Advanced Programming Languages Effective Java Item 1. Spring 2015 Chungnam National Univ Eun-Sun Cho

Transcription

1 Advanced Programming Languages Effective Java Item 1 Spring 2015 Chungnam National Univ Eun-Sun Cho 1

2 1. Introduction 2. Creating and Destroying Objects Item 1: Consider static factory methods instead of constructors Item 2: Consider a builder when faced with many constructor parameters Item 3: Enforce the singleton property with a private constructor or an enum type Item 4: Enforce noninstantiability with a private constructor Item 5: Avoid creating unnecessary objects Item 6: Eliminate obsolete object references Item 7: Avoid finalizers 2

3 Item 1 Consider static factory methods instead of constructors 3

4 Motivation Examples class Boolean { } private final boolean value; public static final Boolean TRUE = new Boolean(true); public static final Boolean FALSE = new Boolean(false); public Boolean(boolean value){ this.value = value; } Usage Boolean condition = null; if (1 == 1) condition = new Boolean(true); else condition = new Boolean(false); if (condition == new Boolean(true)) // always false.. Why? 4

5 Notes: Checks on Basics Two separate type systems in Java values vs. objects (what is an object?) cf. boxing/unboxing Initializing instance variables in Java Execution sequence of a constructor Final instance variables in Java Reference equality 5

6 Motivation Examples conts class Boolean { } private final boolean value; public static final Boolean TRUE = new Boolean(true); public static final Boolean FALSE = new Boolean(false); public Boolean(boolean value){ this.value = value; } Usage Boolean condition = null; if (1 == 1) condition = Boolean.TRUE; else condition = Boolean.FALSE; if (condition == Boolean.TRUE) // true. <- good if (condition == new Boolean(true)) // true? false? 6

7 Related Codes in APIs public final class Boolean implements java.io.serializable, Comparable<Boolean> { private final boolean value; public static final Boolean TRUE = new Boolean(true); public static final Boolean FALSE = new Boolean(false); public static Boolean valueof(boolean b) { return (b? TRUE : FALSE); } // Unless a new instance is required, the static factory valueof(boolean) is generally a better choice. It is likely to yield significantly better space and time performance. } public Boolean(boolean value){ this.value = value; } 7

8 Static Factory Method (SFM) A static method that returns an instance of a class Similar to constructors public final class Boolean.. { }... public static Boolean valueof(boolean b) { return (b? TRUE : FALSE); } 8

9 1. Have names Advantages of SFMs Over Constructors 2. Not required to create a new object each time 3. Return an object of any subtype of their return type 4. Reduce the verbosity of creating parameterized type instances. 9

10 1. Have names Advantages of SFMs Over Constructors easier to use and read eg. Constructor BigInteger(int, int, Random) emits a probable prime number object. Bad readability SFM BigInteger.probablePrime() solves the problem (from 1.4) multiple methods with a given signature eg. If a programmar wants to make more than one different constructors of class A with int and boolean, what he/she can do is to create A(int, boolean) and A(boolean, int). poor readability and non-determinism Two SFMs with different names would solve this problem 10

11 Advantages of SFMs conts Over Constructors 2. Not required to create a new object each time preserve immutability (Item 15) avoid creating unnecessary duplicate objects control the number of instances Why do we need instance-control? 1) singleton (Item 3) 2) noninstantiable (Item 4) 3) immutable class (Item 15) 4) to keep equality constraint like a.equals(b) a == b for performance improvement eg, Enum types improve performance space and time (by caching) easy to check equality (by reference equality) 11

12 Advantages of SFMs conts Over Constructors 3. Return an object of any subtype of their return type create objects without making their classes public hide implementation classes enable interface-based framework s (Item 18) BTW, an interface cannot have static method. solved by interface Type and a noninstantiable class Types (see Item 4) (See More Examples *Later*) 12

13 Advantages of SFMs conts Over Constructors 4. Reduce the verbosity of creating parameterized type instances. Note: basic generic type specification principle (from 1.5) Set<String> s = new HashSet<String>(); // variable types and object creations need type parameters s.add( dumb ); // method invocations do not need type parameters Thus, SFM does not need type parameters while a constructors does Map<String, List<String>> m = new HashMap<String, List<String>> (); vs. Map<String, List<String>> m = HashMap.newInstance(); 13

14 Advantages of SFMs conts Over Constructors 4. Reduce the verbosity of creating parameterized type instances. The Truth (1) Actually, following SFM is not provided in HashMap public static <K, V> HashMap<K, V> newinstance() {return new HashMap<K, V>();} (2) Now, type inference for generic object creation is supported (from 1.7) Map<String, List<String>> mymap = new HashMap<>(); 14

15 More on Advantage #3 of SFM 3. Returning an Object of Subtype of Their Return Type Examples 1) JCF (Java Collection Frameworks) 2) java.util.enumset 3) Service Provider Framework JDBC (Java Database Connection API) JCE (Java Cryptography Extension) 15

16 Example 1 JCF (Java Collection Frameworks) The Java Collections Framework (JCF) eg. class java.util.collections The result of method ncopies() is an instance of interface List but we do not have to know which class (i.e. implementation) of the List it belongs to. ArrayList? LinkedList? Vector? achieves smaller conceptual weight! 16

17 more on JCF (Java Collections Framework) The core collection interfaces General-purpose Implementations Interfaces Hash table Resizable array Implementations Tree Linked list Hash table + Linked list Set HashSet TreeSet LinkedHashSet List ArrayList LinkedList Queue Deque ArrayDeque LinkedList Map HashMap TreeMap LinkedHashMap 17

18 more on JCF (Java Collections Framework) conts JCF reduced the bulk of API but also conceptual weight conceptual weight (mentioned by Bloch) minimize developers overhead of learning new abstract concepts API Should Be As Small As Possible But No Smaller General principles for APIs API should satisfy its requirements When in doubt leave it out Functionality, classes, methods, parameters, etc. You can always add, but you can never remove Conceptual weight more important than bulk Look for a good power-to-weight ratio by Bloch 18

19 Example 2 java.util.enumset abstract class java.util.enumset (from 1.5) represents a set of enum doesn t have public constructors only with static factories two implementations : RegularEnumSet and JumboEnumSet public static <E extends Enum<E>> EnumSet<E> noneof(class<e> elementtype) { //static factory method //Creates an empty enum set with the specified element type.... if (universe.length <= 64) return new RegularEnumSet<E>(elementType, universe); else return new JumboEnumSet<E>(elementType, universe); } 19

20 Example 3 Service Provider Framework Service provider framework : multiple service providers implements a service implementations are available to clients three essential components service interface : service-specific methods that providers implement. provider registration API : system uses to register implementations service access API : clients use to obtain an instance of the service (optional) service provider interface: providers implement to create instances of their service implementation 20

21 // Service interface public interface Service {... // Service-specific methods go here } // Service provider interface public interface Provider { Service newservice(); } // Noninstantiable class for service registration and access public class Services { private Services() { } // Prevents instantiation (Item 4) // Maps service names to services private static final Map<String, Provider> providers = new ConcurrentHashMap<String, Provider>(); public static final String DEFAULT_PROVIDER_NAME = "<def>"; // Provider registration API public static void registerdefaultprovider(provider p) { registerprovider(default_provider_name, p); } public static void registerprovider(string name, Provider p) { providers.put(name, p);} // Service access API public static Service newinstance() {return newinstance(default_provider_name); } public static Service newinstance(string name) { Provider p = providers.get(name); if (p == null) throw new IllegalArgumentException( "No provider registered with name: " + name); return p.newservice(); } } 21

22 Example 3 Service Provider Framework conts JDBC (Java Database Connectivity API) service provider interface service interface Driver adriver = new oracle.jdbc.driver.oracledriver(); DriverManager.registerDriver( adriver ); // or Class.forName( oracle.jdbc.driver.oracledriver ); provider registration String URL = "jdbc:oracle:thin:@amrood:1521:emp"; API String USER = "username"; String PASS = "password ; Connection conn = DriverManager.getConnection(URL, USER, PASS); service access API Statement stmt = conn.createstatement(); String sql= "SELECT id, first, last, age FROM Employees"; ResultSet rs = stmt.executequery(sql); while(rs.next()){ int id = rs.getint( id"); int age =... 22

23 Example 3 Service Provider Framework conts JCE (Java Cryptography Extension) a framework/implementation for encryption, key generation and key agreement, and etc. eg. user (sender) // MessageDigest md = MessageDigest.getInstance( SHA ); //for default usage MessageDigest md = MessageDigest.getInstance( SHA", "ProviderCNU"); ObjectOutputStream oos = new ObjectOutputStream ( new FileOutputStream( test )); String data = This is a sentence. + It is really long. ; oos.writeobject(data); byte[] buf = data.getbytes(); md.update(buf); oos.writeobject(md.digest()); eg. fill provider interface public class ProviderCNU extends Provider { public ProviderCNU () { super( CNU, 1.0, CNU Security Provider v1.0 ); put names of the classes put( KeyGenerator.CNU, kr.ac.cnu.plas.cnukeygenerator ); put( MessageDigest.CNU, kr.ac.cnu.plas.cnumessagedigest );...}}

24 eg. by user (receiver) ObjectInputStream ois = new ObjectInputStream(new FileInputStream( test )); // read delivered data Object o = ois.readobject(); if (!(o instanceof String)) { System.out.println( Unexpected Data in file ); System.exit(-1); } String data = (String) o; // read delivered md value o = ois.readobject(); if (!(o instanceof byte[])) { System.out.println( Unexpected Data in file ); System.exit(-1); } byte[] origdigest = (byte[]) o; // check md md = MessageDigest.getInstance( SHA, ProviderCNU ); md.update(data.getbytes()); if (MessageDigest.isEqual(md.digest(), origdigest)) System.out.println( Message is valid ); else System.our.println( Message was corrupted. ); }catch (Exception e) { System.out.println(e);} 24

25 Example 3 Service Provider Framework conts Comparisons service interface provider registration API service access API service provider interface JDBC class Connection DriverManager.registerDriver() DriverManager.getConnection() class Driver JCE class MessageDigest put() in constructors of class Provider and more MessageDigest.getInstance() class Provider 25

26 Disadvantages of SFMs Compared to Constructors 1. Classes without public or protected constructors cannot be subclassed cf. favor composition over inheritance (Item 16) 2. Not readily distinguishable from other static methods SFMs represent a deviation from the norm solutions adhering to standard naming conventions (e.g. valueof, getinstance and etc.) 26

27 valueof Returns an instance that has, loosely speaking, the same value as its parameters. Such static factories are effectively type-conversion methods. of A concise alternative to valueof, popularized by EnumSet (Item 32). getinstance Returns an instance that is described by the parameters but cannot be said to have the same value. In the case of a singleton, getinstance takes no parameters and returns the sole instance. newinstance Like getinstance, except that newinstance guarantees that each instance returned is distinct from all others. gettype Like getinstance, but used when the factory method is in a different class. Type indicates the type of object returned by the factory method. newtype Like newinstance, but used when the factory method is in a different class. Type indicates the type of object returned by the factory method. 27

28 Related Design Patterns Flyweight Pattern minimizes memory use by sharing as much data as possible with other similar objects; greatly improve performance if equivalent objects are requested often, especially if they are expensive to create Adapter Pattern translates one interface for a class into a compatible interface. service access API in Service Provider Framework can return a richer service interface than the one required of the provider using this pattern 28