Dhananjay Deshmukh

Tapas Chand wrote:You should execute the javac command from the parent directory of the package, not from within the package.
cd Desktop/Kathy_Bates_Practice javac Inheritance/Parent_Class.java

Paweł Baczyński wrote:By the convention, package names should begin with lower-case and class names should not contain underscores.

Please follow these instructions.

Yes i will keep that in mind. Thanks for this, It never occurred to me that since this is under the package i should use the command this way. Following is the output i get now :
java Inheritance/Parent_Class Inside Child class a = 2 b = 3 c = 4 Addition of a, b and c is 9 Addition of a, b and c is 24 Inside child class2 Inside Child class2 b = 5 c = 6 Addition of a, b and c is 15 Addition of a, b and c is 120

Paweł Baczyński wrote:Please post your folder structure and a commend you use to compile.

I compiled your code and it works fine.

By the convention, package names should begin with lower-case and class names should not contain underscores.

I have attached the screenshot for the directory structure and following is the command i use:
ricky@RICKY-Lenovo-G50-70:~$ cd Desktop/Kathy_Bates_Practice/Inheritance ricky@RICKY-Lenovo-G50-70:~/Desktop/Kathy_Bates_Practice/Inheritance$ javac Parent_Class.java

Sorry i m late to reply, had an interview.

So i m basically using Linux as OS and terminal to compile and run project. I also have eclipse installed just in case to understand. So i was just trying a simple inheritance programs. Below is the code:package Inheritance; public class Child_Class1 { private int a; public int b; int c; Child_Class1(int a, int b, int c) { this.a = a; this.b = b; this.c = c; } protected void display() { System.out.println("Inside Child class"); System.out.println("a = "+a+" b = "+b+" c = "+c); } void add() { System.out.println("Addition of a, b and c is "+(a+b+c)); } public void multiply() { System.out.println("Addition of a, b and c is "+(a*b*c)); } }package Inheritance; public class Child_Class2 extends Child_Class1 { public int d,c; Child_Class2(int a, int b, int e) { super(a,b,e); System.out.println("Inside child class2"); d = a; c = b; } public void show() { System.out.println("Inside Child class2"); System.out.println("b = "+d+" c = "+c); this.add(); this.multiply(); } }package Inheritance; public class Parent_Class { public static void main(String []args) { Child_Class1 cc1 = new Child_Class1(2,3,4); cc1.display(); cc1.add(); cc1.multiply(); System.out.println(); Child_Class2 cc2 = new Child_Class2(5,6,4); cc2.show(); } }
Whenever i execute above program through terminal, I get following error:
Parent_Class.java:5: error: cannot find symbol Child_Class1 cc1 = new Child_Class1(2,3,4); ^ symbol: class Child_Class1 location: class Parent_Class Parent_Class.java:5: error: cannot find symbol Child_Class1 cc1 = new Child_Class1(2,3,4); ^ symbol: class Child_Class1 location: class Parent_Class Parent_Class.java:12: error: cannot find symbol Child_Class2 cc2 = new Child_Class2(5,6,4); ^ symbol: class Child_Class2 location: class Parent_Class Parent_Class.java:12: error: cannot find symbol Child_Class2 cc2 = new Child_Class2(5,6,4); ^ symbol: class Child_Class2 location: class Parent_Class 4 errors

However, whenever i execute the above programs in eclipse i get the correct output. I m confused as to why am i not getting the output from terminal. All the package and class i have included are correct. Please help me understand why JVM shows error

Marcus Biel wrote:If the book is to though, you are to weak, sorry.

Start reading Head First Java then.

The exam is tough, the book condensed to the maximum. Make notes, read it several times.

Also, just thinking about it, my Free Java Beginner Video Course is based on all my knowledge I gained from Kathy's books and the certificate.
Whenever possible I go deep down into details which are also important for the certification. Might be helpful for you, too.

Nothing was never too tough for me, it was only hard because i didn't even read java from past 4 years, not a single word. That's why i had to brush up my skill with basics from beginners guide. But this was like month back. I have already finished the OCAJP part - 1 of Kathy. But then i do get the message you wanted to give. Right now i m on binge study kinda thing. And thanks for the video tutorials, i will go through it too.

Marcus Biel wrote:

According to the website

I don't know which website you are talking about. I assume you are talking about the Oracle website, in this case, probably yes.
Additionally, you write "I have all the material". Well that might sound stupid, but more often then not people think possessing something is actually doing the trick - including me and myself ;-).
Really, be honest to yourself. Read Kathy's book, read every line of it. I once read the advice that you would be ready if you had a book full of colored highlighting that would fall apart you would be ready.
I did that. I was there. I even bought a new version (updated from Java 5 to 6) and that one in the end was just as colored and falling apart.
I did all mock exams I could get my hands on, and all simulators. I passed all of them above 90% or so as far as I can remember.
I have actually even now recently bought version 7 of the now called OCA/OCP book you probably have, I didn't yet have the time to read it, however mainly I just wanted to own it.
I love Kathy's books. And you can trust her. Everything that is in the book you will need. Everything that is not in the book, you will not need.
At the time in 2008 when I did my SCJP, I didn't even waste any time looking on any Oracle pages. Trust Kathy. That's all you need!

Was that clear enough?

p.s.: Yes, I am in love with her books

I guess saying that i have all the material was stupid. I did studied her material but was hard to understand , so I started with the basic. I guess i will start reading her material

Marcus Biel wrote:I did not understand what exactly your question is, but let me try.

Yes, as of today, you need to do the OCA before the OCP.
What do you need two month for?

Buy your copy of the
OCA/OCP Java SE 7 Programmer I & II Study Guide (Exams 1Z0-803 & 1Z0-804) (Certification Press)

Read it, best read it several times, make notes, and learn everything inside out you don't know.
Do all the mock exams thorughly until you reach close to a 100% in each question.
Then first you do the OCA and second the OCP.
You might also consider buying a Mock exam simulator, e.g. Wizlabs.

I have a link to this and some further information related to oracle Java certification on my blog:
OCA/OCP Certification

Hope this helps.

Let me put it this way. Are the questions that will appear in the OCAJP will be from the above topics And yes i have the material that yu have advised. I got it from the friend of mine, I m also referring to Java in a nutshell as well as the documentation and tutorial material available on java website.

Greg Charles wrote:

Dhananjay Deshmukh wrote:
I just wanted to check one more thing if i m getting the concept right. Lets just say threads one two and three have started and are in line for the resource. At the moment ob1.t.join() executes , ob1.t is calling sleep in run method and releasing the control making way for ob2.t to execute and then ob3.t. And the process goes on in which case ob1.t comes into life again and the same process goes on. Am i getting it right.

Yes, I think you have it right. When the main thread calls ob1.t.join(), that doesn't actually have any effect at all on the ob1.t thread. Whatever it's doing, it's going to keep doing, whether that's executing or sleeping. All the join does is tell the main thread to wait until the ob1.t thread completes it processing and sleeping entirely, before it (the main thread) continues its own processing.

If an analogy helps, let's say the boss needs some information for a call she will make to the board later in the day. She splits the work of obtaining the information into three pieces and gives one piece to each of her three employees, and she tells them to meet her in the conference room when they finish. As soon as all three return to the conference room with the results of their work, she makes her call to the board. That's like the join() method. It doesn't matter if the three employees slept for most of the time they said they were working. She still waits for all three to be done before going on.

On the other hand, she could have also set her three employees to work, and then herself have gone to sleep on the conference room table for an hour. In that case, when she wakes up, she calls the board immediately without waiting for her employees. That's like the sleep() method. If they had all finished their work by that time, then she has all the information she needs for her phone call. It's not ideal though, because the employees might have all finished their work in five minutes, and she made the board wait 55 extra minutes for no good reason. It's worse though if it took one ore more of those employees more than an hour to finish the work. In that case, the boss calls the board and then doesn't have all the information she promised them.

Thanks i guess i have the general idea of how the join works and it will become more clear with time, i just have to practice more examples.

Greg Charles wrote:The join() method called on a thread (ob2.t for example) blocks the thread making the call from going forward (here your main application thread) until the other thread finishes its processing. When you call Thread.sleep(n), you stop your current thread for about n milliseconds, then go on processing. Since your created threads take about six seconds to complete, the only effective difference in this particular example is that the Thread.sleep(10000) is probably slower. In general, you might not know how long a thread will take to finish its work, and so the join() method can be safer ... that is if it is actually important to wait until a thread finishes before doing some other work.

I just wanted to check one more thing if i m getting the concept right. Lets just say threads one two and three have started and are in line for the resource. At the moment ob1.t.join() executes , ob1.t is calling sleep in run method and releasing the control making way for ob2.t to execute and then ob3.t. And the process goes on in which case ob1.t comes into life again and the same process goes on. Am i getting it right.

Yes I did. I guess the concept becomes clear when I see it in a big picture or atleas a scenario big enough, but I have to study many topics before I can understand and I have to try programming few scenarios. Untill then I will have doubts. So let me try programming and debugging for couple of days. But that's for the link

According to the website, Java SE 8 Programmer I certification is a prerequisite. Als there are topics mentioned in exam section. These topics are as follows:
Java Basics: Define the scope of variables Define the structure of a Java class Create executable Java applications with a main method; run a Java program from the command line; including console output. Import other Java packages to make them accessible in your code Compare and contrast the features and components of Java such as: platform independence, object orientation, encapsulation, etc. Working With Java Data Types: Declare and initialize variables (including casting of primitive data types) Differentiate between object reference variables and primitive variables Know how to read or write to object fields Explain an Object's Lifecycle (creation, "dereference by reassignment" and garbage collection) Develop code that uses wrapper classes such as Boolean, Double, and Integer. Using Operators and Decision Constructs: Use Java operators; including parentheses to override operator precedence Test equality between Strings and other objects using == and equals () Create if and if/else and ternary constructs Use a switch statement Creating and Using Arrays: Declare, instantiate, initialize and use a onedimensional array Declare, instantiate, initialize and use multidimensional array Using Loop Constructs: Create and use while loops Create and use for loops including the enhanced for loop Create and use do/while loops Compare loop constructs Use break and continue Working with Methods and Encapsulation: Create methods with arguments and return values; including overloaded methods Apply the static keyword to methods and fields Create and overload constructors; including impact on default constructors Apply access modifiers Apply encapsulation principles to a class Determine the effect upon object references and primitive values when they are passed into methods that change the values Working with Inheritance: Describe inheritance and its benefits Develop code that demonstrates the use of polymorphism; including overriding and object type versus reference type Determine when casting is necessary Use super and this to access objects and constructors Use abstract classes and interfaces Handling Exceptions: Differentiate among checked exceptions, unchecked exceptions, and Errors Create a trycatch block and determine how exceptions alter normal program flow Describe the advantages of Exception handling Create and invoke a method that throws an exception "Recognize common exception classes (such as NullPointerException, ArithmeticExcpetion, ArrayIndexOutOfBoundsException, ClassCastException)" Working with Selected classes from the Java API: Manipulate data using the StringBuilder class and its methods Creating and manipulating Strings Create and manipulate calendar data using classes from java.time.LocalDateTime, java.time.LocalDate, java.time.LocalTime, java.time.format.DateTimeFormatter, java.time.Period Declare and use an ArrayList of a given type Write a simple Lambda expression that consumes a Lambda Predicate expression


I have a very good knowledge of java and i m pretty sure that i can clear OCJA 8SE certification which is a prerequisite. However i still need 2 months more before i can go for OCJP.
Is it that the question in the OCJA SE8 will be from above topics only or is it otherwise? I just want be sure!!!

Greg Charles wrote:The join() method called on a thread (ob2.t for example) blocks the thread making the call from going forward (here your main application thread) until the other thread finishes its processing. When you call Thread.sleep(n), you stop your current thread for about n milliseconds, then go on processing. Since your created threads take about six seconds to complete, the only effective difference in this particular example is that the Thread.sleep(10000) is probably slower. In general, you might not know how long a thread will take to finish its work, and so the join() method can be safer ... that is if it is actually important to wait until a thread finishes before doing some other work.

All i could muster from your answer is that thread is basically unpredictable. I guess i will have to study more in order to understand the concept. and clear it. Perhaps i will try more programming. May be involving more interfaces will help, i have a plan.

I m getting confused to as what exactly is the function of join. Below is the code that i used in two ways:
1st: Used with Join
2nd: Used with sleep:

Below is the code :
class NewThread implements Runnable { String name; // name of thread Thread t; NewThread(String threadname) { name = threadname; t = new Thread(this, name); System.out.println("New thread: " + t); t.start(); // Start the thread } // This is the entry point for thread. public void run() { try { for(int i = 5; i > 0; i--) { System.out.println(name + ": " + i); Thread.sleep(1000); } } catch (InterruptedException e) { System.out.println(name + " interrupted."); } System.out.println(name + " exiting."); } } class DemoJoin { public static void main(String args[]) { NewThread ob1 = new NewThread("One"); NewThread ob2 = new NewThread("Two"); NewThread ob3 = new NewThread("Three"); System.out.println("Thread One is alive: " + ob1.t.isAlive()); System.out.println("Thread Two is alive: " + ob2.t.isAlive()); System.out.println("Thread Three is alive: " + ob3.t.isAlive()); // wait for threads to finish try { /*System.out.println("Waiting for threads to finish."); ob1.t.join(); ob2.t.join(); ob3.t.join();*/ Thread.sleep(10000); } catch (InterruptedException e) { System.out.println("Main thread Interrupted"); } System.out.println("Thread One is alive: " + ob1.t.isAlive()); System.out.println("Thread Two is alive: " + ob2.t.isAlive()); System.out.println("Thread Three is alive: " + ob3.t.isAlive()); System.out.println("Main thread exiting."); } }

In both the cases I get the same output as follows :
New thread: Thread[One,5,main] New thread: Thread[Two,5,main] One: 5 New thread: Thread[Three,5,main] Two: 5 Thread One is alive: true Three: 5 Thread Two is alive: true Thread Three is alive: true Two: 4 One: 4 Three: 4 Two: 3 Three: 3 One: 3 One: 2 Two: 2 Three: 2 Two: 1 Three: 1 One: 1 One exiting. Three exiting. Two exiting. Thread One is alive: false Thread Two is alive: false Thread Three is alive: false Main thread exiting.

Why is it that both the cases are giving the same output? I feel like there is no purpose to use join here

Paul Clapham wrote:No. Neither of them are ways of creating a thread.

The first one (Thread t = Thread.currentThread) doesn't create a thread. It just gets a reference to a thread which already exists (the thread which is currently running this code) and assigns it to a variable.

And the second one (new NewThread()) creates an object which -- as already mentioned -- isn't a thread. It happens that the constructor of that object creates a new thread, but as I already pointed out it's easy to confuse that thread with the NewThread object when they are actually two different things.

And the fact that a class implements Runnable is nothing to do with the creation of threads, either. All that means is that the class must have a "public void run()" method, and that's all.

Ok now i m clear.

Paul Clapham wrote:It's possible that calling your class "NewThread" has led you into believing that it's some kind of thread. Which it isn't; it only contains a Thread. If you rename your class something else -- like "RanchDemo" -- you'll see that it acts exactly the same way. When you create a RanchDemo object, a Thread is created and started. It makes no difference whether that RanchDemo object is subsequently assigned to a reference variable or not.

One more question that i have for you is that I have observed the statement Thread t = Thread.currentThread below
class CurrentThreadDemo { public static void main(String args[]) { Thread t = Thread.currentThread();//--------------------->Thread Creation, CurrentThreadDemo does not implement Runnable or extend thread. Another way of creating Thread? System.out.println("Current thread: " + t); // change the name of the thread t.setName("My Thread"); System.out.println("After name change: " + t); try { for(int n = 5; n > 0; n--) { System.out.println(n); Thread.sleep(1000); } } catch (InterruptedException e) { System.out.println("Main thread interrupted"); } } }


And another statement new NewThread is used in another class to create object as well as thread that has main and also the fact that NewThread class "implements Runnable" interface. So are these two ways of creating thread?

Paul Clapham wrote:It's possible that calling your class "NewThread" has led you into believing that it's some kind of thread. Which it isn't; it only contains a Thread. If you rename your class something else -- like "RanchDemo" -- you'll see that it acts exactly the same way. When you create a RanchDemo object, a Thread is created and started. It makes no difference whether that RanchDemo object is subsequently assigned to a reference variable or not.

that explains why i was getting confused. Because i tried to create a simple program without either implementing runnable or thread and tried to create an object like "new Classname". And ended up with error i couldn't figure out. Well, so much for trying variation however i think it was worth it. Its like the entire thread chapter just became easy to understand.