CSE 131 Computer Science 1 Module 1: (basics of Java)

Welcome to the World of Java

Eclipse Integrated Development Environment (IDE) »“smart” editing of Java programs – continuous syntax checks »compiles program files automatically as necessary »integrated program testing and debugging tools »has plug-ins for version control system (Subversion) Can be a powerful tool for managing large projects »but, can also be a bit overwhelming and mysterious at first »watch the first few Eclipse videos on the web site before lab works the same way in Windows, Mac and Linux

Your First Program public class HelloWorld { public static void main(String[] args) { System.out.println("Hello, world!"); } } »defines a class called HelloWorld (a class defines an object) »class contains a single method called main in Java, the main method is the starting point of a program the main method has an array of arguments (not used here) Any program has one and only one main() method »program “prints” a brief message and halts

Today: understand basic building blocks of JAVA You will be able to »Output a slogan on the screen »Tell if a year is a Leap Year »Calculate the area of a circle »Output anything you like »Do all kinds of calculation

Basic unit: variable A variable in Java is a piece of information stored in a computer’s memory: like a jar holding food Each variable has a type »Like different kinds of jars (different food, size, usage, etc.) Main types: int a; double b; String s; boolean x;

Basic Data Types in Java Type (keyword)MeaningExamplesPrimitive Operators intInteger3, 0, -5, 256+, -, *, / doubleDecimal number1.5, , 0.0+, -, *, / booleantrue or falsetrue, false&&, ||, ! (return boolean:, =,!=, ==) Stringa sequence of characters “hello world”+ (concatenation)

8 Basic Definitions Type. A category of values and a set of operations on such values. Variable. A name that refers to a value of a declared type. Assignment statement. Associates a value with a variable. type

The int type

Integers This surprises most students. Don’t feel bad: experts forget this too sometimes.

Key points Assignment statement Variable = Expression ; (For example: a = b+ 5/3; ) Expression’s type must match the variable’s type (will come back to this point) Sequential execution »The order of statements matters

The double type

Example: compute the area of a circle

String: a sequence of characters

String operation Meaning of characters depends on context. Example: report +, -, *, / of two integers

Boolean

17 Booleans boolean data type. Useful to control the logic of a program.

18 Comparisons Comparisons. Take two operands of one type ( int or double ) and produce a result of type boolean.

19 Leap Year Q. Is a given year a leap year? A. Yes if either (i) divisible by 400 or (ii) divisible by 4 but not 100. public class LeapYear { public static void main(String[] args) { int year = 2012; boolean isLeapYear; // divisible by 4 but not 100 isLeapYear = (year % 4 == 0) && (year % 100 != 0); // or divisible by 400 isLeapYear = isLeapYear || (year % 400 == 0); System.out.println(isLeapYear); }

Data Type Conversion Operations must be applied to same types, with the following exceptions Java automatically converts some types of values to make them compatible int i, j; double x, y; i = 5; j = i/2; y = 2; x = i/y; - As long as there is a double in an expression, the type of the expression is promoted to double Combining strings and numeric values int i = 5; String S = “two plus two =” + i;

Basic Expressions in Java ExpressionValueType 3+58int 7/2.03.5double 7/23int 3+5-2/4+210int /4+29.5double (3+5-2)/(4+2)1int “Hello”+”there”“Hellothere”String “I have “+5+5+” toes”“I have 55 toes”String true && falsefalseboolean false || truetrueboolean !truefalseboolean (true&&false)||(!false)trueboolean

Variable types A variable can only hold values of the proper type int x; (default 0) int x = 1; … x = false; (error! Type-checking problem) Any expression of type T can appear in an expression where a value of type T is required »E.g. int x = 3+5; int x = 7/2.0; (type error) int x= (int) (7/2.0) (cast, x has value 3)

Method What if we need to find areas of different circles? »Do we have write the code over and over again?

Procedural abstraction (method) Find areas of circles A method is like a blackbox (input  output) »Hide details of computation from outside A java method Use it: double area = circleArea(10.0); double circleArea(double radius) { return Math.PI * radius * radius; }

Method Call and Return Java methods used to decompose program into parts »allows us to solve a problem once and conveniently re-use the solution many times You can invoke the method by using it in an expression »Example: area = circleArea(2.0); »The number of input parameters should match the method definition »The types of input parameters should match the method definition

Method for Length of Hypotenuse We know C=√A 2 +B 2 Forms the basis of the Java method double hypotenuse(double A, double B) { return Math.sqrt(A * A + B * B); } » Math.sqrt(x) returns the square root of x part of a library of mathematical functions, including sin(x), cos(x), tan(x), log(x), exp(x) »Signature of a method: double hypotenuse(double, double) defines method name, return type, types of arguments »Invocation of a method must match its signature Example: double h = hypotenuse(3, 4); C B A