Instance Members and Static Members

Transcription

1 Instance Members and Static Members You may notice that all the members are declared w/o static. These members belong to some specific object. They are called instance members. This implies that these instance members are available only when the object is created. Those declared w/ static are static members, aka class members. Zheng-Liang Lu Java Programming 245 / 270

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3 Static Members Static members mean that there is only one copy of the static members, no matter how many objects of the class are created. The static members belong to the class, and are shared between the instance objects. They are ready once the class is loaded. They can be invoked directly by the class name without creating an instance. For example, Math.random(). Zheng-Liang Lu Java Programming 247 / 270

4 A static method can access other static members. However, static methods cannot access to instance members directly. (Why?) For example, double getdistancefrom(point p) { 3 return Math.sqrt(Math.pow(this.x p.x, 2) + Math.pow( this.y p.y, 2)); 4 } 5 6 static double distancebetween(point p1, Point p2) { 7 return Math.sqrt(Math.pow(p1.x p2.x, 2) + Math.pow(p1. y p2.y, 2)); 8 } 9... Zheng-Liang Lu Java Programming 248 / 270

5 Example: Count of Points 1 class Point { private static int numofpoint = 0; 4 5 Point() { 6 numofpoint++; 7 } 8 9 Point(int x, int y) { 10 this(); // calling the constructor with no input argument; should be placed in the first line in the constructor 11 this.x = x; 12 this.y = y; 13 } } Zheng-Liang Lu Java Programming 249 / 270

6 Exercise: Singleton 1 In some situations, you may create the only instance of the class. 1 class mysingleton { 2 3 // Will be ready as soon as the class is loaded. 4 private static mysingleton theonlyinstance = new mysingleton (); 5 6 // Do now allow to invoke the constructor by other classes. 7 private mysingleton() {} 8 9 // Only way to obtain the singleton from the outside world. 10 public static mysingleton getsingleton() { 11 return theonlyinstance; 12 } 13 } 1 See any textbook for design patterns. Zheng-Liang Lu Java Programming 250 / 270

7 Garbage Collection (GC) 2 Java handles deallocation automatically. Automatic GC is the process of looking at the heap memory, identifying whether or not the objects are in use, and deleting the unreferenced objects. An object is said to be unreferenced if the object is no longer referenced by any part of your program. Simply assign null to the reference to make the object unreferenced. So the memory used by these objects can be reclaimed. 2 java/gc01/index.html Zheng-Liang Lu Java Programming 251 / 270

8 finalize() Method The method finalize() conducts a specific task that will be executed as soon as the object is about to be reclaimed by GC. The finalize() method can be only invoked prior to GC. In practice, it must not rely on the finalize() method for normal operations. (Why?) Zheng-Liang Lu Java Programming 252 / 270

9 Example 1 public class finalizedemo { 2 static int numofpointkilled = 0; 3 4 public void finalize() { 5 numofpointkilled++; 6 } 7 8 public static void main(string[] args) { 9 double n = 1e7; 10 for (int i = 1; i <= n; i++) 11 new finalizedemo(); 12 System.out.println(numOfPointKilled); 13 } 14 } You may try different number for instance creation. The number of the objects reclaimed by GC is not deterministic. Zheng-Liang Lu Java Programming 253 / 270

10 HAS-A Relationship Association is a weak relationship where all objects have their own lifetime and there is no ownership. For example, teacher student; doctor patient. If A uses B, then it is an aggregation, stating that B exists independently from A. For example, knight sword; company employee. If A owns B, then it is a composition, meaning that B has no meaning or purpose in the system without A. For example, house room. Zheng-Liang Lu Java Programming 254 / 270

11 Example: Lines +2: two Point objects used in one Line object. Zheng-Liang Lu Java Programming 255 / 270

12 More Examples More geometric objects, say Circle, Triangle, and Polygon. Complex number (a + bi) equipped with + and so on. Book with Authors. Lecturer and Students in the classroom. Zoo with many creatures, say Dog, Cat, and Bird. Channels played on TV. Zheng-Liang Lu Java Programming 256 / 270

13 More Relationships Between Classes Inheritance: passing down states and behaviors from the parents to their children Interfaces: grouping the methods, which belongs to some classes, as an interface to the outside world Packages: grouping related types, providing access protection and name space management Zheng-Liang Lu Java Programming 257 / 270

14 First IS-A Relationship OO allows classes to inherit commonly used states and behaviors from other classes. This is called inheritance. So the classes exist in some hierarchy. A class can be declared as a subclasses of another class, which is called superclass, by using the extends keyword. Zheng-Liang Lu Java Programming 258 / 270

15 Hence, we can say that a subclass specializes its superclass. Equivalently, one subclass is a special case of the superclass. For example, human beings and dogs are two specific types of animals. Note that a class can extend only one other class. Each superclass has the potential for an unlimited number of subclasses. Zheng-Liang Lu Java Programming 259 / 270

16 Example 1 class Animal { 2 String name; 3 int weight; 4 5 Animal(String s, int w) { name = s; weight = w; } 6 7 void eat() { weight += 1; } 8 void exercise() { weight = 1; } 9 } class Human extends Animal { 12 Human(String s, int w) { super(w, s); } 13 void writecode() { System.out.println("Write codes..."); } 14 } class Dog extends Animal { 17 Dog(String s, int w) { super(w, s); } 18 void watchdoor() { System.out.println("Watch my door..."); } 19 } Zheng-Liang Lu Java Programming 260 / 270

17 The super Keyword Recall that the keyword this is used to refer to the object itself. You can use the keyword super to refer to members of the superclass. Note that this() and super() are used as the constructor of the current class and that of its superclass, respectively. Make sure all the constructor are invoked in the first line in the constructors. Zheng-Liang Lu Java Programming 261 / 270

18 Class Hierarchy 3 3 See Fig. 3-1 in p. 113 of Evans and Flanagan. Zheng-Liang Lu Java Programming 262 / 270

19 Constructor Chaining If a subclass constructor invokes a constructor of its superclass, you might think that there will be a whole chain of constructors called, all the way back to the constructor of the class Object, the topmost class in Java. So every class is an immediate or a distant subclass of Object. Recall that the method finalize() and tostring() are inherited from Object. tostring(): return a string which can be any information stored in the class. Zheng-Liang Lu Java Programming 263 / 270

20 Example 1 class A { 2 A() { System.out.println("A is creating..."); } 3 } 4 5 class B extends A { 6 B() { System.out.println("B is creating..."); } 7 public String tostring() { 8 return "This is inherited from Object." 9 } 10 } public class ConstructorChainingDemo { 13 public static void main(string[] args) { 14 B b = new B(); 15 System.out.println(b); 16 } 17 } The println() method can take an object as input, and invoke tostring() method implicitly. Zheng-Liang Lu Java Programming 264 / 270

21 Field Hiding If one field whose name is identical to the field inherited from the superclass, then the newly field hides that of the superclass. In other words, the shadowed field of the superclass cannot be referenced by the field name. Instead, the field must be accessed through the key word super. Zheng-Liang Lu Java Programming 265 / 270

22 Example 1 class A { 2 int x = 1; 3 } 4 5 class B extends A { 6 int x = 2; 7 } 8 9 class FieldHidingDemo { 10 public static void main(string[] args) { 11 B b = new B(); 12 System.out.println(b.x); // output 2 13 } 14 } Zheng-Liang Lu Java Programming 266 / 270

23 Method Overriding The subclass is allowed to change the behavior inherited from its superclass if needed. As a subclass defines an instance method using the method name, return type, and parameters all identical to the method of its superclass, this method overrides the one of the superclass. 4 Compared to overridden methods, method overloading occurs only in the same class. Similarly if your method overrides one of its superclass s methods, you can invoke the overridden method through the use of the keyword super. 4 The static methods do not follow this rule. Zheng-Liang Lu Java Programming 267 / 270

24 When there are multiple implementations of the method in the inheritance hierarchy, the one in the most derived class (the furthest down the hierarchy) always overrides the others, even if we refer to the object through a reference variable of the superclass type. 5 5 An overridden method in Java acts like a virtual function in C++. See the virtual method table. Zheng-Liang Lu Java Programming 268 / 270

25 Example Zheng-Liang Lu Java Programming 269 / 270

26 Binding Association of method definition to the method call is known as binding. The binding which can be resolved at compile time by compiler is known as static binding or early binding. They are the static, private and final methods. Compiler knows that all such methods cannot be overridden and will always be accessed by object of local class. When the compiler is not able to resolve the binding at compile time, such binding is known as dynamic binding or late binding. For example, method overriding. Zheng-Liang Lu Java Programming 270 / 270