Aside: Running Supplied *.java Programs• Just double clicking on a *.java file may not be too

useful!1. In Eclipse, create a project for this program or

decide to add the new file to an existing project.2. In Windows explorer, copy & paste the file to the

src folder within the project chosen above.3. In Eclipse, choose the project in the Package

Explorer at the left and press <F5> to refresh the contents listed – the new file should show up in the listing.

4. Or, just paste the file into the project in Eclipse.Winter 2016 CMPE212 - Prof. McLeod 1

Today…• Continue Introduction to Java class structure.

• Compare C and Java.

• Get into Building Expressions.

Winter 2016 CMPE212 - Prof. McLeod 2

Aside - varargs• You can use the ellipsis operator (three dots - …) to

create a parameter that can take any number of arguments of that type. For example, suppose you have an object of type Bling:

public void seeBling (Bling... blingers) {// code }

• Within seeBling, you get at the individual arguments of type Bling by pretending that blingers is an array (use [ ] along with an index value.)

Winter 2016 CMPE212 - Prof. McLeod 3

Aside – varargs, Cont.• We’ll need to look at an example of this later.

• It is not used too often, but• See the declaration of the printf method for

example:

public PrintStream printf(String format, Object... args)

Winter 2016 CMPE212 - Prof. McLeod 4

Winter 2016 CMPE212 - Prof. McLeod 5

Methods - the return Statement• Unless the return type is void, the method must

contain at least one return statement. A void method can also use a return statement without anything after it, as a means of terminating the method.

• A method always stops (terminates) when a return statement is encountered.

• Syntax:

return [literal|expression];

Winter 2016 CMPE212 - Prof. McLeod 6

The return Statement - Cont.• The type of “literal|expression” must match

the return type specified in the method declaration statement.

Winter 2016 CMPE212 - Prof. McLeod 7

Method Examplespublic void printHello() {

System.out.println(“Hello”);} // end printHello

public void printHelloName(String yourName) {System.out.println(“Hello “ + yourName);

} // end printHelloName

public void printAvg(int a, int b) {System.out.println((a + b) / 2.0);

} // end printAvg

Winter 2016 CMPE212 - Prof. McLeod 8

Method Examples - Cont.public double average(double a, double b) {

return (a + b) / 2;} // end average

public int lowest(int a, int b) {if (a <= b)return a;

elsereturn b;

} // end lowest

Attribute Examplespublic static double aVar;

public static int aNum = 100;

private static String hello = “Hello”;

Winter 2016 CMPE212 - Prof. McLeod 9

Example: A Simple Class

public class Simple {

public static int aNum = 100;

public static int sumNums(int num1, int num2) {return num1 + num2;

}public static void main(String[] args) {

int anotherNum = 200;System.out.println(sumNums(aNum,

anotherNum));}

}

Winter 2016 CMPE212 - Prof. McLeod 10

Another Simple Class// An example of the use of varargs (the ellipse...)// Also a for each loop!public class AnotherSimple {

public static int sumNums(int... nums) {int sum = 0;for (int num : nums)

sum += num;return sum;

}

public static void main(String[] args) {System.out.println(sumNums(2, 5, 7, 10, 3));

}

} // end AnotherSimple classWinter 2016 CMPE212 - Prof. McLeod 11

CMPE212 - Prof. McLeod 12Winter 2016

Java versus C• What is the main difference between C and Java?

• Code must be contained within an Object. Creating a program is now all about defining Objects (or classes).

• In Java, you cannot have any code outside a class definition.

• The only code you can have outside a method definition are class attribute declarations.

CMPE212 - Prof. McLeod 13Winter 2016

Java versus C, Cont.(Java code examples in white boxes)

• What Java has (and C does not!):

• A class definition line.• All code (methods and attributes) must be contained

within a class (within the { … } brackets).• The class is saved in a text source code file called

“ResistorCodes.java”.• A class is an Object, and an Object is defined in a

class.

CMPE212 - Prof. McLeod 14Winter 2016

Java versus C, Cont.• C has #include, Java has import:

• C has function prototypes, Java does not.• C has global variables, Java does not.• C has functions, Java has methods.

• C has a main() function, Java has a main method:

CMPE212 - Prof. McLeod 15

Java versus C, Cont.• Some stuff is the same:

• { } used to delineate methods.• ; at end of line.• // for in-line comments.• String literal enclosed in “ ”.• ( ) contain arguments and parameter lists and

can be used to establish precedence in expressions.

• Space used to delineate elements of expressions.

Winter 2016

CMPE212 - Prof. McLeod 16Winter 2016

What Makes an Expression?• What are all the components available to a

programmer that can be used to put a line of code together?

– Variables– Literal Values– Keywords– Operators– Method Invocations– Punctuation

CMPE212 - Prof. McLeod 17Winter 2016

Variable Declcaration• Variables

– Both C and Java are declarative languages.– Scope rules are the same.– Syntax of simple variable declaration is the same.

– A variable is “statically typed” in both languages. In other words, once a variable is typed, in cannot be changed to another type.

CMPE212 - Prof. McLeod 18Winter 2016

Primitive Types in Java• Both C and Java have:

– char– short– int– long– float– double

• Only Java has:– boolean– byte

19Winter 2016 CMPE212 - Prof. McLeod

Integer Primitive Types• byte, short, int, long

• For byte, from -128 to 127, inclusive (1 byte).• For short, from -32768 to 32767, inclusive (2

bytes).• For int, from -2147483648 to 2147483647,

inclusive (4 bytes). • For long, from -9223372036854775808 to

9223372036854775807, inclusive (8 bytes).

• A “byte” is 8 bits, where a “bit” is either 1 or 0.

20Winter 2016 CMPE212 - Prof. McLeod

Real Primitive Types• Also called “Floating Point” Types:

• float, double

• For float, (4 bytes) roughly ±1.4 x 10-38 to ±3.4 x 1038 to 7 significant digits.

• For double, (8 bytes) roughly ±4.9 x 10-308 to ±1.7 x 10308 to 15 significant digits.

21Winter 2016 CMPE212 - Prof. McLeod

Character Primitive Type• char

• from '\u0000' to '\uffff' inclusive, that is, from 0 to 65535 (base 10) or 0 to ffff (base 16, or “hexadecimal”). A variable of the “char” type represents a Unicode character. Can also be represented as ‘a’ or ‘8’, etc.

22Winter 2016 CMPE212 - Prof. McLeod

Boolean Primitive Type• boolean is either true or false.

23Winter 2016 CMPE212 - Prof. McLeod

String Objects• String’s are not primitive data types, but are Objects.• A String is declared in the same way as a primitive type

using the keyword: String.• You can also instantiate a String as you would a normal

object. The statement:

String message = “Hello World!”;

aliases the object message to the String literal object “Hello World!”, which is the same as saying:

String message = new String(“Hello World!”);

24Winter 2016 CMPE212 - Prof. McLeod

Legal Variable Names• Java names may contain any number of letters,

numbers and underscore (“_”) characters, but they must begin with a letter.

• Standard Java Naming Conventions:– Names beginning with lowercase letters are variables

or methods.– Names beginning with uppercase letters are class

names.– Successive words within a name are capitalized

(“CamelCase”).– Names in all capital letters are constants.

25Winter 2016 CMPE212 - Prof. McLeod

Literal Values• Integers, for example:

12 -142 0 333444891

• If you write these kinds of numbers into your program, Java will assume them to be of type “int”, and store them accordingly.

• If you want them to be of type “long”, then you must append a “L” to the number:

43L 9999983475L -22233487L

CMPE212 - Prof. McLeod 26

Binary, Octal and Hex Literals• Use the prefix 0b (or 0B) in front of the numbers

to get a binary literal.• Octal – just use 0• Hex use 0x• You can use the underscore _ in-between digits,

to aid in visualizing the number.• System.out.println(), by default displays

the numbers in base 10.• Use printf to show the numbers in another

codex or base. (See Part 2 of Exercise 1.)

Winter 2016

CMPE212 - Prof. McLeod 27

Binary, Octal and Hex Literals, Cont.

• Summary examples of other literals:– binary: 0b111010111– binary with underscores: 0b111_010_111– octal: 07321023– hex: 0xab10fc

Winter 2016

28Winter 2016 CMPE212 - Prof. McLeod

Literal Values - Cont.• Real or “Floating Point” numbers, for example:

4.5 -1.0 3.457E-10 -3.4E45

• These literals will be assumed to be of the “double” type.

• If you want them to be stored as “float” types, append an “F”:

3.456F 5.678E-10F -321.0F

29Winter 2016 CMPE212 - Prof. McLeod

Literal Values - Cont.• char literals:

‘A’ ‘5’ ‘\u0032’

• boolean literals:

true false

• String literals:

“Hello there!” “456.7” “West of North”