COP3502 Programming Fundamentals for CIS Majors 1 Instructor: Parisa Rashidi

Chapter 9: String + Files

 String is a reference type (class) String //Since strings are used frequently, Java provides a //shorthand initializer for creating a string: String message = "Welcome to Java"; //String is a class String message = new String("Welcome to Java"); //You can also create a string from an array of // characters (char) char [] charArray = {‘W’, ‘e’, ‘l’, ‘c’, ‘o’, ‘m’, ‘e’}; String message = new String(charArray);

Immutable String s = "Java"; s = "HTML";

If you use the string initializer, no new object is created, if the interned object is already created. Interned String

Difference between “==“ vs. “equals()” String Comparison String s1 = new String("Welcome"); String s2 = "Welcome"; if (s1.equals(s2)) { // s1 and s2 have the same contents } if (s1 == s2) { // s1 and s2 have the same reference }

To compare strings, do not use >=, <= String Comparison String s1 = new String("Welcome"); String s2 = "Welcome"; if (s1.compareTo(s2) > 0) { // s1 is greater than s2 // lexicographically, i.e. unicode order } else if (s1.compareTo(s2) == 0) { // s1 and s2 have the same contents } else // s1 is less than s2

x.compareTo(y) return the difference of the first distinct character x = a b c y = a b d Unicode code = 81 Unicode code = 82 x.compareTo(y) = 81 – 82 = -1 String Comparison

 The StringBuilder / StringBuffer class is an alternative to the String class.  In general, a StringBuilder/StringBuffer can be used wherever a string is used.  StringBuilder/StringBuffer is more flexible than String.  You can add, insert, or append new contents into a string buffer  whereas the value of a String object is fixed once the string is created. StringBuilder

 Command line arguments Command Line args class TestMain { public static void main(String[] args) {... }

 The File class is intended to provide an abstraction that deals with most of the machine-dependent complexities of files and path names in a machine- independent fashion.  The filename is a string.  The File class is a wrapper class for the file name and its directory path. File

 The file path might be absolute or relative  Absolute  c:\book\Welcome.java  /home/liang/book/Welcome.java  Relative ..\Welcome.java  images/123.jpg File Path

 The directory separator for Windows is a backslash (\).  The backslash is a special character in Java and should be written as \\ in a string literal  The forward slash (/) is the Java directory separator, which is the same as on Unix.  The statement new File("image/us.gif")works on Windows, Unix, and any other platform. File Path

 A File object encapsulates the properties of a file or a path.  But does not contain the methods for reading/writing data from/to a file.  In order to perform I/O, you need to create objects using appropriate Java I/O classes.  Scanner and PrintWriter classes. Text I/O

 What is the printout of the following code? Q >Welcome to Java >Welcabcme tabc Java

 Suppose that s1 and s2 are two strings. Which of the following statements or expressions are incorrect? Q

Chapter 11: Inheritance

 Suppose you want to define classes to model circles, rectangles, and triangles.  These classes have many common features.  What is the best way to design these classes so to avoid redundancy?  The answer is to use inheritance. Motivation

Superclass GeometricObject Circle4 Rectangle1 TestCircleRectangle Run

 A subclass does not inherit the private members of its parent class.  A subclass is not a subset of the superclass  Contains more information! Inheritance

 An abstract class example abstract public abstract class GraphicObject { // declare fields // declare non-abstract methods abstract void getPerimeter(); }

 A constructor may invoke  An overloaded constructor or  Its superclass’s constructor  If none of them is invoked explicitly, the compiler puts super() as the first statement in the constructor. Constructors

 Example Constructors

 The keyword super refers to the superclass of the class in which super appears.  This keyword can be used in two ways:  To call a superclass constructor  super();  To call a superclass method  super.toString(); super

Chaining

 Method overriding Overriding

 Like an instance method, a static method can be inherited.  However, a static method cannot be overridden.  If a static method defined in the superclass is redefined in a subclass, the method defined in the superclass is hidden. Overriding

 Every class in Java is descended from the java.lang.Object class.  If no inheritance is specified when a class is defined, the superclass of the class is Object. Object Class

 Equivalent Object Class

 An object of a subtype can be used wherever its supertype value is required.  This feature is known as polymorphism. Polymorphism

 Polymorphism Code

 Accessibility summary Accessibility

 A final class cannot be extended  A final variable is a constant  A final method cannot be overridden by its subclasses. final

 What is the printout of running the class C? Q

 What problem arises in compiling the program? Q

True or false? 1. A subclass is a subset of a superclass. 2. When invoking a constructor from a subclass, its superclass’s no-arg constructor is always invoked. 3. You can override a private method defined in a superclass. 4. You can override a static method defined in a superclass. Q

What is the output? Q

Chapter 13: Exceptions

 Exception Types Exception Hierarchy

 RuntimeException, Error and their subclasses are known as unchecked exceptions.  All other exceptions are known as checked exceptions, meaning that  the compiler forces the programmer to check and deal with the exceptions. Checked

 Exception handling process 1. Declare exception 2. Throw exception 3. Catch exception Exception Handling

 Every method must state the types of checked exceptions it might throw.  This is known as declaring exceptions. Declaring Exception

 When the program detects an error, the program can create an instance of an appropriate exception type and throw it.  This is known as throwing an exception. Throwing Exception

 Example Throwing Exception /** Set a new radius */ public void setRadius(double newRadius) throws IllegalArgumentException { if (newRadius >= 0) radius = newRadius; else throw new IllegalArgumentException( "Radius cannot be negative"); }

 Example Catching Exception

 Exception handling example Catching Exception

 To deal with a checked exception, you have to write the code as shown in (a) or (b) Catching Exception

 The order in which exceptions are specified in catch blocks is important.  A compile error will result if a catch block for a superclass type appears before a catch block for a subclass type. Catching Exception

 Occasionally, you may want some code to be executed regardless of whether an exception occurs or is caught. finally

 When to use exceptions: 1. An exception occurs in a method:  If you want the exception to be processed by its caller, you should create an exception object and throw it.  If you can handle the exception in the method where it occurs, there is no need to throw it. When?

 When should you use the try-catch block in the code?  You should use it to deal with unexpected error conditions.  Do not use it to deal with simple, expected situations. When?

 Output? Q

Q

 Suppose that statement2 causes an exception in the following try-catch block: Q

Answer the following questions: ■ Will statement3 be executed? ■ If the exception is not caught, will statement4 be executed? ■ If the exception is caught in the catch block, will statement4 be executed? ■ If the exception is passed to the caller, will statement4 be executed? Q