java strings - stringbuilder

1Strings in Java

2OOP2 - Strings in Java

Strings in Java

String basics String methods Immutability and performance Formatting strings


String basics

A string in Java is a sequence of characters

Instead of character arrays, Java uses class representationof strings, hiding the internal structure that holds the data

String is one of the basic types, used not only for printingmessages, but also for advanced processing of input data


Java String class

String class is located in the default package, java.lang

String class is immutable: once created, a string cannot bechanged

- None of its methods actually change the string!

Additionally, String class is final (i.e. it cannot be extended)


Strings in Java



String methods

The String class is the only privileged class in Java

- An object can be created without the new keyword:

- Strings can be concatenated without calling a specialmethod:

In total, there are 15 constructors for creating a stringfrom another string, a character array, a byte array, etc.

The String class and other utility classes provide a big poolof methods, enabling easy string manipulation


Information methods

int length(): returns the length of this string int indexOf(): several different argument lists enable

searching for a position of a substring in the string fromthe beginning or specified position onwards

int lastIndexOf(): search for the last occurrence of asubstring from the end or specific position backwards

boolean endsWith(String suff) boolean startsWith(String pref) boolean equals(Object anObject) boolean equalsIgnoreCase(String anotherString) int compareTo(String anotherString) int compareToIgnoreCase(String anotherString)


Transformation methods Used for producing new string(s) from the original string Simpler methods include:

- String substring(int beginIndex, int endIndex)- String trim()- String concat(String str)- String toLowerCase()- String toUpperCase()

More advanced methods are:- String replace(CharSequence target, CharSequence replacement):

replaces every occurrence of target with replacement- String replaceFirst(String regex, String replacement): replaces the

first occurrence matching the specified regular expression- String replaceAll(String regex, String replacement)- String[] split(String regex): splits the string into sub-strings, using

the specified regular expression as a separator


Regular expressions

Regular expressions enable short representation of stringswith similar structure

The following constructs can be used to build a regex:- [abc]: characters a, b, and c- [^abc]: any character except a, b, and c (negation)- [a-z]: range- [a-zA-Z]: union of ranges- [a-z&&[^df]]: intersection (a through z, except d and f)- \d: digit, [0-9]- \s: whitespace character- \w: word character [a-zA-Z_0-9]-


String manipulation example


String manipulation example (cont)

Output:


Strings in Java



Immutability and performance

Immutable objects are convenient because severalreferences can safely point to the same object- That is, there is no danger of changing the object through

one reference without the others being aware of thechange

Disadvantages: less efficient, as for any change a new stringneeds to be created and the old one thrown away:

str Strings

Strings in Java


StringBuilder

A companion of the String class, StringBuilder representsa mutable sequence of characters

The principal operations on a StringBuilder are the appendand insert methods, which are overloaded so as to acceptdata of any type

The class also offers methods similar to those of the Stringclass, e.g. indexOf(), lastIndexOf(), charAt(), length(),replace(), substring(), etc.

In addition, there are methods that employ the mutabilityof StringBuilder, and that cannot be found in the Stringclass: delete(), deleteCharAt(), reverse(), setCharAt(), etc.


Advantages of using StringBuilder

Because strings are immutable, frequent updates of astring can severely affect the performance

Performance-wise, it is much better to use a StringBuilderin place of a String to store the statement underconstruction

To improve performance even further, the appropriatecapacity of the StringBuilder instance should be set- The capacity determines the size of memory to be pre-

allocated

- The more memory is pre-allocated, the less will therebe need for re-allocation (a slow operation)

- However, the memory should not be wasted!


Speed test: StringBuilder vs. String


Speed test results

Output:


Strings in Java


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Formatting strings

As of version 1.5, Java provides a powerful tool forgenerating formatted outputs

The functionality is available in form of a (but not limitedto):- Static format() method of the String class- System.out.printf() method- New Formatter class

Heavily inspired by the Cs printf() function, stringformatting provides support for:- Layout justification and alignment- Common formats for number, string, and date/time

data- Locale specific output


Formatting strings example

Output:

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Formatting functions

All mentioned formatting approaches operate in a similarfashion, and accept the same set of input parameters:- Format string: a string which may contain fixed text and

one or more format specifiers

- Argument list (optional): values to be inserted into theformat string

- Locale (optional): an object representing a specificgeographical, political, or cultural region (e.g. forcorrect date-to-string conversion)


Format specifiers

Format specifiers determine how the arguments should beformatted and inserted into the destination string

General syntax:

- %[index$][flags][width][.precision]conversion

- Note: specifiers within [ and ] are optional

Index: index of the argument to be used with this specifier- The first argument is 1$, the second 2$, etc.

Flags: a set of characters that modify the output format

- E.g. + indicates always show the sign of thenumber; - means left-justify the output, etc.

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Format specifiers (cont)

Width: The minimum number of characters to output

Precision: used to restrict the number of characters- E.g. show up to 3 decimal points

Conversion: indicates how the argument should beformatted

Note: not all specifiers work for all arguments- E.g. the precision cannot be used with date/time data


Conversion indicators

There exists around 50 conversion indicators, some ofwhich are:

hour for the 12-hour clocktimeth

hour of the day for the 24-hour clocktimetH

locale-specific short month namedatetb

locale-specific full month namedatetB

hexadecimal integerintegralx

scientific notationfloating pointe

formatted as a decimal numberfloating pointf

decimal integerintegrald

true or falseBooleanb or B

OutputRequired argument typeIndicator

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Formatting examples

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