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Strings. An extension of types A class that encompasses a character array and provides many useful behaviors Chapter 9 Strings are IMMUTABLE. String Accessors (What does this mean?). message = "Welcome"; message.length() (gives us what?). Retrieving Characters in a String.
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Strings • An extension of types • A class that encompasses a character array and provides many useful behaviors • Chapter 9 • Strings are IMMUTABLE
String Accessors (What does this mean?) message = "Welcome"; message.length() (gives us what?)
Retrieving Characters in a String • Cannot use message[0](strings are immutable) • Use message.charAt(index) • Index starts from 0 Is charAt() a public or private method? • Can you write a loop for a string to print out each char?
String Comparisons String s1 = new String("Welcome“); String s2 = "Welcome"; if (s1.equals(s2)){ // do s1 and s2 have the same contents? } if (s1 == s2) { // do s1 and s2 have the same reference? }
String Comparisons, cont. • compareTo(Object object) String s1 = new String("Welcome“); String s2 = "hello"; if (s1.compareTo(s2) > 0) { // is s1 greater than s2? } else if (s1.compareTo(s2) == 0) { // do s1 and s2 have the same contents? } else { // s1 is lexicographically less than s2 }
Strings are Immutable! • Immutable means we can’t change them. • Can’t do: String str = “hello”; str[3] = ‘b’; • So then why does this work? String str = "Hello"; System.out.println(str); //Prints Hello str = "Help!"; System.out.println(str); //Prints Help!
Why can I do this?(Or, isn’t this mutating the string?) String str = "Mississippi"; System.out.println(str); //Prints Mississippi str = str.replace("i", "!"); System.out.println(str); //Prints M!ss!ss!pp!
String methods: charAt(int index) – gets the character at the index within the string and returns the char indexOf(char c)- sees if a character is within a string and returns an int representing the index indexOf(char c, intfromindex) - sees if a character is within a string from fromindex onwards length() – returns number of characters in a string (note that for strings this is a method. What is it for arrays?) substring(intbeginindex) – extracts and returns a substring from a string, starting at beginindex and going to the end of the string substring(intbeginindex, intendindex) – extracts and returns a substring from beginindex to endindex toLowerCase() – returns a string that is the lower case version of the string toUpperCase() – returns a string that is the upper case version of the string trim() - removes white space (space char) from both ends of a string and returns that string toCharArray() – converts string to character array and returns that array equalsIgnoreCase(String otherstring) – compares string with otherstring and returns Boolean value (true if equals, false if doesn’t) compareToIgnoreCase(String otherstring) – compares string with otherstring, and returns int (1 if string is “greater”, 0 if equal, and -1 if “less than”.
toString() method • Printing objects: • If you print an object that doesn’t have a toString() method, you print out the hash representation of the object • Not what we want. • All java built-in objects have a toString method implemented. • E.g., Color x = new Color(255,0,0); //Color is a built-in java class -RGBSystem.out.println(x.toString()); • Will print out: java.awt.Color[r=255,g=0,b=0] (this is the string that the toString method explicitly created and returned.) System.out.println(x)
toString() • The println() method can take an Object as an argument. • This version will implicitly call toString() for you and print the result. • Meaning, you can do: Color x = new Color( 255, 0, 0 );System.out.println( x ); • Also happens with concatenation, e.g.,: Color x = new Color( 255, 0, 0 );String str = “This is color: “; str += x; • toString() is a method that you should write when you create a class definition. • will automatically be used as above.
Wanna Try? (Quickly write toString()) public class StudentInfo { private String first; private String last; private String[] schedule; public StudentInfo(String f, String l, String[] sched) { first = f; last=l; schedule = sched; } //… }
My toString() public String toString() { String str = ""; str += first + "\t"+ last + "\n"; for (inti = 0; i < schedule.length; i++) { str+= schedule[i] "\t"; } return(str); } Note the \t and \n These are known as “escape sequences” • \t adds a tab • \n adds a new line (makes the printout go to the next line.
