When we call start() method → a new thread is created in a separate Stack Memory.
Thread class provides a predefined non-static method:
public final boolean isAlive()
Return type → boolean
Used to verify:
Whether a thread has started or not
Whether the thread is alive or not
Behavior:
If the thread is not started → isAlive() returns false
If the thread is started and running → returns true
If the thread completed execution → returns false
⚠ In Java, we cannot restart a thread. If we try → Runtime Exception:
java.lang.IllegalThreadStateException
Example 1
package com.ravi.basic;
class Test extends Thread { @Override public void run() { System.out.println("Child thread is running..."); System.out.println("It is running with separate stack memory"); } } public class IsAliveDemo { public static void main(String[] args) throws InterruptedException { System.out.println("Main thread started!!!!"); Test t1 = new Test(); System.out.println("Is thread started OR Alive before start : "+t1.isAlive()); t1.start(); System.out.println("Is thread started OR Alive after start : "+t1.isAlive()); Test t2 = new Test(); t2.start(); t1.start(); // java.lang.IllegalThreadStateException } }
🔴 Thread Name & Exception Demo
package com.ravi.basic;
class Foo extends Thread { @Override public void run() { String name = Thread.currentThread().getName(); System.out.println(name+" thread is running..."); } } public class ExceptionDemo { public static void main(String[] args) { String name = Thread.currentThread().getName(); System.out.println(name+" thread is running!!!!"); Foo f1 = new Foo(); f1.start(); Foo f2 = new Foo(); f2.start(); System.out.println(10/0); // Exception System.out.println(name+" thread completed"); } }
NOTE:
If the main thread is interrupted due to an ArithmeticException, child threads will still execute because they run in separate stack memory.
🔴 Thread Loop Example
package com.ravi.basic;
class Sample extends Thread { @Override public void run() { String name = Thread.currentThread().getName(); for(int i=1; i<=10; i++) { System.out.println(i+" by "+name+" thread"); } } } public class ThreadLoop { public static void main(String[] args) { new Sample().start(); String name = Thread.currentThread().getName(); for(int i=1; i<=10; i++) { System.out.println(i+" by "+name+" thread"); } int x = 1; do { System.out.println("India"); x++; } while(x<=10); } }
NOTE:
The processor frequently switches between the main thread & Thread-0. So the output is unpredictable.
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The thread scheduler decides which thread executes first.
🔴 How to Set and Get Thread Name
By default JVM assigns names:
Thread-0 Thread-1 Thread-2
Methods:
public final synchronized void setName(String name)
public final String getName()
Example 1 (Default Names)
package com.ravi.basic;
class DoStuff extends Thread { @Override public void run() { String name = Thread.currentThread().getName(); System.out.println("Running Thread name is : "+name); } } public class ThreadName1 { public static void main(String[] args) { DoStuff d1 = new DoStuff(); d1.start(); DoStuff d2 = new DoStuff(); d2.start(); System.out.println("Current thread name is : "+Thread.currentThread().getName()); } }
Example 2 (User-Defined Names)
package com.ravi.basic;
class Demo extends Thread { @Override public void run() { String name = Thread.currentThread().getName(); System.out.println("Running Thread name is : "+name); } } public class ThreadName2 { public static void main(String[] args) { Thread t = Thread.currentThread(); t.setName("Parent"); Demo d1 = new Demo(); Demo d2 = new Demo(); d1.setName("Child1"); d2.setName("Child2"); d1.start(); d2.start(); System.out.println("Current thread name is :"+t.getName()); } }
🔴 Batch Assignment Example
package com.ravi.basic;
class BatchAssignment extends Thread { @Override public void run() { String name = Thread.currentThread().getName(); if(name != null && name.equalsIgnoreCase("Placement")) { this.placementBatch(); } else if(name != null && name.equalsIgnoreCase("Regular")) { this.regularBatch(); } else { throw new NullPointerException("Name can't be null"); } } public void placementBatch() { System.out.println("I am a placement batch student."); } public void regularBatch() { System.out.println("I am a Regular batch student."); } }
public class ThreadName3 { public static void main(String[] args) { Scanner sc = new Scanner(System.in); try(sc) { System.out.println("Enter your Batch Title [Placement/Regular] :"); String title = sc.next();
BatchAssignment b = new BatchAssignment(); b.setName(title); b.start(); } catch(InputMismatchException e) { System.out.println("Invalid Input"); } } }
🔴 Thread.sleep(long millis)
public static void sleep(long millis) throws InterruptedException
Static method of the Thread class
Puts the thread into a temporary waiting/sleeping state
Time is given in milliseconds
Example:
Thread.sleep(1000); // sleeps for 1 second
Must handle using:
try-catch OR
throws InterruptedException
Example 1
package com.ravi.sleep;
class Sleep extends Thread { @Override public void run() { String name = Thread.currentThread().getName(); for(int i=1; i<=10; i++) { System.out.println(i+" value is :-"+i+" by "+name+" thread"); try { Thread.sleep(1000); } catch(InterruptedException e) { System.out.println(e); } } } } public class SleepDemo1 { public static void main(String[] args) { Sleep s = new Sleep(); s.setName("Child"); s.start(); } }
Example 2
class MyTest extends Thread { @Override public void run() { for(int i=1; i<=5; i++) { System.out.println("i value is :"+i); try { Thread.sleep(1000); } catch(InterruptedException e) { e.printStackTrace(); } } } }
public class SleepDemo2 { public static void main(String[] args) { MyTest m1 = new MyTest(); MyTest m2 = new MyTest(); m1.setName("M1"); m2.setName("M2"); m1.start(); m2.start(); } }
threadId() Method (JDK 19+)
Introduced in JDK 19
Returns the unique ID of the current thread
Defined in the Thread class
Final method
long id = Thread.currentThread().threadId();
Example
package com.ravi.sleep; class MyThread1 extends Thread { @Override public void run() { System.out.println("Child Thread is running"); Thread t = Thread.currentThread(); System.out.println("Id of child thread is :"+t.threadId()); } } public class SleepDemo3 { public static void main(String[] args) { System.out.println("Main thread is Started"); Thread t = Thread.currentThread(); System.out.println("Id of main thread is :"+t.getId()); MyThread1 mt = new MyThread1(); mt.start(); try { Thread.sleep(4000); } catch(InterruptedException e) { e.printStackTrace(); } System.out.println("Main thread is Ended"); } }
MPORTANT DIFFERENCES (Console Points)
start() → creates a new thread run() → normal method call (no new thread)
