Characters and Strings Characters In Java a char is a primitive type that can hold one single character A character can be A letter or digit A punctuation mark A space tab newline or other whitespace A control character Control characters are holdovers from the days of teletypes char literals A char literal is written between single quotes a A 5 Some characters cannot be typed directly and must be written as an escape sequence Tab is t Newline is n Some characters must be escaped to prevent ambiguity Single quote is quote backslash quote quote Backslash is Additional character literals n newline t tab b backspace r return f form feed backslash single quote double quote Character encodings A character is represented as a pattern of bits The number of characters that can be represented depends on the number of bits used For a long time ASCII American Standard Code for Information Interchange has been used ASCII is a seven bit code allows 128 characters ASCII is barely enough for English Omits many useful characters Unicode Unicode is a new 16 bit two byte standard that is designed to replace ASCII Unicode provides a unique number for every character no matter what the platform no matter what the program no matter what the language Java uses Unicode to represent characters You should know that Java uses Unicode but Except for having these extra characters available it seldom makes any difference to how you program Unicode character literals The rest of the ASCII characters can be written as octal numbers from 0 to 377 Any Unicode character can be written as a hexadecimal number between u0000 and uFFFF Since there are over 64000 possible Unicode characters the list occupies an entire book This makes it hard to look up characters Unicode letters in any alphabet can be used in identifiers Glyphs and fonts A glyph is the printed representation of a character For example the letter A can be represented by any of the glyphs A A A A A font is a collection of glyphs Unicode describes characters not glyphs Strings A String is a kind of object and obeys all the rules for objects In addition there is extra syntax for string literals and string concatenation A string is made up of zero or more characters The string containing zero characters is called the empty string String literals A string literal consists of zero or more characters enclosed in double quotes Hello This is a String literal To put a double quote character inside a string it must be backslashed Wait he said Don t go Inside a string a single quote character does not need to be backslashed but it can be String concatenation Strings can be concatenated put together with the operator Hello name Anything added to a String is converted to a string and concatenated Concatenation is done left to right abc 3 5 gives abc35 3 5 abc gives 8abc 3 5 abc gives 35abc Newlines The character n represents a newline When printing to the screen you can go to a new line by printing a newline character You can also go to a new line by using System out println with no argument or with one argument When writing to a file you should avoid n and use System out println instead I ll explain this when we talk about file I O System out print and println System out println can be called with no arguments parameters or with one argument System out println is called with one argument The argument may be any of the 8 primitive types The argument may be any object Java can print any object but it doesn t always do a good job Java does a good job printing Strings Java typically does a poor job printing types you define Printing your objects In any class you can define the following instance method public String toString This method can return any string you choose If you have an instance x you can get its string representation by calling x toString If you define your toString method exactly as above it will be used by System out print and System out println Constructing a String You can construct a string by writing it as a literal This is special syntax to construct a String Since a string is an object you could construct it with new new String This also constructs a String But using new for constructing a string is foolish because you have to write the string as a literal to pass it in to the constructor You re doing the same work twice String methods This is only a sampling of string methods All are called as myString method params length the number of characters in the String charAt index the character at integer position index where index is between 0 and length 1 equals anotherString equality test because doesn t do quite what you expect Don t learn all 48 String methods unless you use them a lot instead learn to use the API Vocabulary escape sequence a code sequence for a character beginning with a backslash ASCII an 7 bit standard for encoding characters Unicode a 16 bit standard for encoding characters glyph the printed representation of a character font a collection of glyphs empty string a string containing no characters concatenate to join strings together The End
View Full Document
Unlocking...