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Java in a Nutshell

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Classes and Objects in Java

3.11 Summary

This has been a long and detailed chapter. The following list summarizes the most important points to remember. This summary is not intended to simplify the complex material we've covered, but it may allow you to test your comprehension of the material now, and may help jog your memory later:

  • A class is a collection of data and methods that operate on that data.

  • An object is a particular instance of a class.

  • Object members (fields and methods) are accessed with a dot between the object name and the member name.

  • Instance (non-static) variables occur in each instance of a class.

  • Class (static) variables are associated with the class. There is one copy of a class variable regardless of the number of instances of a class.

  • Instance (non-static) methods of a class are passed an implicit this argument that identifies the object being operated on.

  • Class (static) methods are not passed a this argument and therefore do not have a current instance of the class that can be used to implicitly refer to instance variables or invoke instance methods.

  • Objects are created with the new keyword, which invokes a class constructor method with a list of arguments.

  • Objects are not explicitly freed or destroyed in any way. The Java garbage collector automatically reclaims objects that are no longer being used.

  • If the first line of a constructor method does not invoke another constructor with a this() call, or a superclass constructor with a super() call, Java automatically inserts a call to the superclass constructor that takes no arguments. This enforces "constructor chaining."

  • If a class does not define a constructor, Java provides a default constructor.

  • A class may inherit the non-private methods and variables of another class by "subclassing"--i.e., by declaring that class in its extends clause.

  • java.lang.Object is the default superclass for a class. It is the root of the Java class hierarchy and has no superclass itself. All Java classes inherit the methods defined by Object.

  • Method overloading is the practice of defining multiple methods which have the same name but have different argument lists.

  • Method overriding occurs when a class redefines a method inherited from its superclass.

  • Dynamic method lookup ensures that the correct method is invoked for an object, even when the object is an instance of a class that has overridden the method.

  • static, private, and final methods cannot be overridden and are not subject to dynamic method lookup. This allows compiler optimizations such as inlining.

  • From a subclass, you can explicitly invoke an overridden method of a superclass with the super keyword.

  • You can explicitly refer to a shadowed variable with the super keyword.

  • Data and methods may be hidden or encapsulated within a class by specifying the private or protected visibility modifiers. Members declared public are visible everywhere. Members with no visibility modifiers are visible only within the package.

  • An abstract method has no method body (i.e., no implementation).

  • An abstract class contains abstract methods. The methods must be implemented in a subclass before the subclass can be instantiated.

  • An interface is a collection of abstract methods and constants (static final variables). Declaring an interface creates a new data type.

  • A class implements an interface by declaring the interface in its implements clause and by providing a method body for each of the abstract methods in the interface.

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