RAQ: Rarely asked questions

I guess these questions are not really very difficult so you should be able to answer them just by reading them or try it yourself. I have prepared these from various sources(none of this is mine) so if the question seem your original then I thank you for the contribution duely respecting copyrights.

Here, I have not tried to explain each answer elaborately as I prepared this on the fly along the way I work and code. Please see me as just one of you while reading questions and answers and forgive me if I sound terrible before you suggest correction to what you see as errornous.

The reason I name these as RAQ is- they are not FAQ.


1. What happens when we try to compile the following code?

class GoogleMagic {
    public static void main(String[] s) {
        System.out.println("Here starts the magic...");
        http://WWW.Google.COM;
        System.out.println("Here ends the magic.");
    }
}

Ans: "http:" here is considered to be a label and rest of the line is ignored as comment!

2. What would be the output of the following program in following scenarios,
a) <<replace>> = private
b) <<replace>> = public

Explain reason for each output about "what happens".

class A {
    <<replace>> void m1() {
        System.out.println("A.m1");
    }
    public  void m2() {
        m1();
        System.out.println("A.m2");
    }
}

class B extends A {
    public void m1() {
        System.out.println("B.m1");
    }
    public void m2() {
        System.out.println("B.m2");
    }
}

class test {
    public static void main(String[] s) {
        B b = new B();
        b.m2();   
    }
}


3. Explain what happens when we try to compile the following.
   What would be the output if we are able to compile the code?

class test {
    public void trimIt(String s) {
        s.trim();
    }
    public static void main(String[] s) {
        test t = new test();
        String String = "  Trim Me  ";
        t.trimIt(String);
        System.out.println(String);
    }
}


4. Write the output of the following code.

class test {
    public static void main(String[] s) {
        System.out.println("Start");
        for(int i=0; i < 10; i = i++ ) {
            System.out.println("Processing...");
        }
        System.out.println("End");
    }
}


5. Consider the following code.

class A {
        public void m1(int i) {
                System.out.println("A.m1:"+i);
        }
}

class B extends A {
        public void m1(long i) {
                System.out.println("B.m1:"+i);
        }
}

class test {
        public static void main(String[] s) {
                B b = new B();
                b.m1(999999); // Line-1

                A a = new B();
                a.m1(999999);    // Line-2
        }
}

5.1 What would be the result of the compilation of the above code?
5.2 What would be the result of the compilation of the above code when Statement at "Line-1" is commented fully?

Explain results in each scenario.


6. Explain the output of the program upon compiling and if it is able to get
   compiled then what would be the output at runtime?

import java.util.Vector;

class Person {
    private String name;
    private String ssn;

    Person() {
    }
   
    Person(String name,String ssn) {
        this.name = name;
        this.ssn = ssn;
    }
    public void setName(String name) { this.name = name; }
    public void setSSN(String ssn) { this.ssn = ssn; }
    public String toString() {
        return name+":"+ssn;
    }
}

class vectorTest {
    public static void main(String[] s) {
        Vector roster = new Vector();
        Person tempPerson = new Person();

        for(int i=0; i < 5; i++) {
            tempPerson.setName("Person"+i);
            tempPerson.setSSN(i+"");
            roster.add(tempPerson);
        }

        http://www.Yahoo.com;

        for(int j=0; j < 5 ; j++) {
            System.out.println(((Person)roster.elementAt(j)).toString());
        }
    }   
}

Ans:
----
http://www.Yahoo.com line will be interpreted as,
http: = label
// = comment

7. What will be the output of the following? Will you complete your Masters Degree? :)

class test {
    public static void main(String[] s) {
       
        byte b = 0;
        System.out.println("Good Luck. You are starting Masters Degree at USC");
        while( ++b > 0 );
        System.out.println("Congrats!!! You got graduated from School!");
    }
}

Good Luck. You are starting Masters Degree at USC
Congrats!!! You got graduated from School!


8. Declare a constant array of constant int in Java.

Can't do it.



9. Explain System.out.println().

Ans:
----
println() is a instance method in PrintStream class
out is static reference in System class of type PrintStream
When we say System.out.println() it is accessing static out
member of System class and then calling instance method on that reference.


10. Write the output of the following program.

abstract class base {
        abstract void getName(base b);
}

class test2 extends base {
       
        void main() {
                getName(this);
        }
        void getName(base b) { System.out.println(b.getClass().getName()); }

        static void main(String[] s) {
                test2 t2 = new test2();
                t2.main();
        }
}

Ans: test2



11. What would happen when we run the following program?

class test {
    public static void method1() {
        System.out.println("Hi! I am test.method1");
    }
    public static void main(String[] s) {
        test t = new test();
        t = null;
        t.method1();
    }
}

Ans: Hi! I am test.method1

As method1 is static it doesn't raise NullPointerException.



12. What will be the output of the following code when run?

import java.util.*;

class test4 {

        public static void main(String[] s) {

                String temp = "1, 2, 3 , 4, 5, 6, 7, 8";
                StringTokenizer stk = new StringTokenizer(temp,",");

                while (stk.hasMoreTokens() ) {
                        stk.hasMoreTokens();
                        System.out.println(stk.nextToken().trim());
                }
        }
}

Ans:
----
1
2
3
4
5
6
7
8

hasMoreTokens() is atomic. It doesn't increase the pointer. Only nextToken() does that.


13. Write the output of the following.

class test {
    static void main(String[] s) {
        StringBuffer sb1 = new StringBuffer("Java");
        StringBuffer sb2 = new StringBuffer("Java");
        System.out.println(sb1==sb2);
        System.out.println(sb1.equals(sb2));
    }
}

Ans:
----
false
false

As sb1 == sb2 equates references which are different and equals() method
is not overridden in StringBuffer by which it could return true for the second SOP.



14. What will be the output of following given the below folder hierarchy? (this is on Unix platform)

maulin (folder)
|___dir1 (folder)
|___temp.txt (file)
|___test.java (file)

test.java:

import java.io.*;

class test {
    public static void main(String[] args) throws Exception {
        System.out.println(System.getProperty("user.dir"));
        File f = new File("temp.txt");
        System.out.println(f.getCanonicalPath());

        System.out.println(System.setProperty("user.dir","/maulin/dir1"));

        File f1 = new File("temp.txt");
        System.out.println(f1.getCanonicalPath());
        System.out.println(System.getProperty("user.dir"));
    }
}


15.
A) What does the following code do?
B) What will be the output of the following program?
C) What is the flow in the code that can cause misbehavior when we add Entry objects into the TreeMap structure?

import java.util.*;

class Entry implements Comparable {
    String name;
    int value;

    public Entry(String name, int value ) {
        this.name = name;
        this.value = value;
    }

    public int compareTo(Object o) {
        return this.value - ((Entry)o).value;
    }

    public String toString() {
        return "{ " +name+" , "+value+" }";
    }
}

public class testTreeMap {

    public static void main(String[] args) {

        TreeMap tm = new TreeMap();

        String[] names = new String[]{"zebra","maulin","soham"};
        int[] values = new int[]{0,2,2};

        for(int i=0; i < names.length; i++) {
            System.out.println(tm.put(new Entry(names[i],values[i]),new Integer(values[i])));
        }

        Set ks = tm.keySet();
        Iterator iter = ks.iterator();
        while ( iter.hasNext() ) {
            System.out.println(iter.next());
        }
    }
}

ANS:
----
A) It sorts the Entry objects based on 'value'
B) null
   null
   2
   { zebra, 0 }
   { maulin, 2 }
C) Flaw is that compareTo() is not compatible with equals() which causes
   only first two Entry objects go in the TreeMap instead all of three.



16. What is the difference between API and Implementation?
e.g. JAXP and Xerces-J. Do we need to have both or only one?

ANS: JAXP is the API, Xerces-J is the Implementation of JAXP. We need both.



17. What will be the output of the following program? Explain your answer.

class TestParent {

    int a=100;

    static {
        System.out.println("TestParent.static init");
    }

    TestParent() {
        System.out.println("TestParent.constructor: a="+a);
        incr();
    }

    void incr() {
        a++;
        System.out.println("TestParent.incr: a="+a);
    }
    int getVar() { return a; }
}

class TestChild extends TestParent {

    int a=-10;

    static {
        System.out.println("TestChild.static init");
    }

    TestChild() {
        System.out.println("TestChild.constructor: a="+a);
        incr();
    }

    void incr() {
        a+=10;
        System.out.println("TestChild.incr: a="+a);
    }
    int getVar() { return a; }
}

public class TestInheritance1 {
   
    public static void main(String[] args) {
        TestChild tc = new TestChild();
        System.out.println("Result:"+tc.getVar());
    }
}

ANS:
----
TestParent.static init
TestChild.static init
TestParent.constructor: a=100
TestChild.incr: a=10
TestChild.constructor: a=-10
TestChild.incr: a=0
Result:0

Explanation:

The sequence of initialization of any class,
1. static vars
2. static blocks
3. parent class (if extends relationship)
4. instance vars
5. rest of the constructor code




18. What is a "Magic Word" in computer arena? Do we have one for Java?
What is that "magic word" for Java?

ANS: magic word is a special code so that OS can identify the file types based on the file content. for java it is "CAFEBABE".




19. What is "green thread"? If we say some code won't work on "green thread" based JVMs what does that mean?

ANS: Green Thread is a thread that doesn't get mapped to the OS level thread. Its an application level thread.

When somebody says that some code won't work on 'green thread' based JVMs it means that the code is not taking care of calling yeild() at appropriate times and as JVM is executing the threads at application level it can hog the application for indefinite amount of time.



20. What is the output of following program if we try to compile it and run it?

public class Test
{

    public Test(Object obj)
    {
        System.out.println("Constructor A.");
    }

    public Test(String s)
    {
        System.out.println("Constructor B.");
    }

    public static void main(String args[])
    {
        new Test(null);
    }
}

ANS:
----
Constructor B.

Because String is a subclass of Object and so it is more specific class that matches null before Object.



21. If you have a shell script then write a code in Java to invoke it and run it in the background. e.g. if the shell script name is, shscript then your code should achieve the same effect as, "$shscript &" command.

ANS: Hint: you can't put a process in background directly from java using Runtime.exec()...


22. Consider the below code,

public class arrayTest1 {

    public static void main(String[] args) {

        // line:arr1
        int[][][][][] a = new int[3][4][5][10][98];
        //line:arr2
        //int[][][][][] a = new int[3][4][5][10][];

        // line:1
        a[2][0][0][0] = new int[100];

        // line:2
        a[3][0][0][0] = new int[100];

        // line:3
        a[0][0][0][0] = new int[1000];
        a[0][0][0][0][98] = 10;

        System.out.println("Done");
    }
}

what happens when,
1. we comment line below "line:2". Explain the reason.
2. we comment line below "line:3". Explain the reason.
3. what happens when we comment line below "line:arr1" and
uncomment line below "line:arr2" and comment line below "line:3" simulataneously?

ANS:
----

HINT: the last dimension index doesn't matter so we can say what we do in line:3 but if we remove it then the below statement is not applicable as the original size was 98.

When we don't specify the last index limit (like line:arr2) then the default array ref is NULL so the 2nd line after "line:3" gives NPE if we remove line below "line:3"


23. What does the following code do on Windows?

import java.io.*;

public class dir {

        public static void main(String[] s) throws Exception {

                Runtime rt = Runtime.getRuntime();
                Process p = rt.exec("dir");
                p.waitFor();
                BufferedReader br = new BufferedReader(new InputStreamReader(p.getInputStream()));
                String line = "";
                line = br.readLine();
                while ( line != null ) {
                        System.out.println(line);
                        line = br.readLine();
                }
                br.close();
        }
}

ANS:
----
Doesn't work and throws IOException, error=2 as "dir" is "internal dos" command and can't be executed by windows directly hence JRE also can't execute it. (NOTE: Similar program with "ls" command on Unix will work).


24. Write a method to swap two strings in Java.


25. What happens when we run the following code.

class myThread extends Thread {

    public void run() {
        System.out.println("In myThread.run()");
    }
}

class myRunnable implements Runnable {

    public void run() {
        System.out.println("In myRunnable.run()");
    }
}

public class test {

    public static void main(String[] args) {

        myThread mth = new myThread(new myRunnable());
        mth.start();
    }
}


26.
a) What is meant by "immutable"/"mutable" objects?
b) If we provide only "Getter" methods in our code for the data members then does
that mean our class is immutable? (Provided we don't have any other methods manipulating the data members).

c) Where do we need the "mutability"?

ANS:
----
b)
No. Because if we have mutable attributes in the code then it is a problem, e.g. if we have a method,

private Point p;
public Point getPoint() {
    return p;
}

main()...
Point p = getPoint();
p.setX(300); // we are messed up!!

c)
awt/swing can't add one component at two places. we have to do clone() them to "drag and drop".



27.
a) How would you clone an object?
b) Can you use something like "copy constructor" for clonning? Explain answer.
c) What could be some other ways for clonning instead using Clonnable interface?
d) Modify the below code to have a clonning capability,

class list {

    list l;
    int i;

    list(int i) {
        this.i = i;
        if ( --i > 0 )
            l = new list();
    }

    void incr() {
        i++;
        if ( l != null )
            l.incr();
    }
}



28. If we have 256 MB RAM then what is the maximum length of double array we
can create? (Ignore the jvm memory occupied and everything else)

Ans: Integer.MAX_VALUE  as the length() method must return correct 'int' length.

29. What does this return?
Integer.parseInt("Jack",27);

Ans: 381611



30. In a servlet/jsp which is the "current dir" when you run a code from the
servlet/jsp that refers to the "current dir" something like,
File f = new File("test.txt");

Ans:
----
The one from where the webserver was started actually, e.g. on Tomcat it would be /bin directory


31. What is serialVersionUID and what problems we might face w.r.t to this?



32. Express your views about when to use Singleton and a class with all static methods?


33. If we have a class e.g. Employee, object of which, we want to pass between client and server (written based on sockets) written in java with the following constraint, - we don't want Employee.class file on theee client side The reason being, if we change Employee.java file (hence Employee.class file) then it becomes difficult to make client have the latest .class file always. How we will go about it?
You can add/remove/rename objects/files here.