Chapter 2  Using Objects 1 Chapter 2 Using Objects.

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  • Slide 1
  • Chapter 2 Using Objects 1 Chapter 2 Using Objects
  • Slide 2
  • Chapter 2 Using Objects 2 Chapter Goals Learn about variables Type of variables, assignment operators, etc. Classes, objects and methods Methods parameters and return values Accessor and mutator methods Number types
  • Slide 3
  • Chapter 2 Using Objects 3 Chapter Goals Constructing objects Write test programs Browse the API documentation Objects versus object references
  • Slide 4
  • Chapter 2 Using Objects 4 Types and Variables Every value has a type and a name Variable declaration examples: Variables Store values Can be used in place of the objects they store String greeting = "Hello, World!"; PrintStream printer = System.out; int luckyNumber = 13;
  • Slide 5
  • Chapter 2 Using Objects 5 Syntax 2.1: Variable Definition typeName variableName = value; or typeName variableName; Example: String greeting = "Hello, Dave!"; Purpose: To define a new variable of a particular type, (optionally) supply an initial value
  • Slide 6
  • Chapter 2 Using Objects 6 Identifiers Identifier Name of a variable, method, or class Rules for identifiers in Java Can be made up of letters, digits, and the underscore (_) character Cannot start with a digit Cannot use other symbols such as ? or % Spaces are not permitted inside identifiers You cannot use reserved words Identifiers are case sensitive
  • Slide 7
  • Chapter 2 Using Objects 7 Identifiers By convention, variable names start with a lowercase letter But things like luckyNumber are OK This is sometimes called camel case By convention, class names start with an uppercase letter
  • Slide 8
  • Chapter 2 Using Objects 8 Self Check 1. What is the type of the values 0 and "0" ? 2. Which of the following are legal identifiers? 3. Define a variable to hold your name. Use camel case in the variable name. Greeting1 g void 101dalmatians Hello, World
  • Slide 9
  • Chapter 2 Using Objects 9 Answers 1. int and String 2. Only the first two are legal identifiers String myName = "John Q. Public";
  • Slide 10
  • Chapter 2 Using Objects 10 The Assignment Operator Assignment operator is = Not used as a statement about equality a = b assigns value of b to a Not the same meaning as = in math Used to change the value of a variable int luckyNumber = 13; luckyNumber = 12;
  • Slide 11
  • Chapter 2 Using Objects 11 Uninitialized Variables Compile time error Figure 2: An Uninitialized Object Variable int luckyNumber; System.out.println(luckyNumber); // ERROR - uninitialized variable
  • Slide 12
  • Chapter 2 Using Objects 12 Initialized Variable To initialize a variable Or, better yet int luckyNumber; luckyNumber = 13; System.out.println(luckyNumber); int luckyNumber = 13; System.out.println(luckyNumber);
  • Slide 13
  • Chapter 2 Using Objects 13 Syntax 2.2: Assignment variableName = value; Example: luckyNumber = 12; Purpose: To assign a new value to a previously defined variable. Assign a value to a variable
  • Slide 14
  • Chapter 2 Using Objects 14 Self Check 4. Is 12 = 12 a valid expression in the Java language? 5. How do you change the value of the greeting variable to "Hello, Nina!" ?
  • Slide 15
  • Chapter 2 Using Objects 15 Answers 4. No, the left-hand side of the = operator must be a variable 5. Note that is not the right answerthat statement defines a new variable greeting = "Hello, Nina!"; String greeting = "Hello, Nina!";
  • Slide 16
  • Chapter 2 Using Objects 16 Objects and Classes Object: entity that you can manipulate in your programs (by calling a method or methods) Each object belongs to a class. For example, System.out belongs to the class PrintStream
  • Slide 17
  • Chapter 2 Using Objects 17 Methods Method: Sequence of instructions that accesses the data of an object You manipulate objects by calling methods Class: Set of objects with the same behavior Class defines methods that can be applied to an object String greeting = "Hello"; greeting.println() // Error greeting.length() // OK println not in String class length is in String class
  • Slide 18
  • Chapter 2 Using Objects 18 Methods Public Interface: Specifies what you can do with the objects of a class Private (hidden) implementation describes how these actions are carried out
  • Slide 19
  • Chapter 2 Using Objects 19 Representation of 2 String Objects Figure 4: A Representation of Two String Objects
  • Slide 20
  • Chapter 2 Using Objects 20 Example of a String Method length: counts the number of characters in a string String greeting = "Hello, World!"; int n = greeting.length(); // sets n to 13
  • Slide 21
  • Chapter 2 Using Objects 21 String Methods toUpperCase: creates another String object that contains the characters of the original string, with lowercase letters converted to uppercase String river = "Mississippi"; String bigRiver = river.toUpperCase(); // sets bigRiver to "MISSISSIPPI"
  • Slide 22
  • Chapter 2 Using Objects 22 String Methods When applying a method to an object, method must be defined in the appropriate class length is not defined in PrintStream class to which System.out belongs System.out.length(); // This method call is an error
  • Slide 23
  • Chapter 2 Using Objects 23 Self Check 6. How can you compute the length of the string "Mississippi" ? 7. How can you print out the uppercase version of "Hello, World!" ? 8. Is it legal to call river.println() ? Why or why not?
  • Slide 24
  • Chapter 2 Using Objects 24 Answers 6. 7. 8. It is not legal. The variable river has type String and the println method is not a method of the String class. river.length() or "Mississippi".length() System.out.println(greeting.toUpperCase());
  • Slide 25
  • Chapter 2 Using Objects 25 Implicit and Explicit Parameters Parameter (explicit parameter) is an input to a method. Not all methods have explicit parameters. The object on which a method is invoked is an implicit parameter System.out.println(greeting) greeting.length() // has no explicit parameter System.out.println(greeting)
  • Slide 26
  • Chapter 2 Using Objects 26 Implicit and Explicit Parameters Figure 5: Passing a parameter to the println method
  • Slide 27
  • Chapter 2 Using Objects 27 Return Values A return value is a result that the method has computed for use by the code that called it int n = greeting.length(); // return value stored in n
  • Slide 28
  • Chapter 2 Using Objects 28 Return Values Figure 6: Invoking the length Method on a String Object
  • Slide 29
  • Chapter 2 Using Objects 29 Passing Return Values You can also use the return value as a parameter of another method Not all methods return values. For example, System.out.println(greeting.length()); println
  • Slide 30
  • Chapter 2 Using Objects 30 Passing Return Values Figure 7: Passing the Result of a Method Call to Another Method
  • Slide 31
  • Chapter 2 Using Objects 31 A More Complex Call replace method carries out a search-and- replace operation This method call has one implicit parameter: string "Mississippi" two explicit parameters: "issipp" and "our" a return value: the string "Missouri" river.replace("issipp", "our") // constructs a new string ("Missouri")
  • Slide 32
  • Chapter 2 Using Objects 32 A More Complex Call Figure 8: Calling the replace Method
  • Slide 33
  • Chapter 2 Using Objects 33 Method Definitions Method definition specifies types of explicit parameters and return value Type of implicit parameter is current class, so not mentioned in method definition
  • Slide 34
  • Chapter 2 Using Objects 34 Method Definitions Example: Class String defines Search for target string and replace with replacement at each occurrence of string public int length() // return type: int // no explicit parameter public String replace(String target, String replacement) // return type: String; // two explicit parameters of type String
  • Slide 35
  • Chapter 2 Using Objects 35 Method Definitions If method returns no value, the return type is declared as void A method name is overloaded if a class has more than one method with the same name (but different parameter types) public void println(String output) // in class PrintStream public void println(String output) public void println(int output)
  • Slide 36
  • Chapter 2 Using Objects 36 Self Check 9. What are the implicit parameters, explicit parameters, and return values in the method call river.length() ? 10. What is the result of the call river.replace("p", "s")? 11. What is the result of the call greeting.replace("World","Dave").length() ? 12. How is the toUpperCase method defined in the String class?
  • Slide 37
  • Chapter 2 Using Objects 37 Answers 9. The implicit parameter is river. There is no explicit parameter and return value is 11 10. "Missississi" 11. 12 12. As public String toUpperCase() with no explicit parameter and return type String
  • Slide 38
  • Chapter 2 Using Objects 38 Number Types Integers: short, int, long For example 13 Floating point numbers: float, double For example 1.3 0.00013
  • Slide 39
  • Chapter 2 Using Objects 39 Number Types When a floating-point number is multiplied or divided by 10, only the position of the decimal point changes; it "floats". This representation is related to the scientific notation 1.3 10 -4. Numbers are not objects; numbers types are primitive types 1.3E-4 // 1.3 10 -4 written in Java
  • Slide 40
  • Chapter 2 Using Objects 40 Arithmetic Operations Math operators: + - As in math, the operator binds more strongly than the + or - operator 10 + n n - 1 10 * n // 10 x n x + y * 2 // means the sum of x and y * 2 (x + y) * 2 // multiplies the sum of x and y with 2
  • Slide 41
  • Chapter 2 Using Objects 41 Self Check 13. Which number type would you use for storing the area of a circle? 14. Why is the expression 13.println() an error? 15. Write an expression to compute the average of the values x and y.
  • Slide 42
  • Chapter 2 Using Objects 42 Answers 13. double 14. An int is not an object, and you cannot call a method on it 15. (x + y) * 0.5
  • Slide 43
  • Chapter 2 Using Objects 43 Rectangular Shapes and Rectangle Objects Objects of type Rectangle describe rectangular shapes Figure 9: Rectangular Shapes
  • Slide 44
  • Chapter 2 Using Objects 44 Rectangular Shapes and Rectangle Objects A Rectangle object is not a shape It is an object that contains a set of numbers that describe a rectangle Figure 10: Rectangular Objects
  • Slide 45
  • Chapter 2 Using Objects 45 Constructing Objects Details 1.The new operator makes a Rectangle object 2.It uses the parameters (in this example, 5, 10, 20, and 30 ) to initialize the data of the object 3.It returns the object Usually the output of the new operator is stored in a variable Rectangle box = new Rectangle(5, 10, 20, 30); new Rectangle(5, 10, 20, 30)
  • Slide 46
  • Chapter 2 Using Objects 46 Constructing Objects The process of creating a new object is called construction (since it uses a constructor method) The four values 5, 10, 20, and 30 are called the construction parameters Some classes let you construct objects in multiple ways new Rectangle() // constructs a rectangle with its top-left corner // at the origin (0, 0), width 0, and height 0
  • Slide 47
  • Chapter 2 Using Objects 47 Syntax 2.3: Object Construction new ClassName(parameters) Example: new Rectangle(5, 10, 20, 30) new Rectangle() Purpose: To construct a new object, initialize it with the construction parameters, and return a reference to the constructed object
  • Slide 48
  • Chapter 2 Using Objects 48 Self Check 16. How do you construct a square with center (100, 100) and side length 20? 17. What does the following statement print? System.out.println(new Rectangle().getWidth());
  • Slide 49
  • Chapter 2 Using Objects 49 Answers 16. 17. 0 new Rectangle(90, 90, 20, 20)
  • Slide 50
  • Chapter 2 Using Objects 50 Accessor and Mutator Methods An accessor method does not change the state of its implicit parameter A mutator method: changes the state of its implicit parameter double width = box.getWidth(); box.translate(15, 25);
  • Slide 51
  • Chapter 2 Using Objects 51 Accessor and Mutator Methods Figure 11: Using the translate Method to Move a Rectangle
  • Slide 52
  • Chapter 2 Using Objects 52 Self Check 18. Is the toUpperCase method of the String class an accessor or a mutator? 19. Which call to translate is needed to move the box rectangle so that its top-left corner is the origin (0, 0)? Recall that Rectangle box = new Rectangle(5, 10, 20, 30);
  • Slide 53
  • Chapter 2 Using Objects 53 Answers 18. An accessor since it does not modify the original string (it returns a new string with uppercase letters) 19. box.translate(-5, -10) (provided the box rectangle has not been changed)
  • Slide 54
  • Chapter 2 Using Objects 54 Implementing a Test Program Create a new class Supply a main method Inside the main method, construct one or more objects Apply methods to the objects Display the results of the method calls
  • Slide 55
  • Chapter 2 Using Objects 55 Importing Packages Must include appropriate packages Java classes are grouped into packages Import library classes by specifying the package and class name, for example, You do not need to import classes in the java.lang package such as String and System import java.awt.Rectangle;
  • Slide 56
  • Chapter 2 Using Objects 56 Importing a Class from a Package import packageName.ClassName; Example: import java.awt.Rectangle; Purpose: To import a class from a package for use in a program.
  • Slide 57
  • Chapter 2 Using Objects 57 File MoveTester.java 01: import java.awt.Rectangle; 02: 03: public class MoveTester 04: { 05: public static void main(String[] args) 06: { 07: Rectangle box = new Rectangle(5, 10, 20, 30); 08: 09: // Move the rectangle 10: box.translate(15, 25); 11: 12: // Print information about the moved rectangle 13: System.out.println("After moving, the top-left corner is:"); 14: System.out.println(box.getX()); 15: System.out.println(box.getY()); 16: } 17: }
  • Slide 58
  • Chapter 2 Using Objects 58 Self Check 20. The Random class is defined in the java.util package. What do you need to do...

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