Checked and Unchecked Exceptions

Except for RuntimeException, Error, and their subclasses, all exceptions are called checked exceptions. The compiler ensures that if a method can throw a checked exception, directly or indirectly, then the method must explicitly deal with it. The method must either catch the exception and take the appropriate action, or pass the exception on to its caller .

Exceptions defined by Error and RuntimeException classes and their subclasses are known as unchecked exceptions, meaning that a method is not obliged to deal with these kinds of exceptions. They are either irrecoverable (exemplified by the Error class) and the program should not attempt to deal with them, or they are programming errors (examplified by the RuntimeException class) and should be dealt with as such and not as exceptions.

Multitasking Vs. Multithreading

Multitasking allows several activities to occur concurrently on the computer. A distinction is usually made between:

• Process-based multitasking

• Thread-based multitasking

At the coarse-grain level there is process-based multitasking, which allows processes (i.e., programs) to run concurrently on the computer. A familiar example is running the spreadsheet program while also working with the word-processor. At the fine-grain level there is thread-based multitasking, which allows parts of the same program to run concurrently on the computer. A familiar example is a word-processor that is printing and formatting text at the same time. This is only feasible if the two tasks are performed by two independent paths of execution at runtime. The two tasks would correspond to executing parts of the program concurrently. The sequence of code executed for each task defines a separate path of execution, and is called a thread (of execution).

In a single-threaded environment only one task at a time can be performed. CPU cycles are wasted, for example, when waiting for user input. Multitasking allows idle CPU time to be put to good use.

Some advantages of thread-based multitasking as compared to process-based multitasking are

• threads share the same address space

• context switching between threads is usually less expensive than between processes

• cost of communication between threads is relatively low

Java supports thread-based multitasking and provides high-level facilities for multithreaded programming. Thread safety is the term used to describe the design of classes that ensure that the state of their objects is always consistent, even when the objects are used concurrently by multiple threads.

Threads

A thread is an independent sequential path of execution within a program. Many threads can run concurrently within a program. At runtime, threads in a program exist in a common memory space and can, therefore, share both data and code, that is, they are lightweight compared to processes. They also share the process running the program.

Every thread in Java is created and controlled by a unique object of the java.lang.Thread class. Often the thread and its associated Thread object are thought of as being synonymous.

The Main Thread

The runtime environment distinguishes between user threads and daemon threads. As long as a user thread is alive, the JVM does not terminate. A daemon thread is at the mercy of the runtime system: it is stopped if there are no more user threads running, thus terminating the program. Daemon threads exist only to serve user threads

Thread Creation

A thread in Java is represented by an object of the Thread class. Implementing threads is achieved in one of two ways:

• implementing the java.lang.Runnable interface

• extending the java.lang.Thread class