Stefan Andrei

Programming Fundamentals I (COSC-1336), Lecture 6 (prepared after Chapter 9 of Liang’s 2011 textbook)Stefan Andrei

04/21/23 1COSC-1336, Lecture 6

Overview of Previous Lecture To define methods, invoke methods, and pass

arguments to a method (§5.2-5.5). To develop reusable code that is modular, easy-

to-read, easy-to-debug, and easy-to-maintain. (§5.6).

To use method overloading and understand ambiguous overloading (§5.7).

To design and implement overloaded methods (§5.8).

04/21/23 2COSC-1336, Lecture 6

Overview of Previous Lecture (cont) To determine the scope of variables (§5.9). To know how to use the methods in the Math

class (§§5.10-5.11). To learn the concept of method abstraction

(§5.12). To design and implement methods using

stepwise refinement (§5.12).

04/21/23 3COSC-1336, Lecture 6

Motivation of the current lecture Often you encounter the problems that involve string processing and file input and output. Suppose you need to write a program to replace all occurrences of a word with a new word in a file. How do you solve this problem? This chapter introduces strings and text files, which will enable you to solve this problem.

04/21/23 4COSC-1336, Lecture 6

Overview of This Lecture To use the String class to process fixed strings

(§9.2). To use the Character class to process a single

character (§9.3). To use the StringBuilder/StringBuffer class to

process flexible strings (§9.4). To distinguish among the String, StringBuilder,

and StringBuffer classes (§9.2-9.4). To learn how to pass arguments to the main

method from the command line (§9.5).

04/21/23 5COSC-1336, Lecture 6

Overview of This Lecture (cont.) To discover file properties and to delete

and rename files using the File class (§9.6).

To write data to a file using the PrintWriter class (§9.7.1).

To read data from a file using the Scanner class (§9.7.2).

(GUI) To open files using a dialog box (§9.8).

04/21/23 6COSC-1336, Lecture 6

The String Class Constructing a String:

String message = "Welcome to Java“; String message = new String("Welcome to Java“); String s = new String();

Obtaining String length and Retrieving Individual Characters in a string

String Concatenation (concat) Substrings (substring(index), substring(start, end)) Comparisons (equals, compareTo) String Conversions Finding a Character or a Substring in a String Conversions between Strings and Arrays Converting Characters and Numeric Values to Strings

04/21/23 7COSC-1336, Lecture 6

Constructing StringsString newString = new String(stringLiteral);

String message = new String("Welcome to Java");

Since Strings are used frequently, Java provides a shorthand initializer for creating a String:

String message = "Welcome to Java";

04/21/23 8COSC-1336, Lecture 6

Strings Are Immutable

A String object is immutable: its contents cannot be changed. Does the following code change the contents of the String s?

String s = "Java";

s = "HTML";

04/21/23 9COSC-1336, Lecture 6

Trace Code

String s = "Java";

s = "HTML";

: String

String object for "Java"

s

After executing String s = "Java";

After executing s = "HTML";

: String


: String

String object for "HTML"

Contents cannot be changed

This string object is now unreferenced

s

animation

04/21/23 10COSC-1336, Lecture 6

Trace Code

String s = "Java";

s = "HTML";

: String


s

After executing String s = "Java";

After executing s = "HTML";

: String


: String

String object for "HTML"

Contents cannot be changed

This string object is now unreferenced

s

animation

04/21/23 11COSC-1336, Lecture 6

Interned Strings Since strings are immutable and are frequently used, to improve efficiency and save memory, the JVM uses a unique instance for string literals with the same character sequence. Such an instance is called interned. For example, the following statements:

04/21/23 12COSC-1336, Lecture 6

Examples

displays s1 == s2 is false

s1 == s3 is true

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

String s1 = "Welcome to Java"; String s2 = new String("Welcome to Java"); String s3 = "Welcome to Java"; System.out.println("s1 == s2 is " + (s1 == s2)); System.out.println("s1 == s3 is " + (s1 == s3));

: String

Interned string object for "Welcome to Java"

: String

A string object for "Welcome to Java"

s1

s2

s3

04/21/23 13COSC-1336, Lecture 6

Trace Code String s1 = "Welcome to Java"; String s2 = new String("Welcome to Java"); String s3 = "Welcome to Java";

: String


s1

animation

04/21/23 14COSC-1336, Lecture 6


: String


: String


s1

s2

04/21/23 15COSC-1336, Lecture 6


: String


: String


s1

s2

s3

04/21/23 16COSC-1336, Lecture 6

String Comparisons java.lang.String

+equals(s1: String): boolean

+equalsIgnoreCase(s1: String): boolean

+compareTo(s1: String): int

+compareToIgnoreCase(s1: String): int

+regionMatches(toffset: int, s1: String, offset: int, len: int): boolean

+regionMatches(ignoreCase: boolean, toffset: int, s1: String, offset: int, len: int): boolean

+startsWith(prefix: String): boolean

+endsWith(suffix: String): boolean

Returns true if this string is equal to string s1.

Returns true if this string is equal to string s1 case-insensitive.

Returns an integer greater than 0, equal to 0, or less than 0 to indicate whether this string is greater than, equal to, or less than s1.

Same as compareTo except that the comparison is case-insensitive.

Returns true if the specified subregion of this string exactly matches the specified subregion in string s1.

Same as the preceding method except that you can specify whether the match is case-sensitive.

Returns true if this string starts with the specified prefix.

Returns true if this string ends with the specified suffix.

04/21/23 17COSC-1336, Lecture 6

String Comparisons equals()

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 }

04/21/23 18COSC-1336, Lecture 6

The String Class – no need for new Because strings are so common, we do not have

to use the new operator to create a String object:

title = "Java programming language";

This is special syntax that works only for strings.

Each string literal (enclosed in double quotes) represents a String object.

04/21/23 19COSC-1336, Lecture 6

Creating String objects Using the new operator:

String s1 = new String("tree");

Will create a new String object whose value is "tree"

and assign its address to s1; Without using the new operator:

String s1 = "tree"; The Java compiler will check whether there exists already

in memory a String object whose value is "tree", not created using the new operator.

If yes, then s1 will have the same reference as that String;

If no, the Java compiler will create a new String object whose value is "tree", its address will be referred by s1.

04/21/23 COSC-1336, Lecture 6 20

ExampleString s1 = new String("tree"), s2 = "tree", s3 = "tree";

if (s1.equals(s2))

System.out.println("Same name for s1 and s2");

if (s1 == s2)

System.out.println("Same address for s1 and s2");

if (s2.equals(s3))

System.out.println("Same name for s2 and s3");

if (s2 == s3)

System.out.println("Same address for s2 and s3");

This snippet of Java code will display:Same name for s1 and s2

Same name for s2 and s3

Same address for s2 and s3

04/21/23 COSC-1336, Lecture 6 21

String Comparisons (cont.) compareTo(Object object)

String s1 = new String("Welcome“);String s2 = "welcome";

if (s1.compareTo(s2) > 0) { // s1 is greater than s2 } else if (s1.compareTo(s2) == 0) { // s1 and s2 have the same contents } else // s1 is less than s2

04/21/23 22COSC-1336, Lecture 6

String Length, Characters, and Combining Strings

java.lang.String

+length(): int

+charAt(index: int): char

+concat(s1: String): String

Returns the number of characters in this string.

Returns the character at the specified index from this string.

Returns a new string that concatenate this string with string s1. string.

04/21/23 23COSC-1336, Lecture 6

Finding a String’s Length

Finding string length using the length() method:

message = "Welcome";

message.length() (returns 7)

04/21/23 24COSC-1336, Lecture 6

Retrieving Individual Characters in a String Do not use message[0] Use message.charAt(index) Index starts from 0

W e l c o m e t o J a v a

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

message

Indices

message.charAt(0) message.charAt(14) message.length() is 15

04/21/23 25COSC-1336, Lecture 6

String Concatenation

String s3 = s1.concat(s2);

String s3 = s1 + s2;

s1 + s2 + s3 + s4 + s5 is equivalent to:

(((s1.concat(s2)).concat(s3)).concat(s4)).concat(s5).

04/21/23 26COSC-1336, Lecture 6

Extracting Substrings

java.lang.String

+subString(beginIndex: int): String

+subString(beginIndex: int, endIndex: int): String

Returns this string’s substring that begins with the character at the specified beginIndex and extends to the end of the string, as shown in Figure 8.6.

Returns this string’s substring that begins at the specified beginIndex and extends to the character at index endIndex – 1, as shown in Figure 8.6. Note that the character at endIndex is not part of the substring.

04/21/23 27COSC-1336, Lecture 6

Extracting Substrings

You can extract a single character from a string using the charAt() method. You can also extract a substring from a string using the substring() method in the String class. String s1 = "Welcome to Java";String s2 = s1.substring(0, 11) + "HTML";

W e l c o m e t o J a v a

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

message

Indices

message.substring(0, 11) message.substring(11)

04/21/23 28COSC-1336, Lecture 6

Converting, Replacing, and Splitting Strings

java.lang.String

+toLowerCase(): String

+toUpperCase(): String

+trim(): String

+replace(oldChar: char, newChar: char): String

+replaceFirst(oldString: String, newString: String): String

+replaceAll(oldString: String, newString: String): String

+split(delimiter: String): String[]

Returns a new string with all characters converted to lowercase.

Returns a new string with all characters converted to uppercase.

Returns a new string with blank characters trimmed on both sides.

Returns a new string that replaces all matching character in this string with the new character.

Returns a new string that replaces the first matching substring in this string with the new substring.

Returns a new string that replace all matching substrings in this string with the new substring.

Returns an array of strings consisting of the substrings split by the delimiter.

04/21/23 29COSC-1336, Lecture 6

Examples "Welcome".toLowerCase() returns a new string:

welcome "Welcome".toUpperCase() returns a new string:

WELCOME "Welcome".trim() returns a new string:

Welcome "Welcome".replace('e', 'A') returns a new string:

WAlcomA "Welcome".replaceFirst("e", "AB") returns a new string:

WABlcome "Welcome".replace("e", "AB") returns a new string:

WABlcomAB "Welcome".replace("el", "AB") returns a new string:

WABcome

04/21/23 30COSC-1336, Lecture 6

Splitting a String

String[] tokens = "Java#HTML#Perl".split("#");

for (int i = 0; i < tokens.length; i++)

System.out.print(tokens[i] + " ");

Java HTML Perl

displays

04/21/23 31COSC-1336, Lecture 6

Matching, Replacing and Splitting by Patterns

You can match, replace, or split a string by specifying a pattern. This is an extremely useful and powerful feature, commonly known as regular expression. Regular expression is complex to beginning students. For this reason, two simple patterns are used in this section.

"Java".matches("Java");

"Java".equals("Java");

"Java is fun".matches("Java.*");

"Java is cool".matches("Java.*");

04/21/23 32COSC-1336, Lecture 6


The replaceAll(), replaceFirst(), and split() methods can be used with a regular expression. For example, the following statement returns a new string that replaces $, +, or # in "a+b$#c" by the string NNN.String s = "a+b$#c".replaceAll("[$+#]", "NNN");

System.out.println(s); Here the regular expression [$+#] specifies a pattern that matches $, +, or #. So, the output is aNNNbNNNNNNc.

04/21/23 33COSC-1336, Lecture 6


The following statement splits the string into an array of strings delimited by some punctuation marks.

String[] tokens = "Java,C?C#,C++".split("[.,:;?]"); for (int i = 0; i < tokens.length; i++) System.out.println(tokens[i]);

04/21/23 34COSC-1336, Lecture 6

Finding a Character or a Substring in a String

java.lang.String

+indexOf(ch: char): int

+indexOf(ch: char, fromIndex: int): int

+indexOf(s: String): int

+indexOf(s: String, fromIndex: int): int

+lastIndexOf(ch: int): int

+lastIndexOf(ch: int,

fromIndex: int): int

+lastIndexOf(s: String): int

+lastIndexOf(s: String,

fromIndex: int): int

Returns the index of the first occurrence of ch in the string. Returns -1 if not matched.

Returns the index of the first occurrence of ch after fromIndex in the string. Returns -1 if not matched.

Returns the index of the first occurrence of string s in this string. Returns -1 if not matched.

Returns the index of the first occurrence of string s in this string after fromIndex. Returns -1 if not matched.

Returns the index of the last occurrence of ch in the string. Returns -1 if not matched.

Returns the index of the last occurrence of ch before fromIndex in this string. Returns -1 if not matched.

Returns the index of the last occurrence of string s. Returns -1 if not matched.

Returns the index of the last occurrence of string s before fromIndex. Returns -1 if not matched.

04/21/23 35COSC-1336, Lecture 6

Finding a Character or a Substring in a String"Welcome to Java".indexOf('W') returns 0.

"Welcome to Java".indexOf('x') returns -1.

"Welcome to Java".indexOf('o', 5) returns 9.

"Welcome to Java".indexOf("come") returns 3.

"Welcome to Java".indexOf("Java", 5) returns 11.

"Welcome to Java".indexOf("java", 5) returns -1.

"Welcome to Java".lastIndexOf('a') returns 14.

04/21/23 36COSC-1336, Lecture 6

Convert Character and Numbers to Strings The String class provides several static valueOf() methods for converting a character, an array of characters, and numeric values to strings. These methods have the same name valueOf() with different argument types char, char[], double, long, int, and float. For example, to convert a double value to a String, use String.valueOf(5.44). The return value is string consists of characters '5', '.', '4', and '4'.

04/21/23 37COSC-1336, Lecture 6

The Character Class

java.lang.Character

+Character(value: char)

+charValue(): char

+compareTo(anotherCharacter: Character): int

+equals(anotherCharacter: Character): boolean

+isDigit(ch: char): boolean

+isLetter(ch: char): boolean

+isLetterOrDigit(ch: char): boolean

+isLowerCase(ch: char): boolean

+isUpperCase(ch: char): boolean

+toLowerCase(ch: char): char

+toUpperCase(ch: char): char

Constructs a character object with char value

Returns the char value from this object

Compares this character with another

Returns true if this character equals to another

Returns true if the specified character is a digit

Returns true if the specified character is a letter

Returns true if the character is a letter or a digit

Returns true if the character is a lowercase letter

Returns true if the character is an uppercase letter

Returns the lowercase of the specified character

Returns the uppercase of the specified character

04/21/23 38COSC-1336, Lecture 6

ExamplesCharacter charObject = new Character('b');

charObject.compareTo(new Character('a')) returns: 1charObject.compareTo(new Character('b')) returns:0charObject.compareTo(new Character('c')) returns: -1charObject.compareTo(new Character('d')) returns:-2charObject.equals(new Character('b')) returns:truecharObject.equals(new Character('d')) returns: false

04/21/23 39COSC-1336, Lecture 6

Problem: Counting Each Letter in a String

This example gives a program that counts the number of occurrence of each letter in a String. Assume the letters are not case-sensitive.

04/21/23 40COSC-1336, Lecture 6

CountEachLetter.java

import java.util.Scanner;

public class CountEachLetter {

public static void main(String[] args) {

Scanner input = new Scanner(System.in);

System.out.print("Enter a string: ");

String s = input.nextLine();

int[] counts = countLetters(s.toLowerCase());

for (int i = 0; i < counts.length; i++) {

if (counts[i] != 0)

System.out.println((char)('a' + i) + " appears “

+ counts[i] + ((counts[i] == 1) ? " time" :

" times"));

}

} // . . .

04/21/23 COSC-1336, Lecture 6 41

CountEachLetter.java (cont.)

/** Count each letter in the string */

public static int[] countLetters(String s) {

int[] counts = new int[26];

for (int i = 0; i < s.length(); i++) {

if (Character.isLetter(s.charAt(i)))

counts[s.charAt(i) - 'a']++;

}

return counts;

}

} // from CountEachLetter class

04/21/23 COSC-1336, Lecture 6 42

Running CountEachLetter.java

04/21/23 COSC-1336, Lecture 6 43

StringBuilder and StringBuffer

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.

04/21/23 44COSC-1336, Lecture 6

StringBuilder Constructors

java.lang.StringBuilder

+StringBuilder()

+StringBuilder(capacity: int)

+StringBuilder(s: String)

Constructs an empty string builder with capacity 16.

Constructs a string builder with the specified capacity.

Constructs a string builder with the specified string.

04/21/23 45COSC-1336, Lecture 6

Modifying Strings in the Builder


+append(data: char[]): StringBuilder

+append(data: char[], offset: int, len: int): StringBuilder

+append(v: aPrimitiveType): StringBuilder

+append(s: String): StringBuilder

+delete(startIndex: int, endIndex: int): StringBuilder

+deleteCharAt(index: int): StringBuilder

+insert(index: int, data: char[], offset: int, len: int): StringBuilder

+insert(offset: int, data: char[]): StringBuilder

+insert(offset: int, b: aPrimitiveType): StringBuilder

+insert(offset: int, s: String): StringBuilder

+replace(startIndex: int, endIndex: int, s: String): StringBuilder

+reverse(): StringBuilder

+setCharAt(index: int, ch: char): void

Appends a char array into this string builder.

Appends a subarray in data into this string builder.

Appends a primitive type value as a string to this

builder.

Appends a string to this string builder.

Deletes characters from startIndex to endIndex.

Deletes a character at the specified index.

Inserts a subarray of the data in the array to the builder at the specified index.

Inserts data into this builder at the position offset.

Inserts a value converted to a string into this builder.

Inserts a string into this builder at the position offset.

Replaces the characters in this builder from startIndex to endIndex with the specified string.

Reverses the characters in the builder.

Sets a new character at the specified index in this builder.

04/21/23 46COSC-1336, Lecture 6

Examples stringBuilder.append("Welcome "); stringBuilder.insert(8, "to Java and"); stringBuilder.delete(16, 19); changes stringBuilder to:

Welcome to Java stringBuilder.deleteCharAt(8); changes stringBuilder to:

Welcome o Java stringBuilder.reverse() changes stringBuilder to:

avaJ o emocleW

04/21/23 47COSC-1336, Lecture 6

Examples (cont.)

stringBuilder.reverse(); stringBuilder.insert(8, "t"); stringBuilder.replace(11, 15, "HTML"); changes stringBuilder to

Welcome to HTML stringBuilder.setCharAt(0, 'w') sets stringBuilder to

welcome to HTML

04/21/23 COSC-1336, Lecture 6 48

The toString(), capacity(), length(), setLength(), and charAt() Methods


+toString(): String

+capacity(): int

+charAt(index: int): char

+length(): int

+setLength(newLength: int): void

+substring(startIndex: int): String

+substring(startIndex: int, endIndex: int): String

+trimToSize(): void

Returns a string object from the string builder.

Returns the capacity of this string builder.

Returns the character at the specified index.

Returns the number of characters in this builder.

Sets a new length in this builder.

Returns a substring starting at startIndex.

Returns a substring from startIndex to endIndex-1.

Reduces the storage size used for the string builder.

04/21/23 49COSC-1336, Lecture 6

Main Method Is Just a Regular Method

public class A { public static void main(String[] args) { String[] strings = {"New York", "Boston", "Atlanta"}; B.main(strings); } }

class B { public static void main(String[] args) { for (int i = 0; i < args.length; i++) System.out.println(args[i]); } }

You can call a regular method by passing actual parameters. Can you pass arguments to main()? Of course, yes. For example, the main() method in class B is invoked by a method in A, as shown below:

04/21/23 50COSC-1336, Lecture 6

Command-Line Parametersclass TestMain {


System.out.println(args[0]);



}

} By calling the Java bytecode like this:java TestMain arg0 arg1 arg2 args[0] becomes arg0, args[1] becomes arg1,

and args[2] becomes arg2.

04/21/23 51COSC-1336, Lecture 6

Running TestMain.java

04/21/23 COSC-1336, Lecture 6 52

Processing Command-Line Parameters In the main() method, get the arguments from args[0], args[1], ..., args[n], which corresponds to arg0, arg1, ..., argn in the command line.

04/21/23 53COSC-1336, Lecture 6

The File Class

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.

04/21/23 54COSC-1336, Lecture 6

java.io.File

+File(pathname: String)

+File(parent: String, child: String)

+File(parent: File, child: String)

+exists(): boolean

+canRead(): boolean

+canWrite(): boolean

+isDirectory(): boolean

+isFile(): boolean

+isAbsolute(): boolean

+isHidden(): boolean

+getAbsolutePath(): String

+getCanonicalPath(): String

+getName(): String

+getPath(): String

+getParent(): String

+lastModified(): long

+delete(): boolean

+renameTo(dest: File): boolean

Creates a File object for the specified pathname. The pathname may be a directory or a file.

Creates a File object for the child under the directory parent. child may be a filename or a subdirectory.

Creates a File object for the child under the directory parent. parent is a File object. In the preceding constructor, the parent is a string.

Returns true if the file or the directory represented by the File object exists.

Returns true if the file represented by the File object exists and can be read.

Returns true if the file represented by the File object exists and can be written.

Returns true if the File object represents a directory.

Returns true if the File object represents a file.

Returns true if the File object is created using an absolute path name.

Returns true if the file represented in the File object is hidden. The exact definition of hidden is system-dependent. On Windows, you can mark a file hidden in the File Properties dialog box. On Unix systems, a file is hidden if its name begins with a period character '.'.

Returns the complete absolute file or directory name represented by the File object.

Returns the same as getAbsolutePath() except that it removes redundant names, such as "." and "..", from the pathname, resolves symbolic links (on Unix platforms), and converts drive letters to standard uppercase (on Win32 platforms).

Returns the last name of the complete directory and file name represented by the File object. For example, new File("c:\\book\\test.dat").getName() returns test.dat.

Returns the complete directory and file name represented by the File object. For example, new File("c:\\book\\test.dat").getPath() returns c:\book\test.dat.

Returns the complete parent directory of the current directory or the file represented by the File object. For example, new File("c:\\book\\test.dat").getParent() returns c:\book.

Returns the time that the file was last modified.

Deletes this file. The method returns true if the deletion succeeds.

Renames this file. The method returns true if the operation succeeds.

Obtaining file properties and manipulating files

04/21/23 55COSC-1336, Lecture 6

Problem: Explore File Properties Objective: Write a program that demonstrates how to create files in a platform-independent way and use the methods in the File class to obtain their properties.

04/21/23 COSC-1336, Lecture 6 56

public class TestFileClass {


java.io.File file = new java.io.File("image/us.gif");

System.out.println("Does it exist? " + file.exists());

System.out.println("The file has " + file.length() + " bytes");

System.out.println("Can it be read? " + file.canRead());

System.out.println("Can it be written? " + file.canWrite());

System.out.println("Is it a directory? " + file.isDirectory());

System.out.println("Is it a file? " + file.isFile());

System.out.println("Is it absolute? " + file.isAbsolute());

System.out.println("Is it hidden? " + file.isHidden());

System.out.println("Absolute path is " + file.getAbsolutePath());

System.out.println("Last modified on " + new java.util.Date(file.lastModified()));

}

}

04/21/23 COSC-1336, Lecture 6 57

Left: run on Windows; right: run on Unix

04/21/23 COSC-1336, Lecture 6 58

Text Input/Output (I/O) 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. The objects contain the methods for reading/writing data from/to a file. This section introduces how to read/write strings and numeric values from/to a text file using the Scanner and PrintWriter classes.

04/21/23 59COSC-1336, Lecture 6

Writing Data Using PrintWriter

java.io.PrintWriter

+PrintWriter(filename: String)

+print(s: String): void

+print(c: char): void

+print(cArray: char[]): void

+print(i: int): void

+print(l: long): void

+print(f: float): void

+print(d: double): void

+print(b: boolean): void

Also contains the overloaded println methods.

Also contains the overloaded printf methods.

.

Creates a PrintWriter for the specified file.

Writes a string.

Writes a character.

Writes an array of character.

Writes an int value.

Writes a long value.

Writes a float value.

Writes a double value.

Writes a boolean value.

A println method acts like a print method; additionally it prints a line separator. The line separator string is defined by the system. It is \r\n on Windows and \n on Unix.

The printf method was introduced in §3.6, “Formatting Console Output and Strings.”

04/21/23 60COSC-1336, Lecture 6

WriteData.javapublic class WriteData {

public static void main(String[] args) throws Exception {

java.io.File file = new java.io.File("scores.txt");

if (file.exists()) {

System.out.println("File already exists");

System.exit(0); }

// Create a file

java.io.PrintWriter output = new java.io.PrintWriter(file);

output.print("John T Smith ");

output.println(90);

output.print("Eric K Jones ");

output.println(85);

output.close();

}

}

04/21/23 COSC-1336, Lecture 6 61

Running and testing WriteData.java

04/21/23 COSC-1336, Lecture 6 62

Reading Data Using Scanner

java.util.Scanner

+Scanner(source: File)

+Scanner(source: String)

+close()

+hasNext(): boolean

+next(): String

+nextByte(): byte

+nextShort(): short

+nextInt(): int

+nextLong(): long

+nextFloat(): float

+nextDouble(): double

+useDelimiter(pattern: String): Scanner

Creates a Scanner that produces values scanned from the specified file.

Creates a Scanner that produces values scanned from the specified string.

Closes this scanner.

Returns true if this scanner has another token in its input.

Returns next token as a string.

Returns next token as a byte.

Returns next token as a short.

Returns next token as an int.

Returns next token as a long.

Returns next token as a float.

Returns next token as a double.

Sets this scanner’s delimiting pattern.

04/21/23 63COSC-1336, Lecture 6


public class ReadData {


java.io.File file = new java.io.File("scores.txt");

Scanner input = new Scanner(file);

while (input.hasNext()) {

String firstName = input.next();

String mi = input.next();

String lastName = input.next();

int score = input.nextInt();

System.out.println(firstName + " " + mi + " " +

lastName + " " + score);

}

input.close();

}

}04/21/23 COSC-1336, Lecture 6 64

Running and testing ReadData.java

04/21/23 COSC-1336, Lecture 6 65

ReadFileUsingJFileChooser.java


import javax.swing.JFileChooser;

public class ReadFileUsingJFileChooser {


JFileChooser fileChooser = new JFileChooser();

if (fileChooser.showOpenDialog(null) ==

JFileChooser.APPROVE_OPTION) {

// Get the selected file

java.io.File file = fileChooser.getSelectedFile();

// Create a Scanner for the file

Scanner input = new Scanner(file);

04/21/23 COSC-1336, Lecture 6 66

ReadFileUsingJFileChooser.java

(cont.) // Read text from the file

while (input.hasNext()) {

System.out.println(input.nextLine());

}

// Close the file

input.close();

}

else {

System.out.println("No file selected");

}

}

}

04/21/23 COSC-1336, Lecture 6 67

Running and testing ReadFileUsingJFileChooser.java

04/21/23 COSC-1336, Lecture 6 68

Summary To use the String class to process fixed

strings (§9.2). To use the Character class to process a

single character (§9.3). To use the StringBuilder/StringBuffer class

to process flexible strings (§9.4). To distinguish among the String,

StringBuilder, and StringBuffer classes (§9.2-9.4).

To learn how to pass arguments to the main method from the command line (§9.5).

04/21/23 69COSC-1336, Lecture 6

Summary (cont.) To discover file properties and to delete

and rename files using the File class (§9.6).

To write data to a file using the PrintWriter class (§9.7.1).

To read data from a file using the Scanner class (§9.7.2).

(GUI) To open files using a dialog box (§9.8).

04/21/23 70COSC-1336, Lecture 6

Reading suggestions

From [Liang: Introduction to Java programming: Eight Edition, 2011 Pearson Education, 0132130807]

Chapter 9 (Strings and Text I/O)

04/21/23 71COSC-1336, Lecture 6

Coming up next

From [Liang: Introduction to Java programming: Eight Edition, 2011 Pearson Education, 0132130807] Chapter 6 (Arrays)

04/21/23 72COSC-1336, Lecture 6

Thank you for your attention!

Questions?

04/21/23 73COSC-1336, Lecture 6

Documents

Stefan Andrei