java basics - city university of new yorkcwang/212/java_tutorial.pdfinput and output: file i/o using...
TRANSCRIPT
Overview
1 Variables in JAVA
2 Methods
3 Overloading Methods
4 Input/Output statements
5 Strings
6 Exception Handling
7 Summary
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Variables in JAVA
Variables
Java programming language defines the following kinds of variables:
1 Instance Variables (Non-Static Fields)
2 Class Variables (Static Fields)
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Variables in JAVA
Instance Variables
Their values are unique to each instance of a class i.e., each object
Example: StudentID is an instance variable. Because every studenthas a different StudentID.
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Variables in JAVA
Class Variables
A class variable is any field declared with the static modifier, this tellsthe compiler that there is exactly one copy of this variable inexistence, regardless of how many times the class has beeninstantiated/created.
Example: The lecturer of this workshop is always me regardless ofany student. StudentID can be different, but the lecturer is alwaysthe same.
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Variables in JAVA
Local Variables
Similar to how an object stores its state in fields, a method will oftenstore its temporary state in local variables.
There is no special keyword designating a variable as local, thatdetermination comes entirely from the location in which the variableis declared-which is between the opening and closing braces of amethod.
Example: for (int i=0; i <10; i++) .... The variable i here is thelocal variable. It is only valid within the opening and closing braces ofthe for loop. It is invalid outside the for loop.
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Variables in JAVA
Examples
public void swap( int x, int y )
{
int temp = x; // temp is a local variable
x = y;
y = temp;
}
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Methods
Defining Methods
Method declarations have six components:
1 Modifiers—such as public, private, protected.
2 The return type—the data type of the value returned by the method,or void if the method does not return a value.
3 The method name—the rules for naming are same as C++
4 The parameter list in parenthesis—a comma-delimited list of inputparameters, preceded by their data types, enclosed by parentheses. Ifthere is no parameters, you must use empty parentheses.
5 An exception list- to be discussed later.
6 The method body, enclosed between braces- the methods code,including the declaration of local variables.
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Methods
Method: Modifiers and Access Levels
Modifier Class Package Subclass World
public Y Y Y Yprotected Y Y Y Nno modifier Y Y N Nprivate Y N N N
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Overloading Methods
Overloading Methods
Methods within a class can have the same name if they have differentparameter lists.
Example:
Suppose that you have a class that can use calligraphy to drawvarious types of data and that contains a method for drawing eachdata type.
Option 1: New name for each method—for example,drawString(String s), drawInteger(int i), drawFloat(float f), and so on.
Option 2: Same name but different parameters (Overloading) Youcan use the same name for the drawing methods but pass a differentargument list to each method (see example in next slide)
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Overloading Methods
Overloading Methods: Example
public classDataArtist
{
public void draw ( String s )
{
.
}
public void draw ( int i )
{
.
}
public void draw (int i, double f)
{
.
}
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Overloading Methods
Caution: Overloading Methods
The compiler does NOT consider return type when differentiating methods.You cannot declare two methods with the same parameters even if theyhave a different return type.
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Input/Output statements
Input and Output: Standard I/O
C++ JAVA
cout << ”Hello World” << endl; System.out.println(”Hello World”);cin >> name; System.in(name);
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Input/Output statements
Input and Output: File I/O using ByteStream
CopyBytes spends most of its time in a simple loop that reads theinput stream and writes the output stream, one byte at a time.
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
public class CopyBytes
{
public static void main(String[]args) throwsIOException
{
FileInputStream in = null;
FileOutputStream out = null;
try
{ in = newFileInputStream("input.txt");
out = newFileOutputStream("output.txt");
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Input/Output statements
Input and Output: File I/O using ByteStream
int c;
while ((c =in.read()) != -1)
{
out.write(c);
}
}
finally
{
if (in != null)
{
in.close();
}
if (out != null)
{
out.close();
}
}
}
}
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Input/Output statements
When Not to Use Byte Streams
CopyBytes seems like a normal program, but it actually represents a kindof low-level I/O that you should avoid.Byte streams should only be used for the most primitive I/O.Byte streams use unbuffered I/O. This means each read or write requestis handled directly by the underlying OS.This can make a program much less efficient, since each such requestoften triggers disk access, network activity, or some other operation that isrelatively expensive.So why talk about byte streams?Because all other stream types are built on byte streams.
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Input/Output statements
Input and Output: File I/O using BufferedStream
To reduce this kind of overhead, the Java platform implements bufferedI/O streams.Buffered input streams read data from a memory area known as a buffer;the native input API is called only when the buffer is empty.Similarly, buffered output streams write data to a buffer, and the nativeoutput API is called only when the buffer is full.A program can convert an unbuffered stream into a buffered stream .There are four buffered stream classes used to wrap unbuffered streams:BufferedInputStream and BufferedOutputStream create buffered bytestreams, while BufferedReader and BufferedWriter create bufferedcharacter streams.(example below)
BufferedReader in, out;
in = new BufferedReader(newFileReader("input.txt"));
out = newBufferedWriter(newFileWriter("output.txt"));
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Strings
Strings
Strings are widely used in Java programming.
A string is a sequence of characters.
String is class and hence each instance is an object of type String.
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Strings
Creating Strings
Two options:
1 String greeting = "Hello World!" ; //string literal
2 char[] helloArray = { ’h’, ’e’, ’l’, ’l’, ’o’, ’.’ };
String helloString = new String(helloArray);
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Strings
String Conversion
Converting Strings to Numbers
int x= Integer.parseInt("3");
Converting Numbers to Strings
Three options:
int i;
String s1 = " " + i;
String s2 = String.valueOf(i);
String s3 = Integer.toString(i);
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Strings
Manipulating Characters in a String
Getting Characters and Substrings by Index:
String a = "hello!";
//gets the character at position index 1
char b = a.charAt(1);
//gets the substring starting at position index 1 (including)
//to index 3 (excluding)
String c = a.substring(1,3);
//gets the string starting at position index 3 up to the end
String d = a.substring(3);
System.out.println (a + " " + b + " " + c + " " + d);
The output is
hello! e e1 lo!
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Exception Handling
What is an Exception
Definition:
An exception is an event, which occurs during the execution of aprogram, that disrupts the normal flow of the program’s instruc-tions.
When an error occurs within a method, the method creates an objectand hands it off to the runtime system.
The object, called an exception object, contains information aboutthe error, including its type and the state of the program when theerror occurred.
Creating an exception object and handing it to the runtime system iscalled throwing an exception.
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Exception Handling
Types of Exceptions
There are three kinds of Exceptions:
1 Checked exception
2 Error
3 Runtime Exception
Errors and runtime exceptions are collectively known as uncheckedexceptions.
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Exception Handling
Checked Exceptions
These are exceptional conditions that a well-written application shouldanticipate and recover from.Example
Suppose an application prompts a user for an input file name,then opens the file by passing the name to the constructor forjava.io.FileReader.
Normally, the user provides the name of an existing, readable file, sothe construction of the FileReader object succeeds, and theexecution of the application proceeds normally.
But sometimes the user supplies the name of a nonexistent file, andthe constructor throws java.io.FileNotFoundException.
A well-written program will catch this exception and notify the user ofthe mistake, possibly prompting for a corrected file name.
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Exception Handling
Error
These are exceptional conditions that are external to the application, andthat the application usually cannot anticipate or recover from.Example
Suppose that an application successfully opens a file for input,but is unable to read the file because of a hardware or systemmalfunction. The unsuccessful read will throw java.io.IOError.
An application might choose to catch this exception, in order tonotify the user of the problem
It also might make sense for the program to print a stack trace andexit.
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Exception Handling
Runtime Exception
These are exceptional conditions that are internal to the application, andthat the application usually cannot anticipate or recover from. Theseusually indicate programming bugs, such as logic errors or improper use ofan API.Example
Consider the application described previously that passes a filename to the constructor for FileReader. If a logic error causes anull to be passed to the constructor, the constructor will throwthe NullPointerException.
The application can catch this exception
It probably makes more sense to eliminate the bug that caused theexception to occur
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Exception Handling
try-catch Statement for Exception Handling: The try
block
The first step in constructing an exception handler is to enclose the codethat might throw an exception within a try block.
private List<Integer> list;
private static final int SIZE = 10;
PrintWriter out = null;
try
{
System.out.println("Entered try statement");
out = new PrintWriter ( newFileWriter ( "OutFile.txt" ) );
for ( int i = 0; i < SIZE; i++)
out.println("Value at: " +i+ " = " +list.get(i));
}
// put catch and finally statements . . .
Question: Why do we add the try block here? 27 / 33
Exception Handling
try-catch Statements for Exception Handling: The try
block
Answer: Why do we add the try block here?
The second line in try block is a call to a constructor.
The constructor initializes an output stream on a file.
If the file cannot be opened, the constructor throws an IOException.
The forth line in the try block is a call to the arrayList class’s get method.
This call throws an IndexOutOfBoundsException if the value of itsargument is too small (less than 0) or too large (more than thenumber of elements currently contained in the array)
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Exception Handling
try-catch Statements for Exception Handling: The catch
block
try
{
//see the try block slide
} catch (FileNotFoundExceptione)
{
System.err.println("FileNotFoundException: "
+ e.getMessage());
throw newSampleException(e);
} catch (IOExceptione)
{
System.err.println("CaughtIOException: "
+ e.getMessage());
}
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Exception Handling
try-catch Statements for Exception Handling: Thefinally block
The finally block always executes when the try block exitsregardless of what happens within the try block.
finally
{
if (out != null)
{
System.out.println("Closing PrintWriter");
out.close();
}
else
{
System.out.println("PrintWriter not open");
}
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Exception Handling
Example: try-catch and finally put togetherpublic void writeList()
{
PrintWriterout = null;
try
{
System.out.println("Entering" + " try statement");
out = new PrintWriter(new FileWriter("OutFile.txt"));
for (int i= 0; i < SIZE; i++)
out.println("Value at: " +i+ " = " +vector.elementAt(i));
}
catch (ArrayIndexOutOfBoundsExceptione)
{
System.err.println("CaughtArrayIndexOutOfBoundsException:"
+ e.getMessage());
}
catch (IOExceptione)
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Exception Handling
Example: try-catch and finally put togetherSystem.err.println("CaughtIOException: "
+ e.getMessage());
}
finally
{
if (out != null)
{
System.out.println("ClosingPrintWriter");
out.close();
}
else
{
System.out.println("PrintWriternot open");
}
}
}
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Summary
Summary
Variables: instance variables, class variables, local variables,parameters.
Methods: method modifiers(public, private, protected), methodssignature(method name, parameter type), methods overloading.
Input/Output statements: Byte Streams, Buffered Streams
Strings: creating Strings, converting strings to numbers, manipulatingcharacters in strings
Try-Catch statements: Exceptions, three types of exceptions,try-catch-finally blocks.
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