Lecture 8 and 9 - Strings, Arrays, Exceptions

Question 1

What is an immutable object ?

Answer 1

if its contents or state cannot be changed once it has been created.

Question 2

Array characteristics :

Answer 2

size ==> fixed
data type ==> primitives, objects
get size ==> .length
get item ==> [i] 
dimension ==> multi-dimensional

Question 3

ArrayList characteristics :

Answer 3

size ==> re-sizable
data type ==> objects
get size ==> .size()
get item ==> .get(i)
dimension ==> one-dimensional

Question 4

String objects are special. Why?

Answer 4

Creating a string: String s=”ok”;
String objects are immutable.
Contactinating strings: String s=”Ms “+”Elly”;

Question 5

What is an exception ?

Answer 5

An exception is an abnormal event that arises during the execution of the program and disrupts the normal flow of the program.

When an error occurs during the running of a program in Java, an Exception object is automatically generated.

Question 6

What is exception handling ?

Answer 6

Exception handling is to handle the runtime errors so that normal flow of the application can be maintained:
try-catch
try-catch-finally

Question 7

Java exception classes are categorised into:

Answer 7

unchecked exceptions => is not checked during compilation.

checked exceptions => is checked during compilation.

Question 8

Unchecked exceptions explenation

Answer 8

any exception that IS A SUBCLASS of RuntimeException (as well as RuntimeException itself).

are not checked by the compiler.

examples:
ArithmeticException
int a = 2/0;
NullPointerException
Rectangle r;
r.getWidth();
NumberFormatException
int i=Integer.parseInt("book");
ArrayIndexOutOfBoundsException
int[] anArray = new int[3];
System.out.println(anArray[3]);

Question 9

When an unchecked exception might occur, two options:

Answer 9

Don’t handle it - Do nothing.

Handle it with a try-catch or try-catch-finally

Question 10

When a checked exception occurs, two options:

Answer 10

Don’t handle it - declare it, i.e. does not handle it inside this method, but propagate it down to the next method in the call stack.

public void methodsX() throws IOException{
____methodY();
}

Note: The keyword for declaring is throws, NOT throw.

catch it, i.e. handle the exception using try-catch or
try-catch-finally.
public void methodX(){
\_\_\_\_try {
\_\_\_\_\_\_\_\_methodY();
\_\_\_\_} catch (IOException ex) { ...}
}

Note: The exception categorization (unchecked or checked) affect compile-time behavior only; they are handled identically at runtime.

Question 11

What is exception propagation?

Answer 11

When a method throws an exception, the JVM searches backward through the call stack for a matching exception handler.

If exists an handler, the rest of code is executed.
If no handler, print out the stack trace and exception type,and terminates the program.