SCJP Questions with Answers

Q 1
What is the output of the following code when compiled and run? Select two correct answers.

public class TechnoSample {
public static void main(String[] args){
for(int i = 0; i <>
System.out.println(getPrimitive(127)); //line 1
}
}
public static int getPrimitive(byte b) { //line 2
return (short)(Math.random()*b); //line 3
}
}


(1) Compilation error on line 1
(2) Compilation error on line 2
(3) Compilation error on line 3
(4) Line 3 compiles fine
(5) Prints 10 random numbers between 0 and 127


Answer : 1,4
Explanation :
Line 1 does not compile because getPrimitive() takes a byte and we pass it an int. In a normal assignment (byte b = 127;) this would work because 127 is in the range for byte values and the compiler implicitely does a norrowing conversion.But this is not the case in method invocations. A quote from JLS 5.3: "The designers of the Java programming language felt that including these implicit narrowing conversions would add additional complexity to the overloaded method matchingresolution process". This speaks for itself. Line 3 compiles fine because we have to do with a widening primitive conversion form short to int which is perfectly straightforward.


Q2 Select three correct statements.


(1) A static method may override another static method
(2) A static method cannot override a non-static method
(3) A non-static method cannot override a static method
(4) A non-static method may be overloaded by a static method
(5) A synchronized method cannot be overridden


Answer : 2,3,4
Explanation :
Overriding is for non-static methods and hiding is for static methods. So the following statements are the only true statements about hiding and overriding:

a static method (in a subclass) may hide another static method (in a superclass)
a static method (in a subclass) cannot hide a non-static method (in a superclass)
a non-static method (in a subclass) may override another non-static method (in a superclass)
a non-static method (in a subclass) cannot override a static method (in a superclass)



Q3 Select three correct statements about the following code.
public class TechnoSample {
public static void main(String[] args) {
TechnoSample myref = new TechnoSampleSub();
try{
myref.test();
}
catch(Exception e){}
}
void test() throws Exception{
System.out.println("In TechnoSample");
throw new Exception();
}
}
class TechnoSample Sub extends TechnoSample {
void test() {
System.out.println("In TechnoSampleSub");
}
}



(1) The try-catch block that encloses myref.test(); is mandatory for the code to compile
(2) Prints: In TechnoSample
(3) Prints: In TechnoSampleSub
(4) Method test() in class TechnoSampleSub has no obligation to declare a throws clause
(5) An exception is thrown at runtime


Answer : 1,3,4
Explanation :
myref is an instance of class TechnoSampleSub referenced by a variable of type TechnoSample. Method test() in class TechnoSample is overridden in class TechnoSampleSub, thus the one to be invoked is the one declared in class TechnoSampleSub(Polymorphism!). Moreover, test() has no obligation to declare a throws clause (see overriding rules!). The try-catch block is mandatory because myref could as well reference an instanceof class TechnoSample and in that case the method test() to be invoked would be the one declared in class TechnoSample which throws an exception.


Q4 Given the following code:
import java.util.Date;
public class Example {
public static void main(String args[]) {
Date d1 = new Date (99, 11, 31);
Date d2 = new Date (99, 11, 31);
method(d1, d2);
System.out.println("d1 is " + d1 + "\nd2 is " + d2);
}
public static void method(Date d1, Date d2) {
d2.setYear (100);
d1 = d2;
}
}

Which one or more of the following correctly describe the behavior when this program is compiled and run?


(1) compilation is successful and the output is:
d1 is Fri December 31 00:00:00 GMT 1999 d2 is Fri December 31 00:00:00 GMT 1999

(2) compilation is successful and the output is:
d1 is Fri December 31 00:00:00 GMT 1999 d2 is Sun December 31 00:00:00 GMT 2000

(3) compilation is successful and the output is:
d1 is Sun December 31 00:00:00 GMT 2000 d2 is Sun December 31 00:00:00 GMT 2000

(4) the assignment 'd1 = d2' is rejected by the compiler because the Date class cannot overload the operator '='
(5) the expression (d1 is " + d1 + "\nd2 is " + d2) is rejected by the compiler because the Date class cannot overload the operator '+'


Answer : 2
Explanation :
1) is false because we know that the data in d2 was changed. 3) is false because we know that the data in d1 was not changed. The names d1 and d2 are used in both main and method to be confusing. They are different and stored on the stack in different place. All the interesting stuff that happen in the Example class is in method. main simply initializes some data and prints the results. In method, the following happens:
1.d2 has its year set to 100 (really 2000, as 2.Object d1 is set to be the same as d2. This is a change of the actual reference, not in the data at d1.
Both of these line are perfectly legal, and do not result in a compilation error, so d) is false. I will also point out here that e) is String context. toString() is defined by the Object class and so it is available on all classes in Java. Most non-trivial classes override toString() to return more explicit information about themselves.

LAST MINUTE REVISION POINTS

1. Evaluation and execution –remember that evaluation is from left to right but
execution is from right to left.

2. There must be some statement after do keyword in do – while loop for it to
compile without error.
i.e.
do ; while(false); //correct
do {;}while(false); //correct
do {}while(false); //correct
do while(false); //error

3. If “t” is a reference variable then ,
t.equals(null) is false
(null).equals(t) compiler error
let’s say t2 is some other reference variable then
t.equals(t2) false and not error
consider,
t = null;
t.equals(t2); //not compiler error but runtime error

4. If a class is declared inside a package with public modifier then that class
becomes invisible to all other classes in other packages unless they import the
package or use extended form of addressing the class.

5. The Iterator method of collection interface when invoked returns an instance of
Iterator class.

6. given,
char c = ‘a’;
int i = 1;
c + =i; //correct
c = c+ i; //illegal

7. when use int numbers in basic arithmetic operation then the output is an integer
number. Hence ,
int i = 4/3;
“i” will have the value 1.

8. Native methods can be set to any access level - public , protected, private, default.

9. The methods in the immediate super class in the inheritance tree may be accessed
through the use of keyword “super” , but classes above the immediate super class
are not visible.

10. Valid comments
· /* this is a comment */
· /** this is a comment */
· /*this is a comment **/
· // /** this is a comment */ */
Important:-
· /* //this is a comment */
11. invalid comments
· /** this is a comment */ */

12. If a method declares some exception in throws clause and the subclass of the
given class while overriding the method declares some new exception then before
assuming that it causes compiler error first check whether the new exception
thrown in the subclass’ method is unchecked exception or not.

13. After solving the logic inside the problem before jumping to conclusion check
whether some code is unreachable or not. Because if it happens so then it results
in compiler error.

14. The following form of instantiating a static inner class results in compiler error,
new ().new ();

15. long l = Integer.MAX_VALUE;
float f = l;
double d = l;
then “d==f” is false due to rounding of numbers in float literal.
But when we assign l to Long.MAX_VALUE or to Integer.MIN_VALUE then
we get the result of “d==f” as true.

16. We can place label statements around a block of code wherever we wish , unless
the name used is not a keyword and follows all the rules meant for identifier.
For eg.
labelA:
{
…..some complex code….
…..some complex code….
if(someThingIsTrue)
{
break labelA;
}
}
this way we place break statement with label in any labeled block of code
which may break out of the code if something comes true.
Furthur the same labels can be used for other block of code as long as they
don’t overlap.

17. An abstract method cannot be marked as both
· Abstract and strictfp
· Abstract and native
· Abstract and synchronized
· Abstract and final
· Abstract and private
· Abstract and static

18. Shift operators can be used only on integers .

19. Switch statements can evaluate byte , short, char , int.
but not long, float.double.
i.e. long l = 10;
switch(l){}//causes compiler error
before jumping to conclusion about switch statements , verify whether
the case arguments are lying within the range of the switch argument.
For e.g. byte b = 10;
Switch(b)
{
case 10: …….. complexcode………break;
case 1000: …….. complexcode………break;
}
here second case statement causes compiler error since it is out of range of
byte literal.

20. The case argument must be primitive literal type or final variable.

21. For loop declarations,
valid
· for(int i=0,j=0;i<10;i++);invalid
· for(i=0,int j=0;;);
· int k =1;
for(int i=0,k=0;;);

SCJP 1.5 Dump questions

Q1
class test
{
public static void main(String[] args)
{
test inst_test = new test();
String pig[][] = { {"one little piggy"}, {"two little piggies"}, {"three little piggies"} };
for ( Object []oink : pig )
{
for ( Object piggy : oink )
{
System.out.println(piggy);
}
}
}
}

a. one little piggy two little piggies three little piggies
b. Compile Error incompatible types.
c. java.lang.String;@187c6c7 java.lang.String;@187c6c8 java.lang.String;@187c6c9
( or something like that )
d. Runtime Null Pointer Exception
e. Prints nothing

Answer1:
a. oink refers to every object reference in a one dimensional row of pig[][].
piggy refers to every object within that row.

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Q2
class test
{
public static void main(String[] args)
{
test inst_test = new test();
int i1 = 2000;
int i2 = 2000;
int i3 = 2;
int i4 = 2;
Integer Ithree = new Integer(2); // 1
Integer Ifour = new Integer(2); // 2
System.out.println( Ithree == Ifour );
inst_test.method( i3 , i4 );
inst_test.method( i1 , i2 );

}
public void method( Integer i , Integer eye )
{
System.out.println(i == eye );
}
}

a. true false true
b. false true false
c. false false false
d. true true false
e. Compile error

Answer 2:
b: false true false. lthree and lfour are two seperate objects. if the lines 1 and 2 were
lthree = 2 and lfour = 2 the result would have been true. This is when the objects are created in the pool. When the references I and eye in the pool are compared 2==2 results in true and 2000==2000 is false since it exceeds 127.

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Q3
enum cafe {
BIG ( 10 ) ,
SMALL ( 1 ),
MED ( 5 )
int mySize = 0;
cafe ( int size )
{
mySize = size;

}

}

What happens when this enum is in the code outside a class ?

a. Compiles fine
b. Compiler error
c. Runtime Exception occurs if mySize is accessed.

Answer 3:
a: Compile error: semicolon missing after MED ( 5 ). Watch out for that semicolon when an enum has variables and functions.

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Some Suggestions for SCJP takers

1] Try to give SCJP from Java 5 (SCJP 310-055)

2] If you are feeling that Generics/ Collection is tough then please read Java Complete Reference Book

3] I faced some difficulty while taking the exam bcz of drag and drop questions. Actually the problem is in Exam software. It was not working properly. I was like playing game . So be careful about those questions.

4] For Garbage collection question you must do paper work. Then and only then you will get 100% in these questions.

e.g. MyClass obj1 = new MyClass();
then drow like this...
new Myclass1 ---------obj1
and if I say obj1=null then do this....
new Myclass1 -------------obj1
Then you can guess that one object has been garbage collected. (new Class()) And it will take hardly 10 to 15 sec to draw isn't it?

5] One more thing that many of you doesn't know we can return array from method like this also........keep in mind ..
public int getArr() []
{
return new int[]{1,2};
}

6] I got lots questions on generics and I got 100% in it ..so be prepared.

7] Be a detective ....go through each and every line...you can do this by moving mouse pointer on each line....

8] Go through each and every objectives....because you may feel that this is the right answer but there may be another answer which is better than this....
e.g a] Compiles fine
b] does not compile
c] throws exception
d] error at line number...
e] Compiles with warnings . -------right answer

9] K&B book is enough but for Generics first read Complete Reference and then read K&B.
10 ] Be confident about program flow specially with Exception Handling programs. And don't worry if you are newcomer it doesn't matter.

And I will be always there to help you .

Do write to me in case of any more tips :
wise(dot)mohit(at)gmail(dot)com

Some Questions on Objects for SCJP 5

Q1 What is the result of executing the following fragment of code:
boolean b1 = false;
boolean b2 = false;
if (b2 != b1 = !b2)
{ System.out.println("true");
}
else { System.out.println("false");
}
Select 1 correct option
(1)Compile time error
(2)It will print true
(3)It will print false
(4)Runtime error
(5)It will print nothing
Answer : 1
Explanation : Note that, boolean operators have more precedence than =. (In fact, = has least precedenace) so, in (b2 != b1 = !b2) first b2 != b1 is evaluated which returns a value 'false'. So the expression becomes false = !b2. And this is illegalbecause false is a value and not a variable!Had it been something like (b2 = b1 != b2) then its valid because it will boil down to : b2 = false. Because all an if() needs is a boolean, now b1 != b2 returns false which is a boolean andas b2 = false is an expression and every expression has a return value (which is actually the LHS of the erpression). Here it returns true which is again a boolean.Note, return value of expression (i is int) : i = 10 , is 10 (int).

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Q2
Given two collection objects referenced by c1 and c2, which of these statements are true?Select 2 correct options
(1)c1.retainAll(c2) will not modify c1
(2)c1.removeAll(c2) will not modify c1
(3)c1.addAll(c2) will return a new collection object, containing elements from both c1 and c2
(4)For: c2.retainAll(c1); c1.containsAll(c2); 2nd statement will return true
(5)For: c2.addAll(c1); c1.retainAll(c2); 2nd statement will have no practical effect on c1

Answer : 4,5
Explanation : public boolean retainAll(Collection c) retains only the elts in this collection that are contained in the specified collection. In other words, removes from this collection all of its elts that are not contained in the specified collectionpublic boolean removeAll(Collection c) removes all this collection's elts that are also contained in the specified collection. After this call returns, this collection will contain no elts in common with the specifiedcollectionpublic boolean containsAll(Collection c) returns true if this collection contains all of the elts in the specified collectionpublic boolean addAll(Collection c) adds all the elts in the specified collectionto this collection. The behavior of this opern is undefined if the specified collection is modified while the opern is in progress(ie., the behavior of this call is undefined if the specified collection is this collection, and is nonempty)
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Q 3
What happens when the following code gets executed:
class Sample {
public static void main(String[] args)
{ double d1 = 1.0;
double d2 = 0.0;
byte b =1;
d1 = d1/d2;
b = (byte) d1;
System.out.print(b);
}
}
(1)It results in the throwing of an ArithmeticExcepiton
(2)It results in the throwing of a DivedeByZeroException
(3)It displays the value 1.5
(4)It displays the value –1
Answer : 4
Explanation : 1.0/0.0 results in Double.POSITIVE_INFINITY. Double.POSITIVE_INFINITY is converted to Integer.MAX_VALUE ('0' followed by 31 '1's). Integer.MAX_VALUE is then cast to byte value, which simply takes the last 8 bits(11111111) and is -1.

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Q4
Class finalization can be done by implementing the following method:static void classFinalize() throws Throwable;True Or False?
(1)True
(2)False
Answer : 2
Explanation : PREVIOUSLY: If a class declares a class method classFinalize that takes no arguments and returns no result: static void classFinalize() throws Throwable { . . . } then this method will be invoked before the class is unloaded . Like thefinalize method for objects, this method will be automatically invoked only once. This method may optionally be declared private, protected, or public. NOW: Class finalization has been removed from the Java language. Thefunctionality of JLS 12.7 is subsumed by instance finalization (JLS 12.6).Here is a rationale for this decision. http://java.sun.com/docs/books/jls/class-finalization-rationale.htmlSimilar thing has happend toclass unloading: A class or interface may be unloaded if and only if its class loader is unreachable (the definition of unreachable is given in JLS 12.6.1). Classes loaded by the bootstrap loader may not be unloaded.
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Q5
Consider the following method:
public void getLocks(Object a, Object b)
{ synchronized(a)
{ synchronized(b) { //do something } } }
and the following instantiations:
Object obj1 = new Object();
Object obj2 = new Object();
obj1 and obj2 are accesible to two different threads and the threads are about to call the getLocks() method.Assume the first thread calls the method getLocks(obj1, obj2).
Which of the following is true? Options Select 1 correct option
(1)The second thread should call getLocks(obj2, obj1)
(2)The second thread should call getLocks(obj1, obj2)
(3)The second thread should call getLocks() only after first thread exits out of it
(4)The second thread may call getLocks() any time and passing parameters in any order
(5)None of the above

Answer : 2
Explanation : (1) This may result in a deadlock (3) The is not necessary. Option 2 works just fine.

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SCJP Mock Questions and Answers

Ques1
Given:
1. public class MyThread implements Runnable {
2. private String holdA = "This is ";
3. private int[] holdB = {1,2,3,4,5,6,7,8,9,10};
4.
5. public static void main(String args[]) {
6. MyThread z = new MyThread();
7. (new Thread(z)).start();
8. (new Thread(z)).start();
9. }
10.
11. public synchronized void run() {
12. for(int w = 0;w <>
13. System.out.println(holdA + holdB[w] + ".");
14. }
15. }
16. }

What is the result?


(1) Compilation fails because of an error on line 6
(2) Compilation fails because of an error on line 11
(3) Compilation fails because of errors on lines 7 and 8
(4) Compilation succeeds and the program prints each value in the holdB array at the end of the "This is " line. Each value is printed two times before the program ends, and the values are not printed in sequential order
(5) Compilation succeeds & the prog. prints each val in the holdB array at the end of the "This is " line. Each val is printed in order from 1-10 & after the val 10 prints, it starts printing the vals 1-10 in order again


Answer : 5
Explanation :
Option 5 is correct because the Runnable interface is implemented by declaring a synchronized run() method. The method is declared as synchronized to signify that the object lock must be obtained

Options 1, 2, and 3 are incorrect because compilation succeeds. Option 4 is incorrect, but would be correct if the run() method were not declared as synchronized.
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Ques 2 :Which statement about the Map interface is true?


(1) Entries are placed in a Map using the values() method
(2) Entries are placed in a Map using the entrySet() method
(3) A key/value association is added to a Map using the put() method
(4) A key/value association is added to a Map using the putAll() method


Answer : 3
Explanation :
Option 3 is correct because the put() method is used to add a key/value association to a Map.

Option 1 is incorrect because the values() method returns a Collection of all values in a Map.Option 2 is incorrect because the entrySet() method returns a Set of all mappings in a Map. Option 4 is incorrect because the pubAll() method copies all mappings from one Map to another.
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Ques 3 :
Consider the following class definition:
1. public class Test extends Base {
2. public Test(int j) {
3. }
4. public Test(int j, int k) {
5. super(j, k);
6. }
7. }

Which of the following forms of constructor must exist explicitly in the definition of the Base class?


(1) Base() { }
(2) Base(int j) { }
(3) Base(int j, int k) { }
(4) Base(int j, int k, int l) { }


Answer : 1,3
Explanation :
1 and 3 are correct. In the constructor at lines 2 and 3, there is no explicit call to either this() or super(), which means that the compiler will generate a call to the zero argument superclass constructor, as in 1. The explicit call to super() at line 5 requires that the Base class must have a 7.constructor as in 3. This has two consequences. First, 3 must be one of the required constructors and therefore one of the answers.Second, the Base class must have at least that constructor defined explicitly, so the default constructor is not generated, but must be added explicitly. Therefore the constructor of 1 is also required and must be a correct answer.At no point in the Test class is there a call to either a superclass constructor with one or three arguments, so 2 and 4 need not explicitly exist.

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Threads Some Important Points

1. Methods in object class
· wait : throws interruptedException
· notify
· notifyAll
Remember : they are final and must be called from within a synchronized
context, otherwise illegalmonitorstate exception is thrown at runtime.

*******************************************

2. yield , sleep are static members of Thread class.

********************************************

3. stop , resume , suspend are deprecated members of Thread class.

*********************************************

4. InterruptedException is checked exception.

**********************************************

5. Given
synchronized (expression){…..}
· expression may evaluate to reference to object.
· expression cannot evaluate to primitive type or null.
· If execution block completes ( abruptly or normally )the lock is released.
· synchronized statements can be nested.
· synchronized statements with identical expression can be nested.

*************************************************

6. Thread.yeild method
· may cause the current thread to move from runnable to ready state .
· It isn’t guaranteed i.e. same thread may start executing again.

******************************************************

7. Thread.sleep method
· makes thread to move from running to not – runnable state
· thread takes lock with it if it has got one.

************************************************

8. Thread class inherits Runnable interface.

*************************************************

9. A dead thread can never be restarted.

**************************************************

10. When overriding the ‘run’ method of the Thread class remember the following
points
· the prototype must be public void run(){}
· Access modifier must be public.
· ReturnType must be void.
· No arguments must be specified and if done it doesn’t cause error but it can’t
be used to start a new thread.
. if the class implements Runnable interface then it has to possess run method
with no arguments specified otherwise compiler error is caused.
· it must not declare any new checked exception in its throws clause.
· it must not be declared static
· It can declare unchecked exception in its throws clause.
· If start method is called twice then illegalThreadStateException is thrown.

************************************************

11. the run method in Thread class is empty method which does nothing.

*******************************************************
12. join,sleep,wait methods declare InterruptedException in their throws clause.

*************************************************

13. If we extend thread class or implement runnable interface then the present run()
method must not be marked static ,if it is marked it causes compiler error.

***************************************************

14. The priority of a thread can be altered after the thread has been started using
setPriority method .The code compiles fine.

***************************************************