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String class

String class. What data types have we seen so far?. Data types we’ve seen so far…. integer types: int (also short, char, and byte) real numbers: double and float boolean. Introducing the String. S tring Case sensitive (capital S is required ) Declaring a String

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String class

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  1. String class

  2. What data types have we seen so far?

  3. Data types we’ve seen so far… integer types: int (also short, char, and byte) real numbers: double and float boolean

  4. Introducing the String String Case sensitive (capital S is required) Declaring a String How did we declare variables of the other types (such as int, double, boolean) in the past?

  5. Declarations How have we already declared variables? int i; float f; double x; boolean isAChicken; So how do we declare Strings?

  6. Declaring Strings Declare String s; Declare and initialize (assign) String name = “John”;

  7. The Class String • String is NOT a primitive type! • The class String is a predefined class in Java that is used to store and process strings • Objects of type String are made up of strings of characters that are written within double quotes • Any quoted string is a constant of type String "Live long and prosper." • A variable of type String can be given the value of a String object String blessing = "Live long and prosper.";

  8. Concatenation of Strings • Concatenation: Using the + operator on two strings in order to connect them to form one longer string • If greeting is equal to "Hello", and javaClass is equal to "class", then greeting + javaClass is equal to "Hello class" • Any number of strings can be concatenated together • When a string is combined with almost any other type of item, the result is a string • "The answer is " + 42 evaluates to "The answer is 42"

  9. Output • int answer = 2 + 2; System.out.println(answer); //when argument to println int variable answer is //automatically converted to a string and output as “4” • System.out.println(“2 plus 2 is “ + answer); //“+” operator here “concatenates” (connects) two strings.

  10. Output • int x = 2; int y = 2; System.out.println(x + y); //expresssion is evaluated and “4” is //printed // “+” operator here is the addition operator!!!

  11. Classes, Objects, and Methods • A class is the name for a type whose values are objects • Objects are entities that store data and take actions • Objects of the String class store data consisting of strings of characters • The actions that an object can take are called methods • Methods perform an action • Methods can return (produce) a value of a single type

  12. Classes, Objects, and Methods • Recall: Invoking or calling a method: a method is called into action by writing the name of the calling object, followed by a dot, followed by the method name, followed by parentheses • This is sometimes referred to as sending a message to the object • The parentheses contain the information (if any) needed by the method • This information is called an argument (or arguments)

  13. String Methods • The String class contains many useful methods for string-processing applications • A String method is called by writing a String object, a dot, the name of the method, and a pair of parentheses to enclose any arguments

  14. String methods Useful String method: testing for equality String s = “hello”; s.equals(“help”); s.equals(“Hello”); What type of value does equals(..) return??

  15. String methods Useful String method: testing for equality String s = “hello”; s.equals(“help”); //returns false s.equals(“Hello”); //returns false

  16. String methods The value returned by a method may be stored in a variable of correct type. String s = “hello”; boolean returnValue = s.equals(“help”); //returns false

  17. String methods equals() is case sensitive (i.e., it distinguishes between upper and lower case) “hello” is not the same as “Hello” (Don’t use == to compare two string objects)

  18. String methods Wouldn’t it be nice to have a method that is like equals() but ignores case? “hello” is the same as “Hello” What would be a good name for such a method?

  19. String s = “HeLlo”; s.equalsIgnoreCase(“hello”); //returns true

  20. More String methods length() startsWith() endsWith() toLowerCase() toUpperCase() charAt() substring()

  21. String method: length() What (type of thing) does length() return?

  22. String method: length() What (type of thing) does length() return? An integer int length() Return type is int No arguments

  23. String Methods • If a String method returns a value, then it can be placed anywhere that a value of its type can be used String greeting = "Hello"; int count = greeting.length(); //count’s value is 5

  24. String Methods String greeting = "Hello"; System.out.println("Length is " + greeting.length());

  25. String Indexes

  26. More string methods charAt() char charAt ( int index ) Returns the char value at the specified index (0..N-1). String name = “John”; char c = name.charAt(2); //returns ‘h’

  27. More string methods substring() (Two of them.) String substring ( int beginIndex ) Returns a new string that is a substring of this string (from beginIndex to the end of the string). String substring ( int beginIndex, int endIndex ) Returns a new string that is a substring of this string (from beginIndex..endIndex-1).

  28. substring(..) • String example = “this is a long string.”; String substr = example.substring(8); //returns “a long string.”

  29. substring(..) • String example = “this is a long string.”; String substr = example.substring(1, 4); //returns “his”

  30. More String methods startsWith() and endsWith() Returns true if the string starts with (ends with) the specified string argument. String s1 = “dish”; String s2 = “dishes”; boolean plural = s1.endsWith(“es”);

  31. More String functions: toLowerCase() and toUpperCase() String s1 = “dIsHeS”; String s2 = s1.toLowerCase(); //s2 is “dishes”

  32. And many more!

  33. Escape Sequences • A backslash (\) immediately preceding a character (i.e., without any space) denotes an escape sequence or an escape character • The character following the backslash does not have its usual meaning • Although it is formed using two symbols, it is regarded as a single character

  34. Escape Sequences

  35. String Processing • A String object in Java is considered to be immutable, i.e., the characters it contains cannot be changed • There is another class in Java called StringBuffer that has methods for editing its string objects • However, it is possible to change the value of a String variable by using an assignment statement String name = "Soprano"; name = "Anthony " + name;

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