Strings

Java MethodsJava MethodsA & ABA & AB

Object-Oriented Programmingand Data Structures

Maria Litvin ● Gary Litvin

Copyright © 2006 by Maria Litvin, Gary Litvin, and Skylight Publishing. All rights reserved.

"Chapter 10"

10-2

Objectives:

• Learn about literal strings

• Learn about String constructors and commonly used methods

• Understand immutability of strings

• Learn to format numbers into strings and extract numbers from strings

• Learn several useful methods of the Character class

• Learn about the StringBuffer class

10-3

The String class

• An object of the String class represents a string of characters.

• The String class belongs to the java.lang package, which is built into Java.

• Like other classes, String has constructors and methods.

• Unlike other classes, String has two operators, + and += (used for concatenation).

10-4

Literal Strings

• Literal strings are anonymous constant objects of the String class that are defined as text in double quotes.

• Literal strings don’t have to be constructed: they are “just there.”

10-5

Literal Strings (cont’d)

• can be assigned to String variables.

• can be passed to methods and constructors as parameters.

• have methods you can call:

String fileName = "fish.dat";

button = new JButton("Next slide");

if ("Start".equals(cmd)) ...

10-6

Literal Strings (cont’d)

• The string text may include “escape” characters (described in Section 6.5). For example: \\ stands for \ \n stands for the newline character

String s1 = "Biology”;String s2 = "C:\\jdk1.4\\docs";String s3 = "Hello\n";

10-7

Immutability

• Once created, a string cannot be changed: none of its methods changes the string.

• Such objects are called immutable.

• Immutable objects are convenient because several references can point to the same object safely: there is no danger of changing an object through one reference without the others being aware of the change.

10-8

Immutability (cont’d)

• Advantage: more efficient, no need to copy.

String s1 = "Sun"; String s2 = s1;

String s1 = "Sun"; String s2 = new String(s1);

s1

s2

s1

s2

OK Less efficient: wastes memory

"Sun""Sun"

"Sun"

10-9

Immutability (cont’d)

• Disadvantage: less efficient — you need to create a new string and throw away the old one for every small change.

String s = "sun"; char ch = Character.toUpperCase(s.charAt (0)); s = ch + s.substring (1);

s "sun"

"Sun"

10-10

Empty Strings

• An empty string has no characters; its length is 0.

• Not to be confused with an uninitialized string:

String s1 = ""; String s2 = new String();

private String errorMsg; errorMsg is null

Empty strings

10-11

Constructors

• String’s no-args and copy constructors are not used much.

• Other constructors convert arrays into strings (Chapter 12)

String s1 = new String ();

String s2 = new String (s1);

String s1 = "";

String s2 = s1;

10-12

Methods — length, charAtint length ();

char charAt (k);

• Returns the number of characters in the string

• Returns the k-th char

6

’n'

”Flower".length();

”Wind".charAt (2);

Returns:

Character positions in strings are numbered starting from 0

10-13

Methods — substringString s2 = s.substring (i, k);

returns the substring of chars in positions from i to k-1

String s2 = s.substring (i);

returns the substring from the i-th char to the end

"raw""happy""" (empty string)

”strawberry".substring (2,5); "unhappy".substring (2);"emptiness".substring (9);

Returns:

strawberry

i k

strawberry

i

10-14

Methods — Concatenation

String result = s1 + s2;concatenates s1 and s2

String result = s1.concat (s2);the same as s1 + s2

result += s3;concatenates s3 to result

result += num;converts num to String and concatenates it to result

10-15

Methods — Find (indexOf)

String date ="July 5, 2012 1:28:19 PM";

date.indexOf ('J'); 0

date.indexOf ('2'); 8

date.indexOf ("2012"); 8

date.indexOf ('2', 9); 11

date.indexOf ("2020"); -1

date.lastIndexOf ('2'); 15

Returns:

(not found)

(starts searching at position 9)

0 8 11 15

10-16

Methods — Comparisonsboolean b = s1.equals(s2);

returns true if the string s1 is equal to s2

boolean b = s1.equalsIgnoreCase(s2);returns true if the string s1 matches s2, case-blind

int diff = s1.compareTo(s2);returns the “difference” s1 - s2

int diff = s1.compareToIgnoreCase(s2);returns the “difference” s1 - s2, case-blind

10-17

Methods — ReplacementsString s2 = s1.trim ();

returns a new string formed from s1 by removing white space at both ends

String s2 = s1.replace(oldCh, newCh);returns a new string formed from s1 by replacing all occurrences of oldCh with newCh

String s2 = s1.toUpperCase();String s2 = s1.toLowerCase();

returns a new string formed from s1 by converting its characters to upper (lower) case

10-18

Replacements (cont’d)

• Example: how to convert s1 to upper case

• A common bug:

s1 = s1.toUpperCase();

s1.toUpperCase(); s1 remains unchanged

10-19

Numbers to Strings

• Three ways to convert a number into a string:1.

String s = "" + num;

2.

String s = Integer.toString (i);

String s = Double.toString (d);

3.

String s = String.valueOf (num);

Integer and Double are “wrapper” classes from java.lang that represent numbers as objects. They also provide useful static methods.

10-20

Numbers to Strings (cont’d)

• The DecimalFormat class can be used for formatting numbers into strings.

import java.text.DecimalFormat;... DecimalFormat money = new DecimalFormat("0.00"); ... double amt = 56.7381; ... String s = money.format (amt);

56.7381

"56.74"

10-21

Numbers to Strings (cont’d)

• Java 5.0 added printf and format methods:

int m = 5, d = 19, y = 2007;double amt = 123.5;

System.out.printf ( "Date: %02d/%02d/%d Amount = %7.2f\n", m, d, y, amt);

String s = String. format( "Date: %02d/%02d/%d Amount = %7.2f\n", m, d, y, amt);

Displays, sets s to:

"Date: 05/19/2007 Amount 123.50"

10-22

Numbers from Strings

• These methods throw a NumberFormatException if s does not represent a valid number (integer, real number, respectively).

String s1 = "-123", s2 = "123.45";

int n = Integer.parseInt(s1);

double x = Double.parseDouble(s2);

10-23

Numbers from Strings (cont’d)

• A safer way:

int n;do { try { n = Integer.parseInt(s); } catch (NumberFormatException ex) { System.out.println("Invalid input, reenter"); }} while (...);

10-24

Character Methods

• java.lang.Character is a “wrapper” class that represents characters as objects.

• Character has several useful static methods that determine the type of a character (letter, digit, etc.).

• Character also has methods that convert a letter to the upper or lower case.

10-25

Character Methods (cont’d)if (Character.isDigit (ch)) ...

.isLetter...

.isLetterOrDigit...

.isUpperCase...

.isLowerCase...

.isWhitespace...

return true if ch belongs to the corresponding category

Whitespace is space, tab, newline, etc.

10-26

Character methods (cont’d)char ch2 = Character.toUpperCase (ch1);

.toLowerCase (ch1);

if ch1 is a letter, returns its upper (lower) case; otherwise returns ch1

int d = Character.digit (ch, radix);

returns the int value of the digit ch in the given int radix

char ch = Character.forDigit (d, radix);

returns a char that represents int d in a given int radix

10-27

The StringBuffer Class

• Represents a string of characters as a mutable object

• Constructors:StringBuffer() // empty StringBuffer of the default capacity

StringBuffer(n) // empty StringBuffer of a given capacity

StringBuffer(str) // converts str into a StringBuffer

• Adds setCharAt, insert, append, and delete methods

• The toString method converts this StringBuffer into a String

10-28

Review:

• What makes the String class unusual?

• How can you include a double quote character into a literal string?

• Is "length".length() allowed syntax? If so, what is the returned value?

• Define immutable objects.

• Does immutability of Strings make Java more efficient or less efficient?

10-29

Review (cont’d):• How do you declare an empty string?

• Why are String constructors not used very often?

• If the value of String city is "Boston", what is returned by city.charAt (2)? By city.substring(2, 4)?

• How come String doesn’t have a setCharAt method?

• Is s1 += s2 the same as s1 = s1 + s2 for strings?

10-30

Review (cont’d):

• What do the indexOf methods do? Name a few overloaded versions.

• What is more efficient for strings: == and other relational operators or equals and compareTo methods?

• What does the trim method do?

• What does s.toUpperCase() do to s?

• What does the toString method return for a String object?

10-31

Review (cont’d):

• Name a simple way to convert a number into a string.

• Which class has a method for converting a String into an int?

• Name a few Character methods that help identify the category to which a given character belongs.

• What is the difference between the String and StringBuffer classes?