An Introduction to Java – Part 1 Erin Hamalainen CS 265 Sec 001 October 20, 2010

Some Java Basics Derives most of its syntax from C++ Built almost exclusively as an object- oriented programming language All code is written inside a class Everything is an object, with the exception of the intrinsic data types

Data Types (also called primitive types) Byte –Min value: -128 –Max value: 127 –8 bits (used for memory saving purposes) Short –Min value: -32,768 –Max value: 32,767 –16 bits Int –Min value: -2,147,483,648 –Max value: 2,147,483,647 –32 bits –For integral types, this data type is generally the default Long –Min value: -9,223,372,036,854,775,808 –Max value: 9,223,372,036,854,775,807 –64 bits –Use this only when you need a range of values wider than those provided by int

Data Types (also called primitive types) Float –Use a float (instead of a double) if you need to save memory –Single precision, 32 bit Double –For decimal values this is the default choice –Double precision, 64 bit Boolean –Two possible values: true and false Char –A single character

Arrays A container object that holds a fixed number of values of a single type Length is established when the array is created. After creation, length is fixed. Declaration: int[] anArray; // declares an array of integers The declaration does not actually create an array. It simply tells the compiler that this variable will hold an array of the specified type.

Arrays cont’d You can also declare an array like this: int anArray[]; //this form is discouraged However, convention discourages this form; the brackets identify the array type and should appear with the type designation. int[] anArray;//declares an array of integers anArray = new int[10]; //allocates memory for 10 integers Alternatively you can use the shortcut syntax to create and initialize an array int[] anArray = {1,2,3,4,5,6,7,8,9,10}; Here the length of the array is determined by the number of values between { and }.

Arrays cont’d Elements of an array are accessed using subscripts (just like in C++ and Perl) int[] anArray; //declares an array of integers anArray = new int[10];//allocates memory for 10 integers anArray[0] = 1;//initialize first element anArray[1] = 2;//Initialize second element System.out.println(“Element at index 0: “ + anArray[0]); System.out.println(“Element at index 1: “ + anArray[1]);

Selected Member Functions for Arrays Static void sort(type[] a) –Sorts an array using a QuickSort algorithm Static int binarySearch(type[] a, type v) –Argument a is a sorted array. This function searches for v among the elements of a and returns subscript if v is found. Static boolean equals(type[] a, Object other) –This function returns true if other is an array of the same type as a, if it has the same length, and if the corresponding elements match

Strings Strings are a standard class in Java String str1; //declare a string variable str1 = “Erin”; //initialize string variable String str2 = “My name is” //declaration and initialization in one line String concatenation: + String str3 = str2 + “ “ + str1 + “.”; //str3 holds the string “My name is Erin.”

Selected Member Functions for Strings int compareTo(String other) –Returns a negative value if the implicit argument comes before the explicit argument, a positive value if the explicit argument comes before the implicit argument, and 0 if the strings are equal int cmp = impStr.compareTo(expStr); boolean equals(Object other) –Returns true if the implicit argument equals the explicit argument. Returns false otherwise. boolean eq = impStr.equals(expStr); int length() –Returns the length of the string int length = impStr.length();

Operators Arithmetic Operators +, -, *, /, % Relational Operators ==, !=,, = Logical Operators &&, ||, !

Control Flow If else statements –Same as in C++ –Curly braces are unnecessary if they only enclose one statement While loops –Same as C++ –Curly braces are unnecessary if they only enclose one statement For loops –Same as C++ for(int i=0; i<5; i++) System.out.println(numbers[i]);

Program Structure Public class className { public static void main(String[] args) { program statements } user-defined methods } Public classes are accessible from any class Classes that are not declared public are accessible only within their package –(A package is a namespace for organizing classes and interfaces in a logical manner. Placing your code into packages makes large software projects easier to manage. ) Public methods may be called from any class Static methods do not operate on objects The keyword void indicates that the method does not return a value The array args of the main method stores command line arguments

public class SortJava { public static void main(String[] args) { int n=5; int[] numbers = new int[n]; //The Math.random() method returns a random floating oint number in the range //[0,1). We multiply the random number by n and then we cast the type of the result //to integer. for(int i=0; i<numbers.length; i++) numbers[i]=(int)(Math.random()*n); System.out.println("Random numbers before sorting"); for(int i=0; i<numbers.length; i++) System.out.println(numbers[i]); System.out.println(); System.out.println("Random numbers after sorting"); // We call sort method of Arrays class. // Array class is located in java.util.Arrays. Import //statement java.util.* allows us to access it. Arrays.sort(numbers); for(int i=0; i<numbers.length; i++) System.out.println(numbers[i]); }

Source Code, Compilation, Execution Source code needs to have same as name of public class –There can only be one public class in a Java program Source code needs to have extension.java During compilation, the Java compiler produces a bytecode file –This file has the extension.class The Java interpreter starts execution when we type command java followed by the class name

Questions?