Core Java 2015

1. Which one of the following is NOT a reserved word in Java?

Answers:

• final

• private

• virtual

• public

2. Why is it important to override hashCode() when you override equals()?

Answers:

• equals() will throw an exception if hashCode() is not overridden as well.

• It is NOT important to override hashCode() when overriding equals().

• Overriding equals without hashCode breaks the contract of hashCode().

3. What is the issue with the following code? String s = ""; for(int i = 0; i < 1000000; i++) { s += Integer.toString(i); }

Answers:

• It will not compile.

• It will perform very slowly because strings are immutable.

• It will perform very slowly because Integer.toString() is slow.

• There are no issues with the above code.

4. Complete the sentence: One may extend a class from ...

Answers:

• Many classes through multiple inheritance

• Only one inner class

• Only one interface

• Only one non-final class

5. Which one of these lists contains only Java programming language keywords?

Answers:

• class, if, void, long, int, continue

• try, virtual, throw, final, volatile, transient

• byte, break, assert, switch, include

• strictfp, constant, super, implements, do

• goto, instanceof, native, finally, default, throws

6. How should you create a new class that maps keys to values, using the Java Collections Framework?

Answers:

• Extend the AbstractCollection class, thereby implementing the AbstractCollection interface

• Implement the Map interface, possibly by extending the AbstractMap class

• Implement both the Iterator and Array interfaces

• Implement the Queue, List, and Array interfaces

7. The Object.wait() method:

Answers:

• Resumes from waiting if notifyAll() is invoked for the object

• Resumes from waiting if notify() is invoked for the object

• Causes the current thread to wait

• Resumes from waiting if a specified amount of time has elapsed

8. A java class which extends another class is usually described with the word:

Answers:

• overloaded

• abstract

• subclass

• dynamic

9. Which of these are advantages of encapsulation in Java?

Answers:

• Encapsulated code is easy to change with new requirements

• All of these

• Encapsulation in Java makes unit testing easy

• Encapsulation reduces coupling of modules and increase cohesion inside a module

10. To document an API, which tool do you use?

Answers:

• documentcreate

• javadoc

• javaApi

• apicreate

11. If we want a class not to be overridden,the class must be done as

Answers:

• Class should be static

• Class should be final

• Class should public

• Class should be abstract

12. The "javac" command line tool is used to:

Answers:

• Convert java bytecode files into native executables

• Compile java source files into bytecode class files

• Compress collections of java class files into .jar archives

• Generate C headers and stubs for native methods

13. Which additional keyword may be used with try-catch blocks?

Answers:

• finally

• finish

• final

• finalize

14. The most reliable way to compare two Strings for equality is by:

Answers:

• Using the &= operator on the objects

• Using the == operator on the objects

• Using the == operator on the .value() of each object

• Using the .equals() or .compareTo() method of one object on the other

15. Java handles memory allocation and reuse using a process called:

Answers:

• Garbage Collection

• Buddy Blocks

• Manual Memory Management

• Virtual Memory

16. The part of a "try" block that is always executed is:

Answers:

• "enum"

• "finally"

• "if"

• "import"

17. To define a child class from the Parent class following is used:

Answers:

• class Child :: Parent

• class Child extends Parent

• class Child extends Public Parent

• class Child : Parent

18. What is an example of proper capitalization for a class name?

Answers:

• CAMELcase

• CamelCase

• camELCase

• camelcase

19. If a method or variable is marked as having the "private" access level, then it can only be accessed from:

Answers:

• Inside the same class or its parent class

• Inside the same class

• Inside the same class, or any of its superclasses

• Inside the same class, or a subclass

20. What is the most efficient way to concatenate a large number of strings in Java?

Answers:

• The + operator.

• The StringBuffer object.

21. Finally is used to....

Answers:

• ensure a block of code is executed when the JVM shuts down.

• ensure a block of code is executed only when try/catch completes without an exception

• ensure a block of code is always executed after a try/catch

• ensure a block of code is executed only when try/catch completes with an exception

22. Which of the following is a valid constructor signature?

Answers:

• public static className()

• static className()

• public void className()

• public className()

23. When you create a thread with the “new” operator – which one of the following statements is true about its state

Answers:

• it starts running immediately

• it is blocked until another thread calls notify()

• it is in “runnable” state

• it will be “runnable” when start() method is called

24. The Thread.sleep() method:

Answers:

• Causes all threads to suspend execution

• Suspends execution in synchronized methods only

• Causes the current thread to suspend execution

• Causes the hosted virtual machine to suspend all forms of execution

25. How can you stop your class from being inherited by another class?

Answers:

• It’s not possible.

• Declare the class as abstract.

• Declare the class as final.

• Declare the class default constructor as private.

26. Can an abstract class be a final class?

Answers:

• Yes

• No

27. What is the correct way to create an instance of a class?

Answers:

• ClassName varName = new ClassName(new ClassName);

• ClassName varName = new ClassName(arguments);

• ClassName varName => new ClassName();

• varName ClassName = new varName(arguments);

28. True of False? The strictfp keyword ensures that you get the same result on every platform if you perform operations in the floating point variable.

Answers:

• False

• True

29. Interfaces are useful for...

Answers:

• creating a design contract that encapsulates implementation

• implementing an abstract factory pattern

• reducing heap size

• making an abstract class concrete

30. Which of these is true?

Answers:

• An interface implements another interface and class

• A class implements and extends a class

• An interrface extends a class but implements another interface

• A class implements an interface but extends a class

31. How can we use the class or jar files kept on the network path, within our projects?

Answers:

• mentioning the file names in the Path

• by directly copying and including in the same folder as of the project

• Including the path and class /jar file name in the Classpath

• No the network files can not be used directly

• mentioning the Class /Jar file names during compilation only

32. What is auto boxing?

Answers:

• JVM conversion between primitive types and reference types

• JVM conversion of int to float values

• It doesn’t occur in Java, only in dynamically typed JVM languages like Groovy

• Automatic insertion of brackets by an IDE

33. The "java" command line tool is used to:

Answers:

• Load and execute java .class files

• Disassemble .class files back into readable source code

• Compile java source files into bytecode class files

• Compress collections of java class files into .jar archives

34. The Core Java platform provides many benefits to developers, including:

Answers:

• A purely functional programming language with a minimalist design philosophy

• A consistent programming interface across multiple hardware platforms

• Direct compilation to native code on most platforms

• Superior speed and performance compared to native code

35. What method should you always override when you have overridden the equals() method?

Answers:

• clone()

• toString()

• hashCode()

• wait()

36. JDBC addresses the issue of transactions.

Answers:

• True

• False

37. The "static" keyword marks something as:

Answers:

• No longer able to be subclassed or overloaded

• A constant variable whose value cannot be changed

• Not being mutable after initialization

• Belonging to a class, rather than a specific instance

38. The instanceof operator can be used to determine if an object is:

Answers:

• An instance of a subclass of a class

• An instance of a class

• (All of these)

• An instance of a class that implements a given interface

39. What are all the different types of access modifiers in Java

Answers:

• private, public

• private, protected, default, public

• protected, default, public

• private, protected, public

• private, default, public

40. What is the benefit of ConcurrentHashMap<K,V>?

Answers:

• Allows null to be used a key or value

• Supports locking the entire table in a way that prevents all access

• It maintains a list through all entries to retrieve data in the order it was inserted.

• All operations are thread-safe and retrieval operations do not entail locking

41. You want subclasses in any package to have access to members of a superclass. Which is the most restrictive access that accomplishes this objective?

Answers:

• private

• public

• protected

• transient

42. When creating a user defined class for storing objects in a HashMap, which method(s) should be overridden?

Answers:

• The constructor method

• (You don't need override any methods)

• The equal() method

• The hashCode() method

• Both the equals() and hashCode() methods

43. When the == comparator is used on two objects, it checks to see if they:

Answers:

• Are references to exactly the same object

• Have the same value according to the .equals() method of the first object

• Evaluate to the same value

• Are instances of the same class

44. Java provides a class for mutable sequences of characters, called:

Answers:

• MutableString

• CharSequence

• StringBuffer

• String

45. What is the difference between a checked and unchecked exception?

Answers:

• An unchecked exception extends Throwable and a checked exception does not.

• A checked exception extends Throwable and an unchecked exception does not.

• Unchecked exceptions must be caught while checked do not need to caught

• Checked exceptions must be caught while unchecked do not need to caught

46. An "overloaded" method has what in common with one (or more) methods on the same class?

Answers:

• The same name

• The same number of parameters, regardless of type

• The same return type

• The same number and types of parameters

47. The List interface has which superinterfaces?

Answers:

• Set

• Collection

• Both Collection and Iterable

• Iterable

48. Calling System.gc() when using a modern JVM :

Answers:

• Directly and immediately disposes of all orphaned objects on the heap.

• Is especially important when programming for mobile or memory limited devices.

• Should be done after deleting several elements from a collection.

• Is the most straightforward and reliable way to perform memory management in Java.

• Does not necessarily force garbage collection to occur, and is not idiomatic java.

49. Which one of the following statements is true about Java Beans?

Answers:

• Java Beans cannot be used in server side programming, they are only used for Graphical User Interfaces.

• Java Beans are not permitted to create any threads

• All Java Beans must extend the java.bean base class

• Java Beans are user defined classes.

50. Which class/classes is/are thread safe among these?

Answers:

• StringBuilder

• String and StringBuffer

51. Which option is true for StringBuffer and StringBuilder

Answers:

• StringBuffer are not thread safe and StringBuilder are thread safe.

• StringBuffer are thread safe and StringBuilder are not thread safe

• StringBuffer and StringBuilder are thread safe.

• Neither StringBuffer nor StringBuilder are thread safe.

52. Which option is true for StringBuffer and StringBuilder

Answers:

• StringBuffer are not thread safe and StringBuilder are thread safe.

• StringBuffer are thread safe and StringBuilder are not thread safe

• StringBuffer and StringBuilder are thread safe.

• Neither StringBuffer nor StringBuilder are thread safe.

53. What is the output?  int[] xxx =  {10, 20};          List<String> list = new ArrayList<String>(10); list.add("01"); list.add("02");          System.out.println(xxx.length + ", " +list.size());

Answers:

• 2, 2

• Compile error

• 1, 2

• 2, 10

• 10, 2

54. What is a LinkedHashSet?

Answers:

• A hash set which preserves the order in which objects were inserted.

• A hash set with the performance of a linked list.

• A superclass of the HashSet object.

• A hash set which can easily be converted into a linked list.

55. Immutable objects are always...

Answers:

• memory efficient

• thread safe

• polymorphic

• serializable

56. A method without an access modifier (i.e. public, private, protected) is...

Answers:

• private

• protected

• package-private

• public

• static

57. In addition to CORBA, Core Java also supports network services using:

Answers:

• IPX / SPX Protocol

• Remote Object Access

• Remote Procedure Calls

• Remote Method Invocation

58. Java's automatic memory management:

Answers:

• Can be tuned using Virtual Machine settings

• Uses hardcoded settings inside each Virtual Machine, which should not be altered

• Can be configured to operate statically or dynamically at compile time

• Can be overriden using functions like alloc and dalloc

59. Java's String class is

Answers:

• Immutable, but can be subclassed

• Final, but creates mutable instances

• Mutable, and can be subclassed

• Final, with immutable instances

60. enum Example { ONE, TWO, THREE }    Which statement is true?

Answers:

• The expression (ONE < TWO) is guaranteed to be true and ONE.compareTo(TWO) is guaranteed to be less than one.

• The expressions (ONE == ONE) and ONE.equals(ONE) are both guaranteed to be true.

• The Example values cannot be used in a raw java.util.HashMap; instead, the programmer must use a java.util.EnumMap.

• The Example values can be used in a java.util.SortedSet, but the set will NOT be sorted because enumerated types do NOT implement java.lang.Comparable.

61. Which will compile successfully?

Answers:

• Object o = Old.get(new LinkedList<?>());

• Object o = Old.get(LinkedList());

• String s = Old.get(new LinkedList<String>());

• String s = (String)Old.get(new LinkedList<String>());

62. A class implementing a singleton pattern has...

Answers:

• no public constructors and static factory method and a non-static instance variables.

• public constructors instead of a static factory method and a static instance variable.

• no public constructors, a private static factory method, a static instance variable.

• no public constructors, a public static factory method, a static instance variable.

63. Which of the following is true about overloading vs overriding methods?

Answers:

• Final methods can be overriden, but not overloaded

• The argument list of overloaded methods must be of same data type (unlike overridden methods)

• Overloading happens at compile time, while overriding happens at runtime

• Overloading can arbitrarily change the access of a method, while overriding can only make it more restrictive

64. What is the result when this code is executed:    class One {    public One() { System.out.print(1); }    }    class Two extends One {    public Two() { System.out.print(2); }     }     class Three extends Two {    public Three() { System.out.print(3); }    }    public class Numbers{    public static void main( String[] argv ) { new Three(); }     }

Answers:

• 3

• 123

• 321

• 1

65. How do you convert int[]  to a ArrayList<Integer>?

Answers:

• Using toArrayList()

• Casting

• In a loop, creating new Integers.

• Using the static Arrays.asList method

66. What is a weak reference?

Answers:

• A reference to an object which may have been garbage collected when the object is asked for.

• A reference to an object that cannot be garbage collected.

• A reference to an object which is about to be garbage collected.

• A reference to an object which has been garbage collected.

67. Can the "main" method be overloaded

Answers:

• No

• Yes

68. How many objects are created: String s1="String"; String s2="String"; String s3="String";

Answers:

• Two

• None

• One

• Three

69. What Object would you use to represent currency ?

Answers:

• Currency

• Integer

• BigDecimal

• Double

70. What type should you use for floating point monetary calculations?

Answers:

• BigDecimal

• float

• byte

• double

71. What will be printed out if you attempt to compile and run the following code?  int i=9;  switch (i) {          default:          System.out.println("default ");                  case 0:          System.out.println("zero ");                  break;          case 1:                  System.out.println("one ");          case 2:          System.out.println("two ");  }

Answers:

• default zero

• no output displayed

• default

• error default clause not defined

72. After the following code fragment, what is the value in a? String s; int a; s = "Foolish boy."; a = s.indexOf("fool");

Answers:

• 1

• 0

• -1

• 4

• random value

73. public static void append(List list) { list.add("0042"); }    public static void main(String[] args) {    List<Integer> intList = new ArrayList<Integer>();    append(intList);    System.out.println(intList.get(0));    }      What is the result?

Answers:

• An exception is thrown at runtime.

• Compilation fails because of an error in line 14.

• 42

• 0042

• Compilation fails because of an error in line 13.

74. To create a single instance of a class, we can go with

Answers:

• (none of these)

• Static class

• Final class

• Abstract class

75. When must you override hashcode and equals method?

Answers:

• You should always do so as a matter of best practice

• For better performance when you want to use the object as a key in a HashMap

• When you want to serialize objects

• When you want to sort objects using Comparable

76. what is the output of following code? class A  {  int x=12; public static void main(String args[])  {  x++; System.out.println(x); } }

Answers:

• 12

• Compile time Error.

• Run time Error.

• 13

77. Anonymous inner classes have access to...

Answers:

• Any local variables in the containing scope

• Final, local variables in the containing scope

• Only variables passed into the constructor, and no variables in the containing class

• Only synchronized collections in the containing class

78. Which statement is true?

Answers:

• Any statement that can throw an Exception must be enclosed in a try block.

• catch(X x) can catch subclasses of X where X is a subclass of Exception.

• Any statement that can throw an Error must be enclosed in a try block.

• The Error class is a RuntimeException.

79. In your program, you need to read a zip file (myfile.zip) containing several other data files containing basic Java objects. Which of the following will allow you to construct a InputStream for the task?

Answers:

• new ZipInputStream(new FileInputStream(“myfile.zip”));

• new ObjectInputStream(new ZipInputStream( new FileInputStream((“myfile.zip”)));

• new DataInputStream(new FileInputStream(“myfile.zip”));

• new ZipInputStream(new ObjectInputStream(“myfile.zip”));

80. Does interrupt() always force all threads to terminate?

Answers:

• Yes, the thread gets to a predefined interruption point and stops

• No, if the interruption is not enabled for the thread, it will not terminate

• Yes, after interrupt() is called a thread terminates immediatelly

81. The documentation comment starts and ends with

Answers:

• /** and **/

• /** and */

• /* and **/

• /* and */

82. Float p = new Float(3.14f);    if (p > 3) {    System.out.print("p is bigger than 3. ");     }    else {    System.out.print("p is not bigger than 3. ");     }    finally {     System.out.println("Have a nice day.");     }      What is the result?

Answers:

• p is bigger than 3.

• p is bigger than 3. Have a nice day.

• Compilation fails.

• p is not bigger than 3. Have a nice day.

83. Which of the following statements about static inner classes is true?

Answers:

• A static inner class requires an instance of the enclosing class.

• A static inner class requires a static initializer.

• A static inner class has no reference to an instance of the enclosing class.

• A static inner class has access to the non-static members of the outer class.

84. All of the classes in the Java Collections Framework:

Answers:

• Have methods allowing them to be viewed as both Maps and Lists

• Involve data structures backed by Arrays

• Have methods to retrieve their data as an Array

• Allow elements to be added to the beginning or end of their internal List

85. Given the code: Integer i= new Integer("1"); if (i.toString() == i.toString()) System.out.println("Equal"); else System.out.println("Not Equal");

Answers:

• Compiler error

• Prints "Not Equal"

• None of the above

• Prints "Equal"

86. which statement is True ?

Answers:

• The notifyAll() method must be called from a synchronized context.

• The notify() method is defined in class java.lang.Thread.

• The notify() method causes a thread to immediately release its locks.

• To call wait(), an object must own the lock on the thread.

87. Which one of the following statements is true about threads in Java

Answers:

• the notify() method can only be called from inside a synchronized block or from a synchronized method

• the notify() method can not be called if your class extends Thread class, it can only be called if your class implements the “Runnable” interface

• the notify() must be called to wake up threads that have called wait() and notifyAll() must be called to wake up threads that have called join().

• the notify() method informs the Java Virtual Machine that it has finished executing

88. java.util.Collection is:

Answers:

• An abstract interface representing a group of objects, extended by Set and List

• An interface for iterable groups of objects

• An interface of non-destructive methods used by Set and List classes

• An abstract class representing a group of objects, extended by Set and List

89. A "blank" final variable (defined without an initial value):

Answers:

• Can be initialized later, but only in a single location

• Will raise an exception if its value is accessed or assigned at runtime

• Is illegal, and will cause an error at compile time

• Has a Null value, and will raise an exception if initialized or assigned later

90. LinkedBlockingQueue is useful for...

Answers:

• binary searches of a list of sorted objects

• a synchronized linked list

• implementing a stack

• implementing a producer consumer pattern

91. The keyword that ensures a field is coherently accessed by multiple threads is:

Answers:

• "native"

• "switch"

• "volatile"

• "synchronized"

92. ¿If the class X extends the class Y, wich of these is a polymorphic sentence?

Answers:

• Y y1 = (Y) new X(); y1.methodOfY();

• Y y1= new Y(); y1.methodOfX();

• Y y1= new X(); y1.methodOfY();

• Y y1= new X(); y1.methodOfX();

93. The TreeMap class is Java's implementation of which data structure?

Answers:

• Linked list

• B+ tree

• Red-Black tree

• B-tree

94. I would implement a LRU cache using only JDK classes by...

Answers:

• An ArrayList and binary search over the last accessed timestamp

• A Hashtable and iteration over all values that contain a last accessed timestamp.

• A LinkedList and binary search over the last accessed timestamp

• A TreeMap and iteration over values that containing a last accessed timestamp

95. Which code fragments correctly create and initialize a static array of int elements?

Answers:

• static final int[] a; static { a=new int[2]; a[0]=100; a[1]=200; }

• static final int a = { 100,200 };

• static final int[] a = new int[2]{ 100,200 };

• static final int[] a; static void init() { a = new int[3]; a[0]=100; a[1]=200; }

96. Which of the following are not a valid declarations?

Answers:

• float f = 1.2f;

• float f = (float)1.2;

• float f = 1.2;

• float f =1;

97. You need to keep a list of one million objects sorted at all times having 100 random inserts and delete per second. Reads from the list are always done in sorted order.  You would:

Answers:

• Use a HashMap to keep the list ordered at all times.

• Use a TreeMap for fast inserts and iterate over the list in order to re-sort it after each insert.

• Use a PriorityQueue to keep the list ordered at all times.

• Use a shell sort after each insert.

98. Livelock describes a situation in which two or more threads block each other, because:

Answers:

• Their actions are also responses to the actions of others, such that all are too busy to respond

• They are unable to gain access to shared resources, and cannot make any progress

• A call to Thread.sleep() has suspended the action on all threads in the VM

• Each are waiting on the other thread(s) before acting, such that no thread takes action

99. Which of the following is true about the Cloneable interface?

Answers:

• It changes the behavior of the protected clone method to give a field-by-field (reference) copy of the object.

• Is just a marker interface that does nothing.

• Creates an exact copy of the implementing class by calling its constructor.

• It provides a clone method that must be implemented.

100. You want to listen TCP connections on port 9000. How would you create the socket?

Answers:

• new Socket(9000);

• new ServerSocket(9000);

• new ServerSocket(“localhost”, 9000);

• new Socket(“localhost”, 9000);

101. The TreeMap and LinkedHashMap classes:

Answers:

• Enable iteration of a map's entries based on the insertion order of elements only.

• Enable iteration of a map's entries based either natural ordering of keys OR natural ordering of values - depending on the arguments sent to the constructor.

• Enable iteration of a map's entries based on natural ordering of keys only.

• Enable iteration of a map's entries in a deterministic order.

102. public class Test{ public static void main(String [] arg){ int x=10; if(x++ > 10 && x++ == 12){ ++x; } System.out.println(x); } }  What is the Output of this code?

Answers:

• 10

• 13

• 11

• 12

103. A Guarded Block is a concurrency idiom in which:

Answers:

• A condition is polled before the execution of a code block can proceed

• A block of code will only ever be executed in synchronized mode

• A block of code is protected by an intrinsic lock obtained from a specific Object or Class

• All statements in a block of code are guaranteed to be executed completely, or not at all

104. What's the output of following error ?  class A {   public Number getNumber(){       return 1;   } }  class B extends A {      public int getNumber(){       return 2;   } }   public class Main{       public static void main(String []args){         A a = new B();         System.out.println(a.getNumber());      } }

Answers:

• 1

• RuntimeError

• Compilation error

• 2

105. A class may extend:

Answers:

• Only one interface

• Only one non-final class

• Only one inner class

• Many classes through multiple inheritance

106. What is the difference between a checked and unchecked exception?

Answers:

• An unchecked exception extends Throwable and a checked exception does not.

• Unchecked exceptions must be caught while checked do not need to be caught

• Checked exceptions must be caught while unchecked do not need to be caught

• A checked exception extends Throwable and an unchecked exception does not.

107. What is the name of the method used to start a thread execution?

Answers:

• resume();

• init();

• run();

• start();

108. Java variables are passed into methods as:

Answers:

• Pass-by Value

• Pass-by Reference

• Neither

109. What is @Override annotation used for?

Answers:

• It makes compiler check that method is really overridden

• It just makes your code easier to read

• It makes method virtual

110. What is the correct syntax for importing java.util.Scanner?

Answers:

• import.java.util.scanner;

• import java.util.Scanner;

• import.java.util.scanner.

• import. java.util.Scanner;

111. What is the output?      public static void main(String[] args) {         int x=10;         if(x++ > 9 && x++ == 12){             ++x;         }         System.out.println(x);     }

Answers:

• 11

• 12

• 13

• 10

112. What will be the output of this code?  class Main {    static abstract class Base {                  protected Base() {                       init();                 }                                  abstract void init(); }                   static class Child extends Base {                 private final int value;                           public Child() {                           value = 5;                  }                                  @Override                public void init() {                      System.out.println("value = " + value);              }       }                public static void main(String[] args) {                  Child c = new Child();       } }

Answers:

• Runtime exception

• Compilation error

• value = 5

• value = 0