Download CS61B Lecture #12: Abstraction in Java - Parent Constructors, Errors and Exceptions and more Slides Data Structures and Algorithms in PDF only on Docsity!
CS61B Lecture
Today: Various odds and ends in support of abstraction.
Readings: At this point, we have looked at Chapters 1–8 of Head First Java. Today’s lecture is about Chapters 9 and 11. For Friday, please read Chapter 10 and Chapter 3 (“Numbers”) from Assorted Material on Java.
Parent constructors
- In lecture notes #5, talked about how Java allows implementer of a class to control all manipulation of objects of that class.
- In particular, this means that Java gives the constructor of a class the first shot at each new object.
- When one class extends another, there are two constructors—one for the parent type and one for the new (child) type.
- In this case, Java guarantees that one of the parent’s constructors is called first. In effect, there is a call to a parent constructor at the beginning of every one of the child’s constructors.
- You can call the parent’s constructor yourself. By default, Java calls the “default” (parameterless) constructor. class Figure { class Rectangle extends Figure { public Figure (int sides) { public Rectangle () { ... super (4); }... }... } }
Catching Exceptions
- A throw causes each active method call to terminate abruptly, until (and unless) we come to a try block.
- Catch exceptions and do something corrective with try:
try {
Stuff that might throw exception;
} catch ( SomeException e) {
Do something reasonable;
} catch ( SomeOtherException e) {
Do something else reasonable;
Go on with life;
- When SomeException exception occurs in “Stuff... ,” we immedi- ately “do something reasonable” and then “go on with life.”
- Descriptive string (if any) available as e.getMessage() for error messages and the like.
Exceptions: Checked vs. Unchecked
- The object thrown by throw command must be a subtype of Throwable
(in java.lang).
- Java pre-declares several such subtypes, among them
- Error, used for serious, unrecoverable errors;
- Exception, intended for all other exceptions;
- RuntimeException, a subtype of Exception intended mostly for programming errors too common to be worth declaring.
- Pre-declared exceptions are all subtypes of one of these.
- Any subtype of Error or RuntimeException is said to be unchecked.
- All other exception types are checked.
Checked Exceptions
- Intended to indicate exceptional circumstances that are not neces- sarily programmer errors. Examples: - Attempting to open a file that does not exist. - Input or output errors on a file. - Receiving an interrupt.
- Every checked exception that can occur inside a method must ei-
ther be handled by a try statement, or reported in the method’s
declaration.
void myRead () throws IOException, InterruptedException { ... }
means that myRead (or something it calls) might throw IOException
or InterruptedException.
- Language Design: Why did Java make the following illegal?
class Parent { class Child extends Parent { void f () { ... } void f () throws IOException { ... } } }
Good Practice
- Throw exceptions rather than using print statements and System.exit everywhere, -... because response to an problem may depend on the caller, not just method where problem arises.
- Nice to throw an exception when programmer violates preconditions.
- Particularly good idea to throw an exception rather than let bad input corrupt a data structure.
- Good idea to document when methods throw exceptions.
- To convey information about the cause of exceptional condition, put it into the exception rather than into some global variable: class MyBad extends Exception { try { ... public IntList errs; } catch (MyBad e) { MyBad (IntList nums) { errs=nums; } ... e.errs ... } }
