An Overview of the Java Programming Language

Transcription

1 An Overview of the Java Programming Language Reading list Required: LY99 Chap 2 Java PL Concepts (handout; also online) Bill Pugh s Java crash course slides (on the web) References: Zhong Shao Department of Computer Science Yale University September 15, 1999 AG97: The Java Programming Language (2nd Edition) GJS96: The Java Language Specification (online) Java spec report (a web site discussing dirty issues in Java) Java programmer s FAQ (available online) If you are unfamiliar with Java: read AG97 or David Flanagan s Java in a Nutshell and Java Examples in a Nutshell or Pugh s Learning Java web site. 9/16/99 1 9/16/99 2 1

2 Objectives A thorough understanding of important Java features Focus on their specifications & rules rather than how to program with them Goal: create a Java language specification cheat-sheet Comparison with other languages (ML, Haskell, C, C++) what a modern programming language should support? what are Java s strong points? what are Java s weak aspects? Create a list of topics that we should revisit Create a list of research problems on the Java language design and extension What I will cover? Big picture Java basics unicode; types, values, and variables conversions and promotions names and packages OO support classes; interfaces; constructors; methods; fields; initializers the (modifiers x features) matrix how to program with interfaces? Arrays and exceptions Execution basics (if time permits) Java reflection (if time permits) 9/16/99 3 9/16/99 4 2

3 What I will NOT cover? Statements and expressions syntax (Oct 19, 26) Inner classes (probably a future topic) Execution detail (what this whole seminar is about) Concurrency features (Nov 3) Documentation comments Java coding conventions Java class libraries Binary compatibility (Dec 8) The big picture What is in the JDK? The src-to-bytecode compiler (javac) The java virtual machine (java) Tools: jdb, jar, javap, javah, appletviewer, javadoc,... The class libraries (a file named rt.jar) The native libraries (*.so files used by jre?) Not resolved: The *.h files in the include and include.old directories must be related to writing JNI C programs What is the difference between JDK and JRE? (see FAQ) 9/16/99 5 9/16/99 6 3

4 The Java class libraries (v1.2 std) Java (36 packages) applet (applet creation & communication w. the context) awt(12) (the abstract window toolkit + the gui stuff) beans(2) (java components) io (data streams + object serialization + file system) lang(3) (things in the Java language itself) math (bignum mathematics) net (URL, socket, networking applications) rmi (5) (remote method invocation) security(5) (security tools: access control & code signing...) sql (relational database access) text (internationalization & localization) util(3) (useful data structures, random number, jar/zip) javax (17) (accessibility + swing) why x? org.omg (7) (CORBA + CosNaming) The Java class libraries (extra) Also came with the JDK release (in rt.jar) : com.sun CORBA CosNaming image.codec.jpeg java.swing.plaf.motif & java.swing.plaf.windows sun lots of classes for applet, audio, awt, beans, dc, io, java2d, jdbc, misc, net, security, tools are they machine-dependent parts of the Library? Where are their sources? sunw (for backward compatibility with JDK 1.0.2) io (Serializable.class) util (EventListener.class & EventObject.class) 9/16/99 7 9/16/99 8 4

5 What is in java.lang (v1.2)? Java.lang (75 classes + 3 interfaces) Object; Class (2) String; StringBuffer; Math(3) Wrapper classes for primitive types (12) ClassLoader; Compiler; Package; Process; Runtime; System (6) SecurityManager; RuntimePermission (2) Thread-related classes (4) Throwable: 21 errors + 24 exceptions (46) interfaces: Cloneable; Comparable; Runnable (3) Java.lang.reflect (8 classes + 1 interface) AccessibleObject (Constructor + Field + Method) / Member + Array; Modifier; ReflectPermission + InvocationTargetException Java.lang.ref (5 classes) pointer to objects; weak pointers;... What is 100%-pure Java? Checkout the cookbook on the web; roughly, Contain no native methods No use of undocumented parts of a Java implementation No use of tunnel methods (no access to machine code) No use of hardwired platform-specifc constants 9/16/99 9 9/16/

6 The Java programming language The basics unicode, identifiers, literals, types, values, variables initialization of local variables (definite assignment) names, packages conversions and promotions statement and expressions (not covered) OO features Arrays classes and interfaces w. modifiers methods, fields, constructors, and initializers w. modifiers inner classes and interfaces (not covered) field shadowing; method overloading, overriding; implementing Exceptions Classes: String; Object; Class 9/16/99 11 The basics: lexical structure Unicode (16 bits char rather than 8 bits) used for comments, identifiers, character and string literals the rest are in ASCII (but what are the rest?) Identifiers Letter followed by sequence of letters and digits Not keywords and not true and false Literals ints (1, -1); hex ints (0x0f28); unicode hex (\u003f) A leading 0 on an integer is an octal (027) --- potential bug longs (3L, -99l, 0xf L) floats (1.0345F, 1.04E-12f,.0345f, 1.04e-13f, Float.NaN) doubles (5.6E-120D, 123.4d, 0.1, Double.NaN, Math.PI) Floating point literals without trailing f, F, d or D are considered double. 9/16/

7 More Java literals null type literal (null) boolean (true and false) strings ("ABC ) chars 'A', or integer forms e.g. 45, 0x45, '\u003f' Escape sequences inside char and string literals include: '\u003f' unicode hex, (must be exactly 4 digits) '\n' newline; '\b' backspace; '\f' formfeed '\r' carriage return; '\t' tab; '\\' backslash, '\'' single quote (optional inside " ") '\"' double quote (optional inside ' ') '\377' octal (must be exactly 3 digits) \007 bel; \010 backspace; \013 vt vertical tab; \032 sub, eof 9/16/99 13 Types and values primitive types boolean numeric types signed integral types byte (8 bits); short (16 bits); int (32 bits); long (64 bits) char (16 bits unsigned) floating-point types (IEEE 754) float (32 bits) and double (64 bits) what are the floating-point value sets? void (? ) reference types class types interface types array types null (subtype of any reference types) 9/16/

8 Special types in java.lang String used to type the string literals Object root class Thread interaction with certain modifiers Class for reflection restrict the implementation Variables A variable has type (meaning compile-time type) always stores a value assignment-compatible with the type initialized with default values checked at each assignment no variable of type void Seven kinds of variables in Java a class variable (i.e., static field) --- gone if class unloaded an instance variable --- die when the object is finalized array components (un-named) --- die when the array is gc-ed method parameters constructor parameters an exception-handler parameter (always an Throwable object) local variables 9/16/ /16/

9 Variable initialization Class variable, instance variable, array components, if not initialized, are given default values: 0, 0L, 0.0f, 0.0, \u0000, false, null Initialization blocks (member of the class) static --- executed when the class is loaded non-static --- executed during object creation (before the constructor is run) order is significant; mutual recursion is unclear? Local variables (definite assignment) see chapter 16 of the Java Language Specification look through the control flow paths and check if the variable is assigned on every path Variables have types, objects have classes Every variable has a compile-time type --- type Every object has a runtime type ---- class the class used in the new expression the class whose class object was used to invoke the newinstance method arrays have strangely named classes ( [I for array of int) compile-time type limits the runtime class 9/16/ /16/

10 Conversions and promotions Six broad kinds of conversions: identity widening primitive narrowing primitive widening reference (also w. arrays) narrowing reference (also w. arrays) string conversion (tostring) Five conversion contexts: assignment conversion no exceptions (widening + int-const-narrowing) method/constructor invocation conversion (on arguments) widening only casting conversion string conversion (tostring) numeric promotion (arguments of numerical operators) Names and packages Names refer to entities declared in a program: package, type, field and method of a type, parameter, or local variables simple name (identifier) qualified name (name.identifier) packages subpackages class types fields, methods, nested classes and interfaces (constructors?) interface types (same) array types (those in class Object + the length field) fully qualified name (for all packages and types) array type s name = the component type name + [] 9/16/ /16/

11 The OO features Class (ext. super-class imp. super-interfaces) field method constructor initialization block inner class inner interface Interface (ext. super-interfaces) field method? no inner interface? 9/16/99 21 Java modifiers (10 + 2) public > protected > package (default) > private final vs. non-final (default) - for mutability/extensibility abstract vs. non-abstract (default ) static vs. non-static (default) native vs. non-native(default) - for foreign-function call synchronized vs. non-synchronized (default) volatile vs. non-volatile(default) transient vs. non-transient(default) - for persistence strictfp and widefp 9/16/

12 Features vs. modifiers Class (public, abstract, final) field (pub-prot-priv, static, final, volatile, transient) method (pub-prot -priv, static, final, abs, sync, native) constructor (pub-prot-priv) initializer (static) inner class (pub-prot-priv, static, final, abs) inner interface (public) Interface (public) field (pub-prot-priv?, static, final) implicitly static, final, public; so no package, volatile, transient method (pub-prot-priv?, abs) implicitly abstract, non-static information about final, native, sync., strictfp is lost Subtlety with modifiers What are the conflicting pairs of modifiers? abstract class cannot be final final field cannot be volatile native method cannot be strictfp abstract method cannot be priv., static, final, native, strictfp Methods in a final class are implicitly final. Private methods are implicitly final. Static methods can not directly call and refer to nonstatic methods and fields of the same class 9/16/ /16/

13 A class extends a super-class Overloading methods (w. different signatures) method signature: method name + types of formal params Overriding/hiding methods (w. same signatures) return type must be same the throw clauses must be non-conflicting method n overrides/hides method m if for each exn type in the throws clause of n, the same exn class or one of its superclasses must occur in the throws clause of m. overriding private methods don t have to satisfy these rules must have as much as access as the original method other modifiers: static/final/native/abstract/synchronized? Hiding fields Can a class/interface defines two fields with same name? Fields are accessed based on the compile-time type of the underlying object Hiding fields (w. different or same types) 9/16/ /16/

14 Hiding fields: example class SuperShow { public String str = SuperStr ; public void show() {System.out.println( Super.show: + str); } } class ExtendShow extends SuperShow { public String str = ExtendStr ; Implementing and extending interfaces Same requirements as before Can inherit multiple methods with same signatures all abstract methods (compatible pairwise) all abstract methods except one (that one implements all the other abstract methods) } public void show() {System.out.println( Extend.show: + str); } public static void main(string[] args) { ExtendShow ext = new ExtendShow(); SuperShow sup = ext; sup.show(); ext.show(); System.out.println( sup.str= + sup.str); System.out.println( ext.str= + ext.str); } Inheriting multiple fields compile-time error if any ambiguity arises. 9/16/ /16/

15 Implementing constructors Deafult constructor for Object, the empty body for others, calls super(); same access as the class Abstract class can t have constructor Except for Object, each constructor body starts with an explicit constructor invocation or just super() Method invocation Invoking static methods through class name through super through object or through this (only its static type matters) Invoking non-static methods through super through this through object (only its dynamic class matters) 9/16/ /16/

16 Arrays Language built-in class generator, not in java.lang array classes are hardwired as a sub-class of Object inherit everything in Object override the clone method implements Cloneable and java.io.serializable add an additional final length field array of char is not string; string is immutable 9/16/99 31 Exceptions Throwable Error (unchecked) LinkageError VirtualMachineError other unchecked exn classes Exception RuntimeException (unchecked) ArithmeticException ArrayStoreException ClassCastException NullPointerException other unchecked exn classes ClassNotFoundException IllegalAccessException other checked exn classes Other checked exception classes 9/16/

17 More on Exceptions Try-catch-finally Try-catch Executions VM starting up class and interface type loading, linking, initialization creation finalization unloading exit VM 9/16/ /16/

18 Things I want to find out Method invocation and field access on the null value? Method overriding in the presence of the throw clause? How the throw clause is checked? What are marker interfaces? How the Array class differs from other classes The Java exception hierarchy; is it much better than exceptions in ML or Modula-3? How does the initializer work? Potential topics for discussion What features in Java we really like and what we really hate? Comparison with other programming languages What a modern prog. language should support? What are interesting language extensions over Java? 9/16/ /16/