Utility Package :
1) What is the Vector class?
ANSWER : The Vector class provides the capability to implement a growable array of objects.
2) What is the Set interface?
ANSWER : The Set interface provides methods for accessing the elements of a finite mathematical set.Sets do not allow duplicate elements.
3) What is Dictionary class?
ANSWER : The Dictionary class is the abstarct super class of Hashtable and Properties class.Dictionary provides the abstarct functions used to store and retrieve objects by key-value.This class allows any object to be used as a key or value.
4) What is the Hashtable class?
ANSWER : The Hashtable class implements a hash table data structure. A hash table indexes and stores objects in a dictionary using hash codes as the objects' keys. Hash codes are integer values that identify objects.
5) What is the Properties class?
Answer : The properties class is a subclass of Hashtable that can be read from or written to a stream.It also provides the capability to specify a set of default values to be used if a specified key is not found in the table. We have two methods load() and save().
6) What changes are needed to make the following prg to compile?
import java.util.*;
class Ques{
public static void main (String args[]) {
String s1 = "abc";
String s2 = "def";
Vector v = new Vector();
v.add(s1);
v.add(s2);
String s3 = v.elementAt(0) + v.elementAt(1);
System.out.println(s3);
}
}
ANSWER : Declare Ques as public B) Cast v.elementAt(0) to a String
C) Cast v.elementAt(1) to an Object. D) Import java.lang
ANSWER : B) Cast v.elementAt(0) to a String
8) What is the output of the prg.
import java.util.*;
class Ques{
public static void main (String args[]) {
String s1 = "abc";
String s2 = "def";
Stack stack = new Stack();
stack.push(s1);
stack.push(s2);
try{
String s3 = (String) stack.pop() + (String) stack.pop() ;
System.out.println(s3);
}catch (EmptyStackException ex){}
}
}
ANSWER : abcdef B) defabc C) abcabc D) defdef
ANSWER : B) defabc
9) Which of the following may have duplicate elements?
ANSWER : Collection B) List C) Map D) Set
ANSWER : A and B Neither a Map nor a Set may have duplicate elements.
10) Can null value be added to a List?
ANSWER : Yes.A Null value may be added to any List.
11) What is the output of the following prg.
import java.util.*;
class Ques{
public static void main (String args[]) {
HashSet set = new HashSet();
String s1 = "abc";
String s2 = "def";
String s3 = "";
set.add(s1);
set.add(s2);
set.add(s1);
set.add(s2);
Iterator i = set.iterator();
while(i.hasNext())
{
s3 += (String) i.next();
}
System.out.println(s3);
}
}
A) abcdefabcdef B) defabcdefabc C) fedcbafedcba D) defabc
ANSWER : D) defabc. Sets may not have duplicate elements.
12) Which of the following java.util classes support internationalization?
A) Locale B) ResourceBundle C) Country D) Language
ANSWER : A and B . Country and Language are not java.util classes.
13) What is the ResourceBundle?
The ResourceBundle class also supports internationalization.
ResourceBundle subclasses are used to store locale-specific resources that can be loaded by a program to tailor the program's appearence to the paticular locale in which it is being run. Resource Bundles provide the capability to isolate a program's locale-specific resources in a standard and modular manner.
14) How are Observer Interface and Observable class, in java.util package, used?
ANSWER : Objects that subclass the Observable class maintain a list of Observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects.
15) Which java.util classes and interfaces support event handling?
ANSWER : The EventObject class and the EventListener interface support event processing.
16) Does java provide standard iterator functions for inspecting a collection of objects?
ANSWER : The Enumeration interface in the java.util package provides a framework for stepping once through a collection of objects. We have two methods in that interface.
public interface Enumeration {
boolean hasMoreElements();
Object nextElement();
}
17) The Math.random method is too limited for my needs- How can I generate random numbers more flexibly?
ANSWER : The random method in Math class provide quick, convienient access to random numbers, but more power and flexibility use the Random class in the java.util package.
double doubleval = Math.random();
The Random class provide methods returning float, int, double, and long values.
nextFloat() // type float; 0.0 <= value < 1.0
nextDouble() // type double; 0.0 <= value < 1.0
nextInt() // type int; Integer.MIN_VALUE <= value <= Integer.MAX_VALUE
nextLong() // type long; Long.MIN_VALUE <= value <= Long.MAX_VALUE
nextGaussian() // type double; has Gaussian("normal") distribution with mean 0.0 and standard deviation 1.0)
Eg. Random r = new Random();
float floatval = r.nextFloat();
18) How can we get all public methods of an object dynamically?
ANSWER : By using getMethods(). It return an array of method objects corresponding to the public methods of this class.
getFields() returns an array of Filed objects corresponding to the public Fields(variables) of this class.
getConstructors() returns an array of constructor objects corresponding to the public constructors of this class.
Data types,variables and Arrays :
1) What is meant by variable?
Ans: Variables are locations in memory that can hold values. Before assigning any value to a variable, it must be declared.
2) What are the kinds of variables in Java? What are their uses?
Ans: Java has three kinds of variables namely, the instance variable, the local variable and the class variable.
Local variables are used inside blocks as counters or in methods as temporary variables and are used to store information needed by a single method.
Instance variables are used to define attributes or the state of a particular object and are used to store information needed by multiple methods in the objects.
Class variables are global to a class and to all the instances of the class and are useful for communicating between different objects of all the same class or keeping track of global states.
3) How are the variables declared?
Ans: Variables can be declared anywhere in the method definition and can be initialized during their declaration.They are commonly declared before usage at the beginning of the definition.
Variables with the same data type can be declared together. Local variables must be given a value before usage.
4) What are variable types?
Ans: Variable types can be any data type that java supports, which includes the eight primitive data types, the name of a class or interface and an array.
5) How do you assign values to variables?
Ans: Values are assigned to variables using the assignment operator =.
6) What is a literal? How many types of literals are there?
Ans: A literal represents a value of a certain type where the type describes how that value behaves.
There are different types of literals namely number literals, character literals,
boolean literals, string literals,etc.
7) What is an array?
Ans: An array is an object that stores a list of items.
8) How do you declare an array?
Ans: Array variable indicates the type of object that the array holds.
Ex: int arr[];
9) Java supports multidimensional arrays.
a)True
b)False
Ans: a.
10) An array of arrays can be created.
a)True
b)False
Ans: a.
11) What is a string?
Ans: A combination of characters is called as string.
12) Strings are instances of the class String.
a)True
b)False
Ans: a.
13) When a string literal is used in the program, Java automatically creates instances of the string class.
a)True
b)False
Ans: a.
14) Which operator is to create and concatenate string?
Ans: Addition operator(+).
15) Which of the following declare an array of string objects?
String[ ] s;
String [ ]s:
String[ s]:
String s[ ]:
Ans : a, b and d
16) What is the value of a[3] as the result of the following array declaration?
1
2
3
4
Ans : d
17) Which of the following are primitive types?
byte
String
integer
Float
Ans : a.
18) What is the range of the char type?
0 to 216
0 to 215
0 to 216-1
0 to 215-1
Ans. d
19) What are primitive data types?
Ans : byte, short, int, long
float, double
boolean
char
20) What are default values of different primitive types?
Ans : int - 0
short - 0
byte - 0
long - 0 l
float - 0.0 f
double - 0.0 d
boolean - false
char - null
21) Converting of primitive types to objects can be explicitly.
a)True
b)False
Ans: b.
22) How do we change the values of the elements of the array?
Ans : The array subscript expression can be used to change the values of the elements of the array.
23) What is final varaible?
Ans : If a variable is declared as final variable, then you can not change its value. It becomes constant.
24) What is static variable?
Ans : Static variables are shared by all instances of a class.
1) What is the Vector class?
ANSWER : The Vector class provides the capability to implement a growable array of objects.
2) What is the Set interface?
ANSWER : The Set interface provides methods for accessing the elements of a finite mathematical set.Sets do not allow duplicate elements.
3) What is Dictionary class?
ANSWER : The Dictionary class is the abstarct super class of Hashtable and Properties class.Dictionary provides the abstarct functions used to store and retrieve objects by key-value.This class allows any object to be used as a key or value.
4) What is the Hashtable class?
ANSWER : The Hashtable class implements a hash table data structure. A hash table indexes and stores objects in a dictionary using hash codes as the objects' keys. Hash codes are integer values that identify objects.
5) What is the Properties class?
Answer : The properties class is a subclass of Hashtable that can be read from or written to a stream.It also provides the capability to specify a set of default values to be used if a specified key is not found in the table. We have two methods load() and save().
6) What changes are needed to make the following prg to compile?
import java.util.*;
class Ques{
public static void main (String args[]) {
String s1 = "abc";
String s2 = "def";
Vector v = new Vector();
v.add(s1);
v.add(s2);
String s3 = v.elementAt(0) + v.elementAt(1);
System.out.println(s3);
}
}
ANSWER : Declare Ques as public B) Cast v.elementAt(0) to a String
C) Cast v.elementAt(1) to an Object. D) Import java.lang
ANSWER : B) Cast v.elementAt(0) to a String
8) What is the output of the prg.
import java.util.*;
class Ques{
public static void main (String args[]) {
String s1 = "abc";
String s2 = "def";
Stack stack = new Stack();
stack.push(s1);
stack.push(s2);
try{
String s3 = (String) stack.pop() + (String) stack.pop() ;
System.out.println(s3);
}catch (EmptyStackException ex){}
}
}
ANSWER : abcdef B) defabc C) abcabc D) defdef
ANSWER : B) defabc
9) Which of the following may have duplicate elements?
ANSWER : Collection B) List C) Map D) Set
ANSWER : A and B Neither a Map nor a Set may have duplicate elements.
10) Can null value be added to a List?
ANSWER : Yes.A Null value may be added to any List.
11) What is the output of the following prg.
import java.util.*;
class Ques{
public static void main (String args[]) {
HashSet set = new HashSet();
String s1 = "abc";
String s2 = "def";
String s3 = "";
set.add(s1);
set.add(s2);
set.add(s1);
set.add(s2);
Iterator i = set.iterator();
while(i.hasNext())
{
s3 += (String) i.next();
}
System.out.println(s3);
}
}
A) abcdefabcdef B) defabcdefabc C) fedcbafedcba D) defabc
ANSWER : D) defabc. Sets may not have duplicate elements.
12) Which of the following java.util classes support internationalization?
A) Locale B) ResourceBundle C) Country D) Language
ANSWER : A and B . Country and Language are not java.util classes.
13) What is the ResourceBundle?
The ResourceBundle class also supports internationalization.
ResourceBundle subclasses are used to store locale-specific resources that can be loaded by a program to tailor the program's appearence to the paticular locale in which it is being run. Resource Bundles provide the capability to isolate a program's locale-specific resources in a standard and modular manner.
14) How are Observer Interface and Observable class, in java.util package, used?
ANSWER : Objects that subclass the Observable class maintain a list of Observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects.
15) Which java.util classes and interfaces support event handling?
ANSWER : The EventObject class and the EventListener interface support event processing.
16) Does java provide standard iterator functions for inspecting a collection of objects?
ANSWER : The Enumeration interface in the java.util package provides a framework for stepping once through a collection of objects. We have two methods in that interface.
public interface Enumeration {
boolean hasMoreElements();
Object nextElement();
}
17) The Math.random method is too limited for my needs- How can I generate random numbers more flexibly?
ANSWER : The random method in Math class provide quick, convienient access to random numbers, but more power and flexibility use the Random class in the java.util package.
double doubleval = Math.random();
The Random class provide methods returning float, int, double, and long values.
nextFloat() // type float; 0.0 <= value < 1.0
nextDouble() // type double; 0.0 <= value < 1.0
nextInt() // type int; Integer.MIN_VALUE <= value <= Integer.MAX_VALUE
nextLong() // type long; Long.MIN_VALUE <= value <= Long.MAX_VALUE
nextGaussian() // type double; has Gaussian("normal") distribution with mean 0.0 and standard deviation 1.0)
Eg. Random r = new Random();
float floatval = r.nextFloat();
18) How can we get all public methods of an object dynamically?
ANSWER : By using getMethods(). It return an array of method objects corresponding to the public methods of this class.
getFields() returns an array of Filed objects corresponding to the public Fields(variables) of this class.
getConstructors() returns an array of constructor objects corresponding to the public constructors of this class.
Data types,variables and Arrays :
1) What is meant by variable?
Ans: Variables are locations in memory that can hold values. Before assigning any value to a variable, it must be declared.
2) What are the kinds of variables in Java? What are their uses?
Ans: Java has three kinds of variables namely, the instance variable, the local variable and the class variable.
Local variables are used inside blocks as counters or in methods as temporary variables and are used to store information needed by a single method.
Instance variables are used to define attributes or the state of a particular object and are used to store information needed by multiple methods in the objects.
Class variables are global to a class and to all the instances of the class and are useful for communicating between different objects of all the same class or keeping track of global states.
3) How are the variables declared?
Ans: Variables can be declared anywhere in the method definition and can be initialized during their declaration.They are commonly declared before usage at the beginning of the definition.
Variables with the same data type can be declared together. Local variables must be given a value before usage.
4) What are variable types?
Ans: Variable types can be any data type that java supports, which includes the eight primitive data types, the name of a class or interface and an array.
5) How do you assign values to variables?
Ans: Values are assigned to variables using the assignment operator =.
6) What is a literal? How many types of literals are there?
Ans: A literal represents a value of a certain type where the type describes how that value behaves.
There are different types of literals namely number literals, character literals,
boolean literals, string literals,etc.
7) What is an array?
Ans: An array is an object that stores a list of items.
8) How do you declare an array?
Ans: Array variable indicates the type of object that the array holds.
Ex: int arr[];
9) Java supports multidimensional arrays.
a)True
b)False
Ans: a.
10) An array of arrays can be created.
a)True
b)False
Ans: a.
11) What is a string?
Ans: A combination of characters is called as string.
12) Strings are instances of the class String.
a)True
b)False
Ans: a.
13) When a string literal is used in the program, Java automatically creates instances of the string class.
a)True
b)False
Ans: a.
14) Which operator is to create and concatenate string?
Ans: Addition operator(+).
15) Which of the following declare an array of string objects?
String[ ] s;
String [ ]s:
String[ s]:
String s[ ]:
Ans : a, b and d
16) What is the value of a[3] as the result of the following array declaration?
1
2
3
4
Ans : d
17) Which of the following are primitive types?
byte
String
integer
Float
Ans : a.
18) What is the range of the char type?
0 to 216
0 to 215
0 to 216-1
0 to 215-1
Ans. d
19) What are primitive data types?
Ans : byte, short, int, long
float, double
boolean
char
20) What are default values of different primitive types?
Ans : int - 0
short - 0
byte - 0
long - 0 l
float - 0.0 f
double - 0.0 d
boolean - false
char - null
21) Converting of primitive types to objects can be explicitly.
a)True
b)False
Ans: b.
22) How do we change the values of the elements of the array?
Ans : The array subscript expression can be used to change the values of the elements of the array.
23) What is final varaible?
Ans : If a variable is declared as final variable, then you can not change its value. It becomes constant.
24) What is static variable?
Ans : Static variables are shared by all instances of a class.
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