3. Essential Java Classes. 1. Strings презентация

3. Essential Java Classes 1. Strings

Strings Strings are a sequence of characters. In the Java strings are objects. The simplest way to create a string String greeting = "Hello world!";

String Length String palindrome = "А роза упала на лапу Азора"; int len = palindrome.length();

Concatenation and addition int a = 5; int b = 3; String S = "-output-"; System.out.println (S + a + b); System.out.println (a + b + S); What will be the output?

Converting Numbers to Strings The simplest way: String s = "" + 6; Another way: String tp = String.valueOf(18.3); Formatting: String mes = String.format("Result = %d", 25);

Strings Comparing (1 of 3) String s1 = "hello"; String s2 = "hello"; if (s1 == s2) System.out.println(“Equal”); else System.out.println(“Not equal”); What will be the output?

Strings Comparing (2 of 3) String s1 = "hello"; String s2 = new String("hello“); if (s1 == s2) System.out.println(“Equal”); else System.out.println(“Not equal”); What will be the output?

Strings Comparing (3 of 3) You should use equals method instead of == String s1 = "hello"; String s2 = new String("hello“); if (s1.equals(s2)) System.out.println(“Equal”); else System.out.println(“Not equal”);

Some String Methods indexOf(String subString) substring(int posBeg, int posEnd) toUpperCase(), toLowerCase() trim() replace(CharSequence targ, CharSequence replace) split(String regex) valueOf(value) – static

indexOf method String palindrome = "Niagara. O roar again!"; System.out.println(palindrome.indexOf("roar")); What will be the output?

substring Method String palindrome = "Niagara. O roar again!"; System.out.println(palindrome.substring(11, 15)); System.out.println(palindrome.substring(11)); What will be the output?

replace Method String s = "Niagara. O roar again!"; s = s.replace("a", "A"); System.out.println(s); What will be the output?

split Method String palindrome = "Niagara. O roar again!"; String[] txt = palindrome.split("r"); for (String t : txt){ System.out.println(t); } What will be the output?

String Methods See http://docs.oracle.com/javase/7/docs/api/java/lang/String.html for details

Exercise 3.1.1. Write a program that computes your initials from your full name and displays them

Exercise 3.1.1. See 311Initials project for the full text.

Strings and Arrays It is impossible to work with strings as with arrays: String s="hello"; System.out.println(s[2]); // compile error From string to char array: char[] sArray = s.toCharArray(); From char array to string: String helloString = new String(sArray);

String Formatting String s1 = ""; s1 = String.format("a =%1$3d, b =%2$7.2f, b = %2$6.4e", 12, 122.589); Output: a = 12, b = 122,59, b = 1.2259e+02 s1 = String.format("a =%1$3d, a =%1$4o, a = %1$2x", 43); Output: a = 43, a = 53, a = 2b See http://docs.oracle.com/javase/7/docs/api/java/util/Formatter.html for details

Format String The format string consists of static text embedded with format specifiers Except for the format specifiers, the format string is output unchanged Format specifiers begin with a % and end with a 1- or 2-character conversion that specifies the kind of formatted output being generated

Format Specifiers d formats an integer value as a decimal value. f formats a floating point value as a decimal value. n outputs a platform-specific line terminator s formats any value as a string x formats an integer as a hexadecimal value tD formats date

Examples of Format Specifiers System.out.format("%1f, %1$+012.10f %n", Math.PI); Output is 3.141593, +03.1415926536 System.out.format("%1$5s %2$7.5f", "e = ", Math.E); Output is e = 2.71828 See for detailes http://docs.oracle.com/javase/7/docs/api/java/util/Formatter.html#syntax

Application main Method Every application must contain a main method whose signature is: public static void main(String[] args) The main method accepts a single argument: an array of elements of type String This array is the mechanism through which the runtime system passes information to your application: java MyApp arg1 arg2

How to Run Application with Arguments in Eclipse Right click on project name in the Package Explorer and select Run As > Run Configuration Go to Arguments tab and write argument values in the Program arguments field Press Apply button, then Run button

Exercise 3.1.2. Create a program that will print every other argument given on the command line. If the program was executed with the following on the command line, java ArgumentSkipper one two three a b c d the program would print one three b d Consider how your program would operate when no arguments are given

Run Application java app/E312Arguments one two three a b c d

JAR Files The Java Archive (JAR) file format enables you to bundle multiple files into a single archive file Run JAR-packaged applications with the Java interpreter: java -jar jar-file

Create JAR File in Eclipse Open workspace with necessary project Menu item File / Export Choose Java / Runnable JAR file in “Select an export destination”, then Next Select your project in “Launch configuration” dropdown list Fill “Export destination” field with JAR file name (Browse button can be used) Click Finish button

Run Application java -jar ArgumentSkipper.jar one two three a b c d

String vs StringBuilder Objects of the String class are immutable. A new String object is created during string modification. StringBuilder objects can be modified. StringBuilder objects are more effective when a lot of string modifications are needed

StringBuilder Methods (1 of 2) indexOf(String subString) substring(int posBeg, int posEnd) length() The following are absent: trim() split(String regex) valueOf(value) – static

StringBuilder Methods (2 of 2) append(type arg) - appends the argument to the string insert(int offset, type arg) - inserts the second argument into the string from offset replace(int start, int end, String s) - replaces the specified characters in this string reverse() - reverses the sequence of characters in this string See http://docs.oracle.com/javase/7/docs/api/java/lang/StringBuilder.html for details

append Method StringBuilder s1 = new StringBuilder("Hello"); s1.append(" world!"); System.out.println(s1); What will be the output?

insert Method StringBuilder s1 = new StringBuilder("Niagara again!"); s1.insert(7, ". O roar"); System.out.println(s1); What will be the output?

replace Method StringBuilder s1 = new StringBuilder("Niagara. O roar again!"); s1.replace(7, 16, " "); System.out.println(s1); What will be the output?

reverse Method StringBuilder s1 = new StringBuilder("Niagara. O roar again!"); s1.reverse(); System.out.println(s1); What will be the output?

Exercise 3.1.3. A palindrome is a text phrase that spells the same thing backward and forward. The word redivider is a palindrome, since the word would spell the same even if the character sequence were reversed. Write a program that takes a word as an argument and reports whether the word is a palindrome

Home Exercise 3.1.4. Create a generateString method that gets an integer argument n and returns a string contains first n integer numbers with gaps between them Create a generateStringBuilder method that does the same with StringBuilder class Compare these methods performance (with help of System.nanoTime();)

Manuals http://docs.oracle.com/javase/tutorial/java/data/strings.html