Exceptions Intro

When a method encounters a condition in which it can't continue, it will throw an exception.

• By now, you've almost certainly seen this happen many times.

String str = null;
System.out.println(str.charAt(0));
// Exception in thread "main" java.lang.NullPointerException

str = "abcdef";
System.out.println(str.charAt(6));
// Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 6

float[] array = new float[5];
array[6] = 1.23F;
// Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 6

int x = 1, y = 0;
System.out.println( x / y );
// Exception in thread "main" java.lang.ArithmeticException: / by zero

Scanner scanner = new Scanner(System.in);
System.out.print("Please enter a number: ");
double d = scanner.nextDouble();
// User enters "Giraffe".
// Exception in thread "main" java.util.InputMismatchException

Number iObj = new Integer(42);
Float fObj = (Float)iObj;
// Exception in thread "main" java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.Float

String numStr = "Lasagna";
int iNum = Integer.parseInt(numStr);
// Exception in thread "main" java.lang.NumberFormatException: For input string: "Lasagna"

• Any method or constructor can throw an exception.

An unhandled exception causes execution to immediately stop.

We can handle or catch exceptions using a try/catch block.

• The try block surrounds the method call that might throw an exception.

String numStr = "Lasagna";
try {
  int iNum = Integer.parseInt(numStr); // parseInt() may throw NumberFormatException
}
catch (NumberFormatException e) {
  System.err.println(numStr + " is not a valid number");
}

When a method throws an exception in the try block, execution of the try block stops and control jumps to the catch for that exception.

Scanner scanner = new Scanner(System.in);
double d;
boolean gotValidInput = false;
while ( ! gotValidInput ) {
  System.out.print("Please enter a number: ");
  try {
    d = scanner.nextDouble(); // nextDouble() may throw InputMismatchException
    gotValidInput = true;     // Only executes if no exception from nextDouble().
  }
  catch (InputMismatchException e) {
    System.out.println("Invalid input.");
    scanner.nextLine();       // Clear input buffer.
  }
}
scanner.close();

Like any block, a try or a catch creates a scope.

• Variables that need to be accessed after the try or catch must be declared outside of it.

try {
  int i=0;
}
catch (Exception e) {
  System.out.println(i); // WILL NOT COMPILE: i cannot be resolved to a variable
}

Drill

Exceptions/com.example.exceptions.drills.TryCatchDrill

• Run TryCatchDrill and enter some numbers.
• Try entering text, or a floating-point number. What happens?
• Add a try/catch block at the appropriate location in the code to print an error message and allow the program to continue accepting input and printing results.

(Solution: TryCatchDrill.java)

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