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Computer Science I Classes and Objects Professor: Evan Korth New York University

Computer Science I Classes and Objects Professor: Evan Korth New York University. Road Map. Class modifiers Garbage collection Naming conflicts this Reference members Reading: Liang 5: chapter 6: 6.9, 6.12 Liang 6: chapter 7: 7.10, 7.12, 7.13 Liang 7: chapter 7: 7.9, 7.10, 9.3, 9.4.

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Computer Science I Classes and Objects Professor: Evan Korth New York University

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  1. Computer Science IClasses and ObjectsProfessor: Evan KorthNew York University Evan Korth New York University

  2. Road Map • Class modifiers • Garbage collection • Naming conflicts • this • Reference members • Reading: • Liang 5: chapter 6: 6.9, 6.12 • Liang 6: chapter 7: 7.10, 7.12, 7.13 • Liang 7: chapter 7: 7.9, 7.10, 9.3, 9.4 Evan Korth New York University

  3. review • What does encapsulation mean? • What is a data member? • What is a method member? • What is the difference between an object and a class? • What does the following line of code do? • Integer i; • What is i above? • What happens if you make a class without a constructor? Evan Korth New York University

  4. Review (cont) • What do the following modifiers mean when applied to a data member? • final • static • public • private • What if there is no modifier? • What is the principle of least privilege? Evan Korth New York University

  5. Review (cont) • What data type does a set method usually return? • What parameter does a get method usually take? • A class has 3 objects instantiated, it also has a static variable called x and an instance variable called y. • How many x values are stored in memory? • How many y values are stored in memory? • What is the scope of an instance variable? • Can you call an instance method without an instance of the class? Evan Korth New York University

  6. Class modifiers • No modifier (default) means the class is visible in the package in which it is declared. • public means it is visible to everything. • There are two others (final and abstract) which we will discuss later in the semester. Evan Korth New York University

  7. Garbage collection • When an object is no longer referenced by any reference variable, that object is referred to as garbage. • Java automatically tracks garbage objects and frees its memory when the garbage collector runs. • We do not have direct control over when the garbage is collected. • We can suggest to the compiler to collect garbage but it is not guaranteed that it will run. • To suggest garbage collection we make the following method call: • System.gc(); Evan Korth New York University

  8. Anonymous objects • An object without a reference is called an anonymous object. • It is created, used and immediately marked as garbage. Evan Korth New York University

  9. Variable name conflicts • It is possible to have a variable name in a method with the same name as a data member in a class. • In such case, the local method variable “hides” the data member variable. Evan Korth New York University

  10. Keyword this • The keyword this is used within a class to refer to the specific instance of the class that is being used. • A variable in a class’ method that has the same name as a field will “shadow” the field. You can access the field using the this keyword. • You cannot use the this keyword in static methods. (why?) Evan Korth New York University

  11. Another use for this • this (args) in a constructor will invoke another constructor of that class. • If you call another constructor from a constructor, it must be the first line in the calling constructor. • This is useful when you overload your constructors. In general, a constructor with fewer parameters should call a constructor with more parameters. Evan Korth New York University

  12. Composition • The term composition refers to the practice of having an object as a data member within another object. • What is actually stored is a reference to the member object. (therefore we can have self referential objects) • The default value for a reference variable is null. Evan Korth New York University

  13. Passing reference variables to methods • All variables in Java are passed using call by value. However, since object variables are really references to objects, passing an object is simulated pass by reference. • Objects passed to a method and modified by that method will have the changes reflected in the calling method. • Primitive variables passed to a method and modified by that method will NOT have the changes reflected in the calling method. Evan Korth New York University

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