps its elements sorted) to be able to contain Dog objects.

What is output of below program?

class GeoCache {
public static void main(String[] args) {
String[] s = {"map", "pen", "marble", "key"};
Othello o = new Othello();
Arrays.sort(s,o);//perameters here String[] and class obj which should impliment Comparator
for(String s2: s) System.out.print(s2 + " ");
System.out.println(Arrays.binarySearch(s, "map"));//this method meake use of compareTo of Othello
}
static class Othello implements Comparator<String> {
public int compare(String a, String b) { return a.compareTo(b); }
}
}

Answer:
output:key map marble pen 1

What is output of below program?

Answer:
output:pen marble map key -1

Which are true? (Choose all that apply.)

import java.util.*

public class GeoCache {

public static void main(String[] args) {

String[] s = {"map", "pen", "marble", "key"};

Othello o = new Othello();

Arrays.sort(s,o);

for(String s2: s) System.out.print(s2 + " ");

System.out.println(Arrays.binarySearch(s, "map"));

}

static class Othello implements Comparator<String> {

public int compare(String a, String b) { return b.compareTo(a); }

}

A. Compilation fails
B. The output will contain a 1
C. The output will contain a 2
D. The output will contain a –1
E. An exception is thrown at runtime
F. The output will contain "key map marble pen"
G. The output will contain "pen marble map key"
Answer:
D and G are correct. First, the compareTo() method will reverse the normal sort.
Second, the sort() is valid. Third, the binarySearch() gives –1 because it needs to be
invoked using the same Comparator (o), as was used to sort the array. Note that when the
binarySearch() returns an "undefined result" it doesn’t officially have to be a -1, but it
usually is, so if you selected only G, you get full credit!

Assume you have a class that holds two private variables: a and b. Which of the following pairs can prevent concurrent access problems in that class? (Choose all that apply.)

A. public int read(){return a+b;}
public void set(int a, int b){this.a=a;this.b=b;}
B. public synchronized int read(){return a+b;}
public synchronized void set(int a, int b){this.a=a;this.b=b;}
C. public int read(){synchronized(a){return a+b;}}
public void set(int a, int b){synchronized(a){this.a=a;this.b=b;}}
D. public int read(){synchronized(a){return a+b;}}
public void set(int a, int b){synchronized(b){this.a=a;this.b=b;}}
E. public synchronized(this) int read(){return a+b;}
public synchronized(this) void set(int a, int b){this.a=a;this.b=b;}
F. public int read(){synchronized(this){return a+b;}}
public void set(int a, int b){synchronized(this){this.a=a;this.b=b;}}

Answer:
B and F are correct. By marking the methods as synchronized, the threads will get the
lock of the this object before proceeding. Only one thread will be setting or reading at any
given moment, thereby assuring that read() always returns the addition of a valid pair.
Wrong:
A is incorrect because it is not synchronized; therefore, there is no guarantee that the values
added by the read() method belong to the same pair. C and D are incorrect; only objects
can be used to synchronize on. E fails —it is not possible to select other objects (even this)
to synchronize on when declaring a method as synchronized.

Choose Appropriate option? Given:

public class TwoThreads {
static Thread laurel, hardy;
public static void main(String[] args) {
laurel = new Thread() {
public void run() {
System.out.println("A");
try {
hardy.sleep(1000);
} catch (Exception e) {
System.out.println("B");
}
System.out.println("C");
}
};
hardy = new Thread() {
public void run() {
System.out.println("D");
try {
laurel.wait();
} catch (Exception e) {
System.out.println("E");
}
System.out.println("F");
}
};
laurel.start();
hardy.start();
}
}

Which letters will eventually appear somewhere in the output? (Choose all that apply.)
A. A
B. B
C. C
D. D
E. E
F. F
G. The answer cannot be reliably determined
H. The code does not compile
Answer:
A, C, D, E, and F are correct. This may look like laurel and hardy are battling to cause
the other to sleep() or wait()—but that's not the case. Since sleep() is a static
method, it affects the current thread, which is laurel (even though the method is invoked
using a reference to hardy). That's misleading but perfectly legal, and the Thread laurel
is able to sleep with no exception, printing A and C (after at least a 1-second delay). Meanwhile
hardy tries to call laurel.wait()—but hardy has not synchronized on laurel,
so calling laurel.wait() immediately causes an IllegalMonitorStateException, and
so hardy prints D, E, and F. Although the order of the output is somewhat indeterminate
(we have no way of knowing whether A is printed before D, for example) it is guaranteed
that A, C, D, E, and F will all be printed in some order, eventually—so G is incorrect.

Will below code compiles?

class A1 {
int a=10;
static int add(int i, int j){
return i+j;
}
}
class B1 extends A1{
int a=10;
public static void main(String argv[]){
short s = 9;
System.out.println(add(s,6)+" "+super.a);//error becoz of super key word.
}
}

Answer:
error bcoz of "super" key word.We can not use "this" and "super" key word in side static methods and blocks(Cannot use this(or)super in a static context)

Also see below:

Java Interview questions and answers part1

Java Interview questions and answers part2

SQL Interview Questions