ThreadB that calls the join() on ThreadA
Then ThreadB will have to wait until ThreadA completes its execution

When ThreadA finishes, ThreadB will resume its execution

https://raw.githubusercontent.com/vsaravanan/java22/master/src/main/java/com/saravanjs/java22/console/multithreading/ThreadJoin.java

public class ThreadJoin {
public static void main(String[] args) {
Thread threadA = new Thread(() -> {
System.out.println("ThreadA started");
try {
System.out.println("Sleeping at A");
Thread.sleep(2000); // Simulating some work
} catch (InterruptedException e) {
e.printStackTrace();
            }
System.out.println("ThreadA completed");
        });

Thread threadB = new Thread(() -> {
System.out.println("ThreadB started");
try {
threadA.start();
System.out.println("ThreadA joined with ThreadB");
threadA.join();
            } catch (InterruptedException e) {
e.printStackTrace();
            }
System.out.println("ThreadB completed");
        });


threadB.start();

System.out.println("Main thread completed");
    }
}

Main thread completed

ThreadB started

ThreadA joined with ThreadB

ThreadA started

Sleeping at A

ThreadA completed

ThreadB completed

In case if you don’t join, then both threads will be executed in parallel
ThreadB could complete before the Thread A as below


Main thread completed

ThreadB started

ThreadA joined with ThreadB

ThreadA started

Sleeping at A

ThreadB completed

ThreadA completed