COP 3330

Topics Java Basics - How to write & call your own method - Math methods (static methods) - String methods (instance methods) - Errors Classes Objects

Writing your own methods main() is a method you’ve all already used - public static void main(String[] args) To write your own methods, use the same syntax public static void myMethod() { // Do something here. } public static int myIntMethod() { // Do something here and return an int. }

Writing your own methods Order of methods doesn’t matter (Difference from C) Don’t forget static keyword or you’ll receive error “ Cannot make a static reference to the non-static method ” Write code showing method calls

Math Methods These are static methods. That means they are called on the Math class - abs() : Takes in an int and returns its absolute value - sin(), cos(), tan() : Take in doubles and perform trigonometry. These functions use radians, not degrees - PI : A static final variable that is closer to π than any other double.

Math Methods More Math methods - pow() : Takes in two doubles and returns the first raised to the power of the second - random() : Takes in no parameters and returns a random double in the range from 0.0 to 1.0, including 0, but excluding 1. - sqrt() : Takes in a double and returns its square root - Show example Math code

String Methods Java Strings are immutable. That means that once they're created, they can't be altered Calling methods on Strings will return new Strings, but leave the original intact

String Methods These are instance methods (that means they're called on String objects) - concat() : Takes in a String and returns a new String. Equivalent to + - replace() : Takes in two chars and returns a new String. The first char is the character to be replaced, and the second char is the character to replace it with

String Methods More methods - substring() : Takes in two ints and returns a new String. The first int is the beginning index and the second int is the ending index of the substring - length() : Takes in no parameters. Returns the length of the String - charAt() : Takes in an int and returns the character at that index

Simple grep Program Write a program that reads in a paragraph of text from a file called “ text.in ” and searches for all instances of a given word (pattern) input by the user from standard input, ignoring the case of the text or pattern. Print the line number and index (both 0-based) of each occurrence of the pattern, or print that the pattern was not found in the text. Use either the substring() method or the indexOf() method to search for the pattern.

Errors Compilation Errors Runtime Errors Logic Errors

Compilation Errors A compilation error can occur when the grammar of the language is broken Common examples - Missing semicolon - Unmatched curly brace - Misspelled keyword

Compilation Errors A compilation error can also occur when the syntax is basically valid, but a statement has an invalid meaning Examples - Using undeclared/out-of-scope variables - Not casting when casting is necessary - Using a double to index an array

Runtime Errors A runtime error is when the program crashes In Java, usually an unhandled exception - NullPointerException - ArithmeticException - ArrayIndexOutOfBoundsException - And much, much more… Also, virtual machine errors (e.g. out of memory)

Logical Errors Say what you mean, and mean what you say Logical errors are when the computer understands and can do what you’ve told it, but you’ve told it the wrong thing Logical errors are notoriously difficult to track down

Logical Errors Sources of logical errors - Typo - Programmer didn’t notice special cases - Programmer didn’t understand problem

Quiz #1 Returned Please come to the front of class to claim your Quiz #1 If you think you deserve more points, please see me during office hours - You must have a clear and valid reason to earn more points. “I need to get a better grade on this quiz” is not a valid reason!

A Good Design … features Encapsulation This is a key aspect of object-oriented design Encapsulation means grouping together related data and operations on that data into a unit

Class Basics A class defines a new data type A class is a blueprint for objects Classes are mostly made up of three basic components (members) - Instance variables (fields) - Constructors - Methods

A Good Design …features Information Hiding This is a key aspect of object-oriented design Information hiding means concealing unnecessary details This allows those using a system to more easily understand the big picture This makes it much more difficult to use the system in incorrect ways

Class Basics All members have access modifiers The most common are - public - private The other two will be discussed at a later point - protected - Default access (package)

Instance Variables Instance variables persist throughout the life of the object They can be of the same types as any other variables Example - private double realPart; - private double imaginaryPart;

Constructor A constructor is a method, but it is so important that it needs separate consideration A constructor creates, initializes, and returns a reference to a new object Creation and return are handled automatically The constructor always bears the name of its class

Constructor Example public class Complex { private double realPart; private double imagPart; public Complex() { realPart = imagPart = 0; }... }

Constructor Constructors are called by using the new keyword - Example: Complex foo = new Complex();

Garbage Collection In Java, when an object is no longer referenced, its memory is reclaimed automatically This is a big difference from C

Book Example Store information about a Book - Title - Publisher - ISBN - Number of pages - Number of chapters - The text of the book - Others?

Book Write a class to store this data and create a constructor for the class

Methods Non-static methods act on objects A method is laid out as follows: modifiers return-type identifier(parameters) { body } A method is essentially the same thing as a function in C

Methods The return type of a method can be any type If the method does not return anything, the return type is void

Methods Variables accessible from within a method - Instance variables - Formal parameters - Local variables

Methods Use object.methodName(parameters) to call the method on an object - Example: number = stdin.nextInt(); Add a method to the Book class to add a chapter

.toString() Java uses the toString method on objects to represent it as a string for purposes of string concatenation and such The toString method has the following signature: - public String toString() If you want to be able to print objects of your class to be able to be printed in a meaningful way, write this method in your class!

Overloading Multiple methods (and constructors) with the same name can exist in the same class The formal parameters MUST be different Why do this? - Use different types of information to construct an object (e.g. Scanner constructors) - Perform similar operations (e.g. print() method in PrintStream)

Static Ordinarily, methods and fields are associated with objects The static modifier makes them associate with the class Example - See the class Math in java.lang

Static To access a static member, use the class name instead of the object name - Example: number = Integer.parseInt( “ 5 ” ); Add a static method to the Book class to generate a new ISBN for a new book