Friday, February 25, 2011

Quick Review: Chapters 38 to 51

This chapter is going to be the Quick recap/review of the concepts we learnt in the previous chapters on collections.

So, lets get started!!!

Overriding hashCode() and equals()

* equals(), hashCode(), and toString() are public.

* Override toString() so that System.out.println() or other methods can see something useful, like your object’s state.

* Use == to determine if two reference variables refer to the same object.

* Use equals() to determine if two objects are meaningfully equivalent.

* If you don’t override equals(), your objects won’t be useful hashing keys.

* If you don’t override equals(), different objects can’t be considered equal.

* Strings and wrappers override equals() and make good hashing keys.

* When overriding equals(), use the instanceof operator to be sure you’re evaluating an appropriate class.

* When overriding equals(), compare the objects’ significant attributes.

* Highlights of the equals() contract:

      * Reflexive: x.equals(x) is true.

      * Symmetric: If x.equals(y) is true, then y.equals(x) must be true.

      * Transitive: If x.equals(y) is true, and y.equals(z) is true, then z.equals(x) is true.

      * Consistent: Multiple calls to x.equals(y) will return the same result.

      * Null: If x is not null, then x.equals(null) is false.

* If x.equals(y) is true, then x.hashCode() == y.hashCode() is true.

* If you override equals(), override hashCode().

* HashMap, HashSet, Hashtable, LinkedHashMap, & LinkedHashSet use hashing.

* An appropriate hashCode() override sticks to the hashCode() contract.

* An efficient hashCode() override distributes keys evenly across its buckets.

* An overridden equals() must be at least as precise as its hashCode() mate.

* To reiterate: if two objects are equal, their hashcodes must be equal.

* It’s legal for a hashCode() method to return the same value for all instances (although in practice it’s very inefficient).

* Highlights of the hashCode() contract:

      * Consistent: multiple calls to x.hashCode() return the same integer.

      * If x.equals(y) is true, x.hashCode() == y.hashCode() is true.

      * If x.equals(y) is false, then x.hashCode() == y.hashCode() can be either true or false, but false will tend to create better efficiency.

* transient variables aren’t appropriate for equals() and hashCode().

Collections

* Common collection activities include adding objects, removing objects, verifying object inclusion, retrieving objects, and iterating.

* Three meanings for “collection”:

      * collection Represents the data structure in which objects are stored

      * Collection java.util interface from which Set and List extend

      * Collections A class that holds static collection utility methods

      Four basic flavors of collections include Lists, Sets, Maps, Queues:

      * Lists of things Ordered, duplicates allowed, with an index.

      * Sets of things May or may not be ordered and/or sorted; duplicates not allowed.

      * Maps of things with keys May or may not be ordered and/or sorted; duplicate keys are not allowed.

      * Queues of things to process Ordered by FIFO or by priority.

* Four basic sub-flavors of collections Sorted, Unsorted, Ordered, Unordered.

      * Ordered Iterating through a collection in a specific, non-random order.

      * Sorted Iterating through a collection in a sorted order.

* Sorting can be alphabetic, numeric, or programmer-defined.

Key Attributes of Common Collection Classes

* ArrayList: Fast iteration and fast random access.

* Vector: It’s like a slower ArrayList, but it has synchronized methods.

* LinkedList: Good for adding elements to the ends, i.e., stacks and queues.

* HashSet: Fast access, assures no duplicates, provides no ordering.

* LinkedHashSet: No duplicates; iterates by insertion order.

* TreeSet: No duplicates; iterates in sorted order.

* HashMap: Fastest updates (key/values); allows one null key, many null values.

* Hashtable: Like a slower HashMap (as with Vector, due to its synchronized methods). No null values or null keys allowed.

* LinkedHashMap: Faster iterations; iterates by insertion order or last accessed; allows one null key, many null values.

* TreeMap: A sorted map.

* PriorityQueue: A to-do list ordered by the elements’ priority.

Using Collection Classes

* Collections hold only Objects, but primitives can be autoboxed.

* Iterate with the enhanced for, or with an Iterator via hasNext() & next().

* hasNext() determines if more elements exist; the Iterator does NOT move.

* next() returns the next element AND moves the Iterator forward.

* To work correctly, a Map’s keys must override equals() and hashCode().

* Queues use offer() to add an element, poll() to remove the head of the queue, and peek() to look at the head of a queue.

* As of Java 6 TreeSets and TreeMaps have new navigation methods like floor() and higher().

* You can create/extend “backed” sub-copies of TreeSets and TreeMaps.

Sorting and Searching Arrays and Lists

* Sorting can be in natural order, or via a Comparable or many Comparators.

* Implement Comparable using compareTo(); provides only one sort order.

* Create many Comparators to sort a class many ways; implement compare().

* To be sorted and searched, a List’s elements must be comparable.

* To be searched, an array or List must first be sorted.

Utility Classes: Collections and Arrays

* Both of these java.util classes provide

* A sort() method. Sort using a Comparator or sort using natural order.

* A binarySearch() method. Search a pre-sorted array or List.

* Arrays.asList() creates a List from an array and links them together.

* Collections.reverse() reverses the order of elements in a List.

* Collections.reverseOrder() returns a Comparator that sorts in reverse.

* Lists and Sets have a toArray() method to create arrays.

Generics

* Generics let you enforce compile-time type safety on Collections (or other classes and methods declared using generic type parameters).

* An ArrayList< Car > can accept references of type Ferrari, Bmw, or any other subtype of Car

* When using generic collections, a cast is not needed to get (declared type) elements out of the collection. With non-generic collections, a cast is required:
List< String > gList = new ArrayList< String >();
List list = new ArrayList();
// more code
String s = gList.get(0); // no cast needed
String s = (String)list.get(0); // cast required


* You can pass a generic collection into a method that takes a non-generic collection, but the results may be disastrous. The compiler can’t stop the method from inserting the wrong type into the previously type safe collection.

* If the compiler can recognize that non-type-safe code is potentially endangering something you originally declared as type-safe, you will get a compiler warning. For instance, if you pass a List< String > into a method declared as
void foo(List aList) { aList.add(anInteger); }

* You’ll get a warning because add() is potentially “unsafe”.

* “Compiles without error” is not the same as “compiles without warnings.” A compilation warning is not considered a compilation error or failure.

* Generic type information does not exist at runtime—it is for compile-time safety only. Mixing generics with legacy code can create compiled code that may throw an exception at runtime.

* Polymorphic assignments applies only to the base type, not the generic type parameter. You can say
List< Car > aList = new ArrayList< Car >(); // yes

You can’t say
List< Car > aList = new ArrayList< Ferrari >(); // no


* The polymorphic assignment rule applies everywhere an assignment can be made. The following are NOT allowed:
void foo(List< Car > aList) { } // cannot take a List
List< Car > bar() { } // cannot return a List


* Wildcard syntax allows a generic method, accept subtypes (or supertypes) of the declared type of the method argument:
void addD(List< Car > d) {} // can take only < Car >
void addD(List< ? extends Car >) {} // take a < Car > or < Ferrari >


* The wildcard keyword extends is used to mean either “extends” or “implements.” So in , Car can be a class or an interface.

* When using a wildcard, List< ? extends Car >, the collection can be accessed but not modified.

* When using a wildcard, List< ? >, any generic type can be assigned to the reference, but for access only, no modifications.

* List< Object > refers only to a List< Object >, while List< ? > or List< ? extends Object > can hold any type of object, but for access only.

* Declaration conventions for generics use T for type and E for element:
public interface List< E > // API declaration for List
boolean add(E o) // List.add() declaration


* The generics type identifier can be used in class, method, and variable declarations:
class Foo { } // a class
T anInstance; // an instance variable
Foo(T aRef) {} // a constructor argument
void bar(T aRef) {} // a method argument
T baz() {} // a return type

The compiler will substitute the actual type.

* You can use more than one parameterized type in a declaration:
public class UseTwo { }


* You can declare a generic method using a type not defined in the class:
public void makeList(T t) { }

is NOT using T as the return type. This method has a void return type, but to use T within the method’s argument you must declare the , which happens before the return type.


Previous Chapter: Chapter 51 - Generics

Next Chapter: Self Test - Chapters 38 to 51

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