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Chapter 3 Using Classes and Objects
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2 Creating Objects A variable holds either a primitive type or a reference to an object A class name can be used as a type to declare an object reference variable
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3 Remember… Primitive Data Types X 5 Primitive Data Type int x = 5; A primitive data type variable contains the value itself, but an object variable contains the address of the object
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4 References to Objects An object reference variable holds the address of an object String s = “Hello”; //same as: String s = new String(“Hello”); s “Hello” Memory location An object variable contains the address of the object
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5 Declaring the Object The object itself must be created separately String s; s Memory location
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6 Creating the Object itself The object itself must be created separately String s; s = “Hello”; s “Hello” Memory location
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7 Is this assignment? String s; String t; s = “hello”; t = s; s “Hello” Memory location t
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8 Creating Objects, the formula Generally, we use the new operator to create an object Object objectReference = new Object 0 or more Class name Class name parameters ( ); The identifier or variable name Specifications for the newly created object
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9 Creating Objects Generally, we use the new operator to create an object String name = new String (”Blanca J. Polo"); This keyword new calls the String constructor That is the one that creates/builds the object Creating an object is called instantiation An object is an instance of a particular class
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10 Invoking Methods We've seen that once an object has been instantiated, we can use the dot operator to invoke its methods int nameLength = 0; nameLength = name.length() A method may return a value The returned value can be used in an assignment or expression
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11 The main method is void A void method doesn’t return anything Return types can be of any Primitive Data Type Return types can be of any Object kind, either pre-made by JAVA or made by you
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12 Review: The difference ”Blanca Polo" Srting name int num38
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13 Primitive Data Type Assignment Review The act of assignment takes a copy of a value and stores it in a variable For primitive types: num1 38 num2 96 Before: num2 = num1; num1 38 num2 38 After:
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14 Object Assignment Review For object references, assignment copies the address: name2 = name1; name1 name2 Before: ”Tom Hanks" ”Kurt Russell" name1 name2 After: ”Tom Hanks"
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15 Aliases Two or more references that refer to the same object are called aliases of each other That creates an interesting situation: one object can be accessed using multiple reference variables Aliases can be useful, but should be managed carefully Changing an object through one reference changes it for all of its aliases, because there is really only one object
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16 Questions
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