chapter 2  using objects 1 chapter 2 using objects

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  • Slide 1
  • Chapter 2 Using Objects 1 Chapter 2 Using Objects
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  • 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
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  • Chapter 2 Using Objects 3 Chapter Goals Constructing objects Write test programs Browse the API documentation Objects versus object references
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  • 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;
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  • 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
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  • 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
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  • 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
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  • 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
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  • Chapter 2 Using Objects 9 Answers 1. int and String 2. Only the first two are legal identifiers String myName = "John Q. Public";
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  • 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;
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  • 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
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  • 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);
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  • 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
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  • 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!" ?
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  • 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!";
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  • 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
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  • 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
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  • 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
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  • Chapter 2 Using Objects 19 Representation of 2 String Objects Figure 4: A Representation of Two String Objects
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  • 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
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  • 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"
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  • 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
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  • 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?
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  • 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());
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  • 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)
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  • Chapter 2 Using Objects 26 Implicit and Explicit Parameters Figure 5: Passing a parameter to the println method
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  • 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
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  • Chapter 2 Using Objects 28 Return Values Figure 6: Invoking the length Method on a String Object
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  • 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
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  • Chapter 2 Using Objects 30 Passing Return Values Figure 7: Passing the Result of a Method Call to Another Method
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  • 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")
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  • Chapter 2 Using Objects 32 A More Complex Call Figure 8: Calling the replace Method
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  • 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
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  • 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
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  • 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)
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  • 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?
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  • 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
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  • Chapter 2 Using Objects 38 Number Types Inte


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