How Thread.join() in Java works internally.

How join method works in Java.

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join() method is used for waiting the thread in execution until the thread on which join is called is not completed.

Remember, the thread which will wait is the thread in execution and it will wait until the thread on which join method called is not completed.

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How join method works internally.

Consider a scenario below and we will try to understand join() method by going through example.

package javabypatel;

public class ThreadJoinDemo extends Thread{
 static ThreadJoinDemo thread1;

 public void run(){
  try{
   synchronized(thread1){
    System.out.println(Thread.currentThread().getName()+" acquired a lock on thread1");
    Thread.sleep(5000);
    System.out.println(Thread.currentThread().getName()+" completed");
   }
  }
  catch (InterruptedException e){ }
 }

 public static void main(String[] ar) throws Exception{
  thread1 = new ThreadJoinDemo();
  thread1.setName("thread1");
  thread1.start();

  synchronized(thread1){
   System.out.println(Thread.currentThread().getName()+" acquired a lock on thread1");
   Thread.sleep(1000);
   thread1.join();
   System.out.println(Thread.currentThread().getName()+" completed");
  }
 }
}

Output:
main acquired a lock on thread1
thread1 acquired a lock on thread1 //after 1 second this line is printed
thread1 completed //after 5 second this line is printed
main completed

In above example, we created 2 Threads,

1. "main" thread
2. "thread1" thread

Based on output, the flow went as shown below,

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Who call notify/notifyAll in case of thread waiting on join method?

After run() method of thread is completed, it doesn't mean thread task is completed,  It has to do many other tasks like 

1. Destroying the associated stack, 
2. Setting the necessary threadStatus etc.

One of the task is notifying the waiting threads, So that Thread waiting on join() method will be notified that thread has completed its task and joined threads can resume.

Above task are executed inside native thread call, so it wont be visible in java thread API.

You may also like to see

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Advanced Java Multithreading Interview Question-Answer.

Exception Handling Interview Question-Answer

Method Overloading - Method Hiding Interview Question-Answer

Type Casting Interview Questions In Java

How is ambiguous overloaded method call resolved in java

Method Overriding rules in Java

Interface interview questions and answers in Java

Enjoy !!!! 

If you find any issue in post or face any error while implementing, Please comment.

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Important Java Thread Interview Questions & Answers

Multithreading Tutorial.
Interview Questions on Threads in Java

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Question 1. What are Threads in Java?

In Java, when a program requires more than one task to execute in parallel, say for example,

1. Reading a data from a local file.
2. Reading a data from remote connection.
   

When both of above task need to be executed in parallel at that time Threading will come in picture.
So Java Threads helps creating multiple independent path of execution within a program which can run parallely.  

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Exception Handling Interview Questions.

Exception Handling in Java Tutorial.
Interview Questions on Exception Handling in Java

---

What is Exception handling?

Any program is a set of instructions executing in predefined sequence, but due to some Run-time Error or Exceptions program flow gets disturbed and gives erroneous result and to handle this un-expected behavior or condition is known as Exception handling. 

There are mainly 2 types of problems that can occur at run time,

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How is ambiguous overloaded method call resolved in java?

How compiler resolves ambiguous method overloading call. OR
Which overloaded method will get selected for null parameter in java. OR
Important Java Interview Questions On Method Overloading OR
Method Overloading Tutorial

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What is method overloading?

If a class have multiple methods with same name but with different parameters list, it is known as Method Overloading. Parameters lists should differ in either,

1. Number of parameters.
2. Data type of parameters.
3. Sequence of data type of parameters. 

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Construct a Binary Tree from In-order and Post-order traversals.

How to construct a Binary Tree from given In order and Post order traversals.

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Let us first understand what we want to achieve? what is the input and what will be the expected output?

Question:
Two traversals are given as input,

  int inOrder[] =   {20, 30, 35, 40, 45, 50, 55, 60, 70};
  int postOrder[] = {20, 35, 30, 45, 40, 55, 70, 60, 50};

By using above 2 given In order and Post Order traversal, construct Binary Tree like shown below,

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