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Java provides a String Class

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  1. Java provides a String Class The String class can be used to create String objects. String str1; //declare an object variable str1 = new String (“mary had a little lamb”); Strings are created SO OFTEN that Java provides a short cut for creating them (without using constructor)  str1 = “mary had a little lamb”; ** Object declaration and construction (creation) can be done in one statement: String str1 = new String(“mary had a little lamb”);

  2. Using a String Object String objects have many behaviors. One of these behaviors is called length . The length behavior of a String object provides the number of characters in the String. So: str1.length() provides the number of characters in the String object str1 And: System.out.println(“the length is” + str1.length() ); Prints the length is 22

  3. Design a Greeter Class Greeter : String The message attribute of the Greeter class will have a String for a value ………..

  4. Using a Greeter Class If we had a Greeter class, we could create & use Greeter objects like this: public class Application { public static void main (String[] args ) { Greeter gre1; Greeter gre2; // declare two Greeter object variables gre1 = new Greeter(“Hello, how are you!”); gre2 = new Greeter(“HOWDY!!”); System.out.println( gre1.sayGreeting() ); System.out.println( gre2.sayGreeting() ); } }

  5. Code the Greeter Class public class Greeter { //name of the class we are defining privateString message; //each object of type Greeter will need a variable to store it’s message public String sayGreeting( ) { // this behavior is available to all classes (public) // this behavior gives back a String // this behavior requires no information to be provided return message; //statement that executes when this behavior // is executed { We still need to add the constructor method………….

  6. the constructor method .. public Greeter(String themessage){ //accessible to other classes //requires a String,we’ll call it themessage message = themessage; //store the String which was provided // in the variable ‘message’ } }

  7. The entire Greeter class .. public class Greeter { private String message; public Greeter(String themessage){ message = themessage; } public String sayGreeting() { return message; } }

  8. Updated UML Greeter … the constructor needs to be provided with a String so that the message can be given an initial value … the sayGreeting method returns a String

  9. Instance Fields (Variables) public class Greeter { ...private String message; // called instance field // or instance variable } • access specifier (such as private) • type of variable (such as String) • name of variable (such as message)

  10. Method Definition • access specifier (such as public orprivate) public • return type (such as String or void) public String • method name (such as sayHello) public String sayHello • list of parameters (empty for sayHello) public StringsayHello ( ) • method body in { }

  11. Parameters public Greeter(String themessage){ message = themessage; } * themessage is the name we are giving to the String value which will be provided * we will store the value which is provided in our variable message when an Greeter object is created themessage is called a parameter

  12. Syntax Method Implementation • public class ClassName{ ... accessSpecifier returnType methodName(parameterType parameterName,...) { method body } ... }

  13. Syntax: The return Statement return expression; or return; • Example: return message; • Purpose: To specify the value that a method returns, and exit the method immediately. The return value becomes the value of the method call expression.

  14. The Constructor The constructor is a method used to create and object The constructor has the same name as the class (and no return type) The constructor is a method which provides initial values to the instance variables. public Greeter(String themessage){ message = themessage; }

  15. The Greeter class cannot execute (it is not an application class, there is no main method) • The Greeter class is will be used for creating objects, and then using them. we already saw a class which USES the Greeter class….

  16. this class can be used to test if Greeter works correctly!! public class Application { public static void main (String[] args ) { Greeter gre1; Greeter gre2; // declare two Greeter object variables gre1 = new Greeter(“Hello, how are you!”); gre2 = new Greeter(“HOWDY!!”); System.out.println( gre1.sayGreeting() ); System.out.println( gre2.sayGreeting() ); } }

  17. Another Class for the Greeter Class public class GreeterTest { public static void main(String [] args)) { Greeter worldGreeter = new Greeter(“World”); Greeter daveGreeter = new Greeter(“Dave”); System.out.println(worldGreeter.sayHello()); System.out.println(daveGreeter.sayHello()); } }

  18. Each object of type Greeter is an ‘instance’ of class Greeter. Each instance gets it’s own variable message, so message is called an ‘instance field or variable’.

  19. Accessing Instance Fields • The instance field message is PRIVATE. • That means it can only be used by statements inside the Greeter class definition. • The sayGreeting method of the Greeter class can access the private instance field: • public String sayGreeting() { return message;}

  20. The application class cannot access message public class GreeterTest{ public static void main(String[] args){ System.out.println(daveGreeter.message); // ERROR!!! } } • Encapsulation = Hiding data and providing access through methods