Question 1

Which Java class inherits its equals() and hashCode() directly from java.lang.Object (i.e., does not override them)?

Accepted Answer

Introduction / Context: Many core Java classes override equals() and hashCode() to provide value-based equality. Others, notably mutable sequence classes, keep identity-based equality from Object. Recognizing which is which is a common exam topic. Given Data / Assumptions: We compare several classes from java.lang. We must identify the one that does not override equals() and hashCode(). Concept / Approach: String, Integer, Double, and Character are value types and override equals()/hashCode() to compare by content/value. StringBuffer, being a mutable buffer, inherits identity-based equality from Object, meaning two different buffers with the same characters are not considered equal unless they are the same instance. Step-by-Step Reasoning: Check class docs/behavior: String → overrides (content-based).Double, Integer, Character → wrapper types, also override.StringBuffer → does not override; identity equality. Verification / Alternative check: Printing new StringBuffer("a").equals(new StringBuffer("a")) yields false, confirming identity equality. Why Other Options Are Wrong: Each of them defines value-based equality and a corresponding hash code implementation. Common Pitfalls: Confusing StringBuffer with StringBuilder (neither overrides equals/hashCode) versus String which does. Final Answer: java.lang.StringBuffer

Question 2

Which Java collection lets you associate elements with keys while iterating in insertion (FIFO) order?

Accepted Answer

Introduction / Context: Different Java collections provide different ordering guarantees. The question asks for a map (key–value association) that also preserves insertion order, enabling FIFO-style iteration. Given Data / Assumptions: We need key–value association (i.e., a Map). We want predictable iteration order: specifically insertion order (FIFO-like). Concept / Approach: LinkedHashMap combines a hash table with a doubly-linked list to maintain predictable iteration order. By default, it preserves insertion order; alternatively, it can be configured for access order. Other map implementations either do not preserve order (HashMap) or impose a sort order by key (TreeMap), not insertion order. Step-by-Step Reasoning: Eliminate non-Map types: ArrayList and PriorityQueue are not maps.Consider map types: HashMap → no order guarantee; TreeMap → sorted by key; LinkedHashMap → preserves insertion order. Verification / Alternative check: Quick test inserting keys A, B, C into a LinkedHashMap and iterating shows A, B, C order. Why Other Options Are Wrong: ArrayList lacks keys; HashMap is unordered; TreeMap orders by key comparator; PriorityQueue is a heap, not key–value store. Common Pitfalls: Confusing insertion order with sorted order; assuming HashMap iteration order is stable (it is not guaranteed). Final Answer: java.util.LinkedHashMap

Question 3

Java — Which interface does java.util.Hashtable implement?

Accepted Answer

Introduction / Context: The java.util.Hashtable class is one of the legacy collection classes in Java. Understanding which interfaces it implements is key to using it correctly in modern Java programming. Given Data / Assumptions: Hashtable is a dictionary-like data structure storing key-value pairs. Located in java.util package. Modern usage often prefers HashMap, but Hashtable remains important historically. Concept / Approach: Hashtable implements the Map interface, which defines methods for mapping keys to values without duplicates. Step-by-Step Reasoning: Hashtable extends Dictionary (legacy) but implements Map.Map ensures unique keys, allows value lookup, insertion, and removal.List and Collection are not correct because they do not define key-value mapping. Why Other Options Are Wrong: List → ordered sequence, not key-value mapping. HashTable (spelled incorrectly) is not an interface. Collection is more general, but Hashtable implements Map, not Collection directly. Final Answer: java.util.Map

Question 4

Java — What is the numerical range of the char data type?

Accepted Answer

Introduction / Context: The char type in Java is a 16-bit unsigned integer representing a Unicode character. Unlike C/C++, Java chars are not signed and cover a wide range of Unicode code points. Given Data / Assumptions: char size: 16 bits (2 bytes). Unsigned, not negative. Stores Unicode characters. Concept / Approach: The range of a 16-bit unsigned integer is 0 to (2^16 - 1) = 65535. Step-by-Step Solution: Minimum: 0 → represents null character '\u0000'.Maximum: 65535 → corresponds to '\uffff'. Why Other Options Are Wrong: 32767 is max for signed 16-bit, but char is unsigned. -256 to 255 describes byte range incorrectly. -32768 to 32767 describes signed short, not char. Final Answer: 0 to 65535

Question 5

Java — Store elements with no duplicates and natural order. Which interface provides this capability?

Accepted Answer

Introduction / Context: Different collection interfaces in Java define different rules for storage: duplicates, ordering, and key-value mappings. Given Data / Assumptions: Requirement: no duplicates. Requirement: natural order access. Concept / Approach: Set interface represents collections with no duplicate elements. SortedSet (e.g., TreeSet) provides natural ordering. Step-by-Step Reasoning: Map → stores key-value pairs, not individual elements.List → allows duplicates, preserves insertion order, not natural ordering.Collection → general root interface; does not guarantee uniqueness or order. Why Other Options Are Wrong: Only Set ensures uniqueness; TreeSet (implementation) also provides natural order. Final Answer: java.util.Set

Question 6

Java — Which Map implementation maintains iteration order of an existing Map?

Accepted Answer

Introduction / Context: This question tests knowledge of different Map implementations in Java and their ordering characteristics. Concept / Approach: HashMap → no ordering guarantee. TreeMap → natural or custom comparator order, not iteration order. LinkedHashMap → maintains insertion order or access order. Step-by-Step Reasoning: If you want the iteration order of an existing Map to be preserved, use LinkedHashMap and construct it with the existing Map. Why Other Options Are Wrong: TreeMap re-sorts keys, HashMap randomizes order, and "depends on implementation" is too vague. Final Answer: LinkedHashMap

Question 7

Java — Which collection class allows resizing, provides indexed access, but is not synchronized?

Accepted Answer

Introduction / Context: This checks understanding of ArrayList vs other collections in terms of resizing, indexing, and synchronization. Concept / Approach: ArrayList → resizable array, indexed access, not synchronized. Vector → synchronized, older alternative. HashSet/LinkedHashSet → no indexing, store unique elements. List → interface, not a class. Final Answer: java.util.ArrayList

Question 8

Java — What line of code should replace the missing statement to compile this program? public class foo  {     public static void main(String[]args)throws Exception      {         java.io.PrintWriter out = new java.io.PrintWriter();          new jav…

Accepted Answer

Introduction / Context: This question evaluates knowledge of Java import statements and compilation requirements for using java.io classes. Concept / Approach: In Java, include is invalid (C/C++ feature). If classes are referenced with full package names (java.io.PrintWriter), no import is necessary. But the constructor new java.io.PrintWriter() is invalid without arguments — it requires a Writer or OutputStream. Step-by-Step Reasoning: Given options, the most reasonable fix is using import java.io.*; so that PrintWriter and OutputStreamWriter are available without full qualification. Final Answer: import java.io.*;

Question 9

Java — Which interface provides capability to store objects using key-value pairs?

Accepted Answer

Introduction / Context: This tests knowledge of the Map interface in the Java Collections Framework. Concept / Approach: Map defines methods to store key-value pairs, where keys are unique and values can be retrieved by keys. Why Other Options Are Wrong: Set → stores unique elements only, no key-value. List → ordered collection of elements, allows duplicates. Collection → root interface, but no key-value mapping. Final Answer: java.util.Map

Question 10

Java — Which of the following are Java reserved words? (run, import, default, implement)

Accepted Answer

Introduction / Context: This question checks memory of Java reserved keywords, which cannot be used as identifiers. Concept / Approach: import → reserved word (used for package imports). default → reserved word (used in switch and interfaces). run → not reserved; just a common method name. implement → not reserved; correct word is implements. Final Answer: 2 and 3

Question 11

Java — Which collection class allows key-value associations and provides synchronization?

Accepted Answer

Introduction / Context: This checks knowledge of synchronized collections and legacy classes. Concept / Approach: Hashtable is synchronized, meaning its methods are thread-safe. It stores key-value pairs like HashMap but synchronizes all methods. TreeMap → ordered keys, but not synchronized. SortedMap → interface, not a class. TreeSet → stores unique values, not key-value pairs. Final Answer: java.util.Hashtable

Question 12

Java — Which is a valid declaration of a float value?

Accepted Answer

Introduction / Context: This evaluates the correct way of declaring floating-point literals in Java. Concept / Approach: By default, floating literals like 1.0 are treated as double in Java. To assign to a float, append f or F. d or D indicates double explicitly. String literals like "1" cannot be assigned directly to float. Step-by-Step: Option A: Correct. "1F" explicitly declares a float.Option B: Incorrect. 1.0 defaults to double.Option C: Invalid type mismatch.Option D: Declares double literal with d. Final Answer: float f = 1F;

Question 13

What will be the output of the program? TreeSet map = new TreeSet(); map.add("one"); map.add("two"); map.add("three"); map.add("four"); map.add("one"); Iterator it = map.iterator(); while (it.hasNext() )  {     System.out.print( it.next() + " " ); }

Accepted Answer

TreeSet assures no duplicate entries; also, when it is accessed it will return elements in natural order, which typically means alphabetical.

Question 14

What will be the output of the program? public class Test  {      private static int[] x;      public static void main(String[] args)      {          System.out.println(x[0]);      }  }

Accepted Answer

In the above code the array reference variable x has been declared but it has not been instantiated i.e. the new statement is missing, for example: private static int[]x = new int[5]; private static int[x] declares a static i.e. class level array. the "new" keyword is the word that actually creates said array. int[5] in association with the new sets the size of the array. so since the above code contains no new or size decalarations when you try and access x[0] you are trying to access a member of an array that has been declared but not intialized hence you get a NullPointerException at runtime.

Question 15

What will be the output of the program? import java.util.*;  class H  {     public static void main (String[] args)      {          Object x = new Vector().elements();          System.out.print((x instanceof Enumeration)+",");          System.out.pr…

Accepted Answer

The Vector.elements method returns an Enumeration over the elements of the vector. Vector implements the List interface and extends AbstractList so it is also possible to get an Iterator over a Vector by invoking the iterator or listIterator method.

Question 16

What will be the output of the program? package foo;  import java.util.Vector; /* Line 2 */ private class MyVector extends Vector  {     int i = 1; /* Line 5 */     public MyVector()      {          i = 2;      }  }  public class MyNewVector extends…

Accepted Answer

Option B is correct. The compiler complains with the error "modifier private not allowed here". The class is created private and is being used by another class on line 19.

Question 17

What will be the output of the program? public class Test  {      private static float[] f = new float[2];      public static void main (String[] args)      {         System.out.println("f[0] = " + f[0]);      }  }

Accepted Answer

The choices are between Option A and B, what this question is really testing is your knowledge of default values of an initialized array. This is an array type float i.e. it is a type that uses decimal point numbers therefore its initial value will be 0.0 and not 0

Question 18

What will be the output of the program? import java.util.*;  class I  {     public static void main (String[] args)      {         Object i = new ArrayList().iterator();          System.out.print((i instanceof List)+",");          System.out.print((…

Accepted Answer

The iterator() method returns an iterator over the elements in the list in proper sequence, it doesn't return a List or a ListIterator object. A ListIterator can be obtained by invoking the listIterator method.

Question 19

What will be the output of the program? public class Test  {      public static void main (String args[])      {         String str = NULL;          System.out.println(str);      }  }

Accepted Answer

Option B is correct because to set the value of a String variable to null you must use "null" and not "NULL".

Question 20

What will be the output of the program? public class Test  {      public static void main (String[] args)      {         String foo = args[1];          String bar = args[2];          String baz = args[3];          System.out.println("baz = " + baz);…

Accepted Answer

When running the program you entered 3 arguments "red", "green" and "blue". When dealing with arrays in java you must remember ALL ARRAYS IN JAVA ARE ZERO BASED therefore args[0] becomes "red", args[1] becomes "green" and args[2] becomes "blue". When the program entcounters line 8 above at runtime it looks for args[3] which has never been created therefore you get an ArrayIndexOutOfBoundsException at runtime.

Objects and Collections Questions