Exceptions in java

Definition

An exception is an event, which occurs during the execution of a program, that disrupts the normal flow of the program's instructions.
ex :-

Exception in thread "main" java.io.IOException: Cannot run program "ls": java.io.IOException: error=12, Cannot allocate memory

        at java.lang.ProcessBuilder.start(ProcessBuilder.java:474)
        at java.lang.Runtime.exec(Runtime.java:610)
        at java.lang.Runtime.exec(Runtime.java:448)
        at java.lang.Runtime.exec(Runtime.java:345)
        at prova.main(prova.java:6)
Caused by: java.io.IOException: java.io.IOException: error=12, Cannot allocate memory
        at java.lang.UNIXProcess.<init>(UNIXProcess.java:164)
        at java.lang.ProcessImpl.start(ProcessImpl.java:81)
        at java.lang.ProcessBuilder.start(ProcessBuilder.java:467) 

... 4 more

Exception in thread "main" is mention there is an exception after this java.io.IOException: is type of the exception( class) and Cannot run program "ls":  is the root cause of the exception

An exception can occur for many different reasons, including the following:

• A user has entered invalid data.
• A file that needs to be opened cannot be found.
• A network connection has been lost in the middle of communications, or the JVM has run out of memory.

Some of these exceptions are caused by user error, others by programmer error, and others by physical resources that have failed in some manner.

To understand how exception handling works in Java, you need to understand the three categories of exceptions:

• Checked exceptions: A checked exception is an exception that is typically a user error or a problem that cannot be foreseen by the programmer. For example, if a file is to be opened, but the file cannot be found, an exception occurs. These exceptions cannot simply be ignored at the time of compilation.
• Runtime exceptions: A runtime exception is an exception that occurs that probably could have been avoided by the programmer. As opposed to checked exceptions, runtime exceptions are ignored at the time of compliation.
  
  
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