Java Exceptions, equals/hashCode, enums PDF

Java Exceptions; equals/ hashCode; enums “Orange juice” by Gloria García https://flic.kr/p/4Vz65X Exceptions When an error occurs in program execution, an Exception is thrown (Exceptions are also Java objects like any other; parent class is java.lang.Exc...

Java Exceptions; equals/ hashCode; enums “Orange juice” by Gloria García https://flic.kr/p/4Vz65X Exceptions When an error occurs in program execution, an Exception is thrown (Exceptions are also Java objects like any other; parent class is java.lang.Exception) Unless the exception is caught, the entire program will crash Checked and unchecked exceptions UNCHECKED EXCEPTIONS CHECKED EXCEPTIONS Do not need to be explicitly handled Must be explicitly handled Program will still compile and run without any Program will not compile unless you special handling deal with them somehow Generally indicate programming/logic Generally indicate conditions that a bugs that an application cannot well-written application should reasonably recover from anticipate and recover from Example: Example: ArrayIndexOutOfBoundsExcept FileNotFoundException ion Handling exceptions #1: Catching Wrap a try {} block around any try { code that might throw an Exception // code that might Must be followed by one (or more) // throw Exception catch {} blocks } catch (Exception ex) { First one whose parameter matches the thrown exception is executed // deal with it Optional finally {} block } finally { Executed after entire rest of the try block // clean up } Handling exceptions #2: Passing on If you do something that might public void doSomething() throw an exception, you can add throws IOException that exception to the throws { clause of the current method // code that might Then anyone who calls your // throw IOException method will need to handle the exception (by catching or passing } on) Throwing an Exception Use the throw keyword: throw new Exception (“Invalid input”); You can throw an Exception at any point in your code String parameter indicates the message (available through ex.getMessage()) If you throw a checked Exception, you also need to add it to the header of your method with the throws keyword public String processInput (String input) throws Exception { … } Advantages of using Exceptions 1. Separating out error-handling code Instead of a series of if/then/else statements Just “assume” that things will work and deal with errors elsewhere 2. Propagating errors up the call stack i.e., sending errors along until they reach a method that is prepared to handle them 3. Grouping error types Exception is a class, and can be subclassed => different types of Exceptions can be conceptually grouped together (I/O exceptions, for example) Methods of java.lang.Object protected Object clone() boolean equals (Object obj) protected void finalize() public Class getClass() public int hashCode() public void notify() / notifyAll() public String toString() public void wait() / wait(long timeout) / wait(long timeout, int nanos) java.lang.Object.equals() documentation Indicates whether some other object is “equal to” this one The equals method implements an equivalence relation on non-null object references: It is reflexive: for any non-null reference value x, x.equals(x) should return true. It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true. It is transitive: for any non-null reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true. It is consistent: for any non-null reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified. For any non-null reference value x, x.equals(null) should return false. Default implementation of equals() “The most discriminating possible equivalence relation on objects” Returns true if and only if x and y refer to the same object (i.e., x == y is true) Gives the correct result for primitive types (int, double, char, etc.) Does not check if objects are equivalent – i.e., if their contents are the same ArrayList l1 = new ArrayList(); l1.add (1); ArrayList l2 = new ArrayList(); l2.add (1); boolean result = l1.equals(l2); // Default would return false java.lang.Object.hashCode() documentation Returns a hash code value for the object. This method is supported for the benefit of hash tables such as those provided by HashMap. The general contract of hashCode is: Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application. If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result. It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hash tables. Default implementation “As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects. (This is typically implemented by converting the internal address of the object into an integer, but this implementation technique is not required by the Java™ programming language.)” equals() and hashCode() “If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.” So if you override equals(), you must also override hashCode() Enumerated types An enum type is a special data type that allows a variable to be one of a set of predefined constants Common examples: Compass directions (NORTH, SOUTH, EAST, WEST) Days of week, months of year, etc. Declaring an enum in Java public enum Day { SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, } Note: values are constants ==> conventionally written in ALL_CAPS You use the enum keyword instead of class An enum called Day should be in a class Day.java An enum is a special class It has methods Built-in static method values() that returns an array of all values Built-in static method valueOf() that parses a string into an enum constant Appropriate definitions of compareTo(), equals(), hashCode(), toString() Other methods: ordinal() -- returns the position of this constant in the list name() -- returns the name of this constant Any other methods that you define You can define fields as well if necessary