I want to know what are the differences between runnable and thread class implementation except they are class and interface and how to decide which is better at what time ...

Well it depends on your need. Generally you extend any class when either you want to over ride or use all or most of the methods and interface you want to implement the methods. In Thread class there are tons of methods but we override only one run() method and Runnable interface has only one method to implement that is run(). So its a performance benefit and common sens to use Runnable interface than Thread class. read less

Here are some of my thoughts on whether I should use Thread or Runnable for implementing task in Java, though you have another choice as "Callable" for implementing thread which we will discuss later. 1) Java doesn't support multiple inheritance, which means you can only extend one class in Java so once you extended Thread class you lost your chance and can not extend or inherit another class in Java. 2) In Object oriented programming extending a class generally means adding new functionality, modifying or improving behaviors. If we are not making any modification on Thread than use Runnable interface instead. 3) Runnable interface represent a Task which can be executed by either plain Thread or Executors or any other means. so logical separation of Task as Runnable than Thread is good design decision. 4) Separating task as Runnable means we can reuse the task and also has liberty to execute it from different means. since you can not restart a Thread once it completes. again Runnable vs Thread for task, Runnable is winner. 5) Java designer recognizes this and that's why Executors accept Runnable as Task and they have worker thread which executes those task. 6) Inheriting all Thread methods are additional overhead just for representing a Task which can can be done easily with Runnable. These were some of notable difference between Thread and Runnable in Java, if you know any other differences on Thread vs Runnable than please share it via comments. I personally use Runnable over Thread for this scenario and recommends to use Runnable or Callable interface based on your requirement. read less

The class extending thread class will acts as a thread. But the class which implements runnable is not a thread. We have to externally make it as thread object by passing the class object to the thread class. And it is always better to use for interface since multiple inheritance is not possible in java. but you can implement any no of interfaces. read less

This needs to be understood from two perspectives: 1.As we know that Java does not support multiple inheritance,if we are extending Thread Class to implement the run() method then we are no more able to extend any other class( even if the problem requires to do so).Thus we are restricted due to the fact that we have extended Thread class to implement run() method. 2.The Core idea behind extending a class is to inherit it and enhance some functionalities existing in the base class by overriding them in the derived class.As the idea behind extending Thread class is just to override run() method and we do not modify/enhance any other functionality. Thus there is no point to inherit the whole bulky Thread class in order to get just run () method when we can go for a lighter approach by implementing a Runnable interface and at the same time are free to extend any other class. read less