Unit 3: Exception handling and multithreading

javatry {
    int[] a = new int[3];
    int x = Integer.parseInt("12a");     // NumberFormatException
    a[5] = 10 / 0;
} catch (ArithmeticException | NumberFormatException e) {   // multi-catch
    System.out.println("Error: " + e.getMessage());
} catch (Exception e) {
    System.out.println("Other error");
} finally {
    System.out.println("Always runs — close files here");
}

try (java.io.FileReader fr = new java.io.FileReader("a.txt")) {   // try-with-resources
    System.out.println((char) fr.read());
} catch (java.io.IOException e) { e.printStackTrace(); }

javatry {
    try { int[] a = new int[2]; a[5] = 1; }
    catch (ArrayIndexOutOfBoundsException e) { System.out.println("inner"); }
    throw new IllegalStateException("custom");
} catch (IllegalStateException e) {
    System.out.println("outer: " + e.getMessage());
}

• Checked exceptions must be caught or declared with throws; unchecked (RuntimeException) need not.

javaclass Task implements Runnable {
    public void run() { for (int i = 1; i <= 3; i++) System.out.println(Thread.currentThread().getName() + " " + i); }
}
Thread t1 = new Thread(new Task(), "A"), t2 = new Thread(new Task(), "B");
t1.setPriority(Thread.MAX_PRIORITY);   // 10; MIN 1, NORM 5
t1.start(); t2.start();

• Threads can be created by extending Thread or implementing Runnable (preferred); priorities are hints to the scheduler.

javaclass Counter {
    private int c = 0;
    synchronized void inc() { c++; }   // one thread at a time
}

• Without synchronisation, two threads updating shared data cause a race condition. A synchronized method or block acquires the object's monitor (lock).

• Classic use: producer–consumer — producer waits when the buffer is full; consumer waits when empty.

• suspend(), resume() and stop() are deprecated (deadlock and inconsistent state). Use a volatile flag checked in run(), interrupt(), sleep() and join() instead.

javavolatile boolean running = true;
public void run() { while (running) { /* work */ } }