Basic Java Syntax

Basic Java Syntax CSE301 University of Sunderland Harry R Erwin, PhD

Key Resources to Help You Learn Java • You need to own and read Flanagan, Java in a Nutshell, 5th edition. My lectures are not enough to teach you Java. • You need access to Flanagan’s other Nutshell books. • You need an up-to-date Java 1.5 (or newer) compiler and run-time environment. • You need access to eclipse 3.1.1 • You need to explore the Sun tutorials http://java.sun.com/docs/books/tutorial/index.html

Unicode • Java is written in the Unicode character set. • Unicode characters are stored in 16 bits (some Far Eastern languages require 21) and can represent almost all written languages. – A – å – π

• Java programs can be written in ASCII or Latin-1 characters, which most text editors support. • Unicode can be used anywhere in a Java program.

Comments • Single-line comments: //………….. • Multi-line comments: /*…..*/ • Doc comments: – – – –

Begin with /**…. Provide embedded documentation End with */ Worth investigating further, because they allow you to write self-documenting codefiles. This is an easy way to meet some of the project requirements. – Extended in Java 5 (discussed in two weeks)

Scope of Names • The ‘scope’ of a declaration is the region of the program within which the entity can be referred to using a simple name. • Types must be imported or declared to be in scope. • Members have class or interface scope. • Method parameters have method scope. • Local variables have block scope. • When a name is hidden by another name, you must provide the full name. Sometimes (method parameters, for example) there is no full name you can use.

Identifiers • Begin with a letter, underscore (_), or currency symbol (€$¥£). • Contain any number of letters, numbers, underscores, and currency symbols. • Avoid the currency symbols as compilers use them. • Remember Unicode. The following are legal identifiers: – π – Ö

Primitive Data Types – – – – – – – –

boolean char short byte int long float double

• Have machine-independent formats and default values. Know them! I usually ask a question on them in the TCT or exam.

String • • • •

Contains Unicode text Constant—unlike C and C++ strings. A class, not a primitive type Literals consist of anything between a pair of double quotes. To include a double quote in a string, use \” • Supports operator overloading (+, +=). This is the only place in Java where operator overloading takes place.

Type Conversions • Integer to floating point are automatic. • char to/from integer and floating point • Widening conversions (e.g., short to long) are safe. • Narrowing conversions are dangerous. You need to understand why. I usually ask a question on this in the TCT or exam.

Classes and Arrays • Reference types. • Created using the new operator – new type(args); // an object – new type[dim]; // 1-D array – new type[dim1][dim2] etc; //larger arrays

• A class or array object is null (does not exist) until it is initialized. • Interfaces are reference types, too. • Discussed in the next lecture.

Operators • Generally as in C or C++. See the earlier lecture. • Typecasting is handled by (type)var; , not type(var);!

Statements • Generally as in C/C++. See the earlier lecture. • Additional: – – – –

synchronized (threads) throw (an exception) try/catch/finally (to handle exceptions) All the new Java 5 stuff (discussed in two weeks).

Methods • Correspond to functions in C or C++, but are always associated with a class. • Defined by their signature: – – – – –

Name Arguments Return type (differs from C++, may be void) Checked exceptions thrown Method modifiers: public, static, abstract, final, native, private, protected, synchronized.

Method Modifiers • public—can be called from anywhere • static—defined at the class, not instance level • abstract—must be defined by a subclass to be called • final—no changes allowed • native—calls native code • private—hidden from other classes • protected—visible in same package and subclasses • synchronized—supports multithreading

Summary • Java is not C or C++ • Everything is either a primitive type or a reference type. • Primitive types have default values, while reference types do not exist until they are initialized. • Java smells like C and C++, but a language that smells may not be to your liking… 8)