尝试/捕获/异常

Question

I'l post my three question where the questions occur aswell in the code, aswell as post it up here.

Q 1 what are threads and how do i use them?
Q 2 how do 'try' / 'catch' work? Q 3 what is the exception? see code.

Thank you in advance :)

public void start() {
  //  Question 1: What are these threads and how do i implement them?
  Thread frameTiming = new Thread (this);
  frameTiming.start ();
}

public void run() {
  while (true) {
    xleft-=10;
    yleft-=10;
    xleft2+=10;
    yleft2+=10;
    i-=5;
    repaint();
    //  Question 2: what does try/catch do?
    try {
      Thread.sleep(40);
    // question3: what does this exception do exactly? :0
    } catch (Exception e) {
    }

}

Answer 1

Threads are objects that contain enough support structures to act as a lightweight process.

The process is like a program that is executing, but it is a formalized definition, and is the thing(s) managed by the operating system, which decides if a process gets CPU time or not.

The reason a thread is like a lightweight process is because even though it gets CPU time allocated almost independently of the main program, it doesn't get everything a process gets. It shares the memory with the other threads in the process, while real processes get their own private memory spaces.

The try / catch block you've added is to catch a known, declared exception that it thrown when you call Thread.sleep(...) . Since the thread is asking to be taken off of the CPU for at least 40 milliseconds, there is a chance that while it is off of the CPU, the program shutsdown, or some other Thread tells it to "wake" up for some other reason. If this happens, then the Thread.sleep(...) call will not finish the sleep normally, but instead will terminate the call abnormally with an InterruptedException

Answer 2

Q 1 what are threads and how do i use them?

When you run code it usually all runs linearly. Suppose you ask for user input. The program stops and then awaits user input. What if the user never inputs? The program never moves on. Having multiple threads allows you to do things in the background while you wait for user input. It also allows you to divide work up into multiple simultaneous methods to take advantage of multiple cores, although without the proper algorithm to divide the work you wouldn't be doing yourself much of a favor.

How to start a thread in Java

Q 2 how do 'try' / 'catch' work?

In Java and many other languages try and catch is how you handle possible errors that usually you believe you can recover from. For example if you ask a user for a number it is possible he will give you a letter, therefore you don't ask the user for a number you TRY to ask the user for a number. If the user makes a mistake you CATCH his mistake and then make recourse. This doesn't just apply to user input suppose you open a file but it does not exist or some other error occurs which causes the read operation to fail. This possibility requires you to TRY it rather than go ahead and just do it.

How to use try/catch

Q 3 what is the exception?

An exception is something a method throws when something has gone wrong. In Java there is two types of exceptions, checked and unchecked. Most unchecked exceptions are due to programmer error such as array is out of bounds. However the common exceptions such as trying to open a file but you can't because it does not exist, these Exceptions are called checked exceptions meaning you HAVE TO surround them with a try catch, this is pretty unique to Java.

Click here to learn in a more professional manner the three types of Exceptions in Java