08 Exceptions

Exceptions¶

warnings, not exactly errors

Exception¶

For accessing out of bounds array data, JVM throws ArrayIndexOutOfBoundsException

1. JVM will show a warning
2. it will just skip the exception
3. then proceed with the rest of the program

Exception-Handling¶

use try-catch

• logic in try
• exception will be caught by catch
• finally block contains all statements that must be executed when exception does or does not occurs

IDK

1. neither can exist independently, but not finally is not compulsory
2. nested try is possible, but nested catch is not
3. nothing can come up after finally - unreachable catch block error

try {
  // code to test

  try {
    // something
  }
  catch(Exception E)
  {
    //something
  }
}
catch(ArrayIndexOutOfBoundsException e) {
  //
}
catch(Exception2 e2) {
  //
}
catch(Exception e) { // all exceptions (checked/unchecked)
  //
}
finally {
    //
}

System.out.println("Program done"); // doesn't get executed

here, statement1 runs, but statement2 doesn’t. this is because, the flow of control goes to the catch after the throw

Similarly, the last statement doesn’t get executed because of finally block

throw and throws¶

you can explicitly throw any kind of exception

can come with/without try-catch

IDK¶

error

• Compile time - Syntax errors
• Runtime error - wrong constructor for initialization

exceptions

• Runtime exception
  • unexpected values - divide by 0
  • array index out of bound

Custom Exceptions¶

class CustomException extends IllegalArgumentException
{
  String message = "Blah";
  CustomException(String s)
  {
    super(s); // or super(message);
  }
  @Override
  public String toString() 
  {
    return message;
  }
}

// somewhere else
try {
  throw new CustomExcepiton("specific message"); // prints specific message
} catch(CustomException e) {
  System.out.println(e); // prints Blah
}

Common Exceptions¶

1. ArithmeticException
2. ArrayIndexOutOfBoundException
3. IOException
4. NullPointerException
5. StringIndexOutOfBoundsException
6. FileNotFoundException
7. NumberFormatException

Exception Methods¶

e.func();

public String getMessage();
// inside System.out.println()
// details of why the exception happened
// eg: / by zero

public String toString(); // name + getMessage()
// eg: java.lang.ArithmeticException / by zero

public void printStackTrace();
// outside System.out.println()
// toString() + location of exception
// eg: java.lang.ArithmeticException / by zero at Test.main(Testjava:9)

public Throwable getCause(); // toString()
// eg: java.lang.ArithmeticException / by zero

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