1 class Lecture6 { 2 3 "Methods" / References 8 [1] Ch. 5 in YDL 9 [1] Ch. 20 in YDL 10 / Zheng-Liang Lu Java Programming 185 / 248

Transcription

1 1 class Lecture6 { 2 3 "Methods" 4 5 } 6 7 / References 8 [1] Ch. 5 in YDL 9 [1] Ch. 20 in YDL 10 / Zheng-Liang Lu Java Programming 185 / 248

2 All roads lead to Rome. Anonymous 但如你根本並無招式, 敵人如何來破你的招式? 風清揚, 笑傲江湖 第十回 傳劍 Zheng-Liang Lu Java Programming 186 / 248

3 Methods 2 Methods can be used to define reusable code, and organize and simplify code. The idea of functions originates from the definition of mathematical functions, that is, y = f (x), where x is the input parameter 1 and y is the function value. In computer science, each input parameter should be declared with a specific type, and a function should be assigned with a return type. 1 Recall the multivariate functions. The variable x can be a vector, say the position vector (x, y, z). 2 Some programming languages refer to methods as procedures and functions. Zheng-Liang Lu Java Programming 187 / 248

4 Example: max Zheng-Liang Lu Java Programming 188 / 248

5 1 modifier returntype methodname(listofparameters) { 2 // method body 3 } So far, the modifier could be static and public. returntype could be primitive data types, referential data types 3, and void. 4 listofparameters is the inputs of the method, separated by commas. Note that the parameters are optional; that is, a method doesn t have to contain any parameter. 5 The method name and the parameter list together constitute the method signature. 6 3 A method can return an object. 4 Recall that a void method does not return a value. 5 For example, Math.random(). 6 It is the key to the method overloading. We will see it soon. Zheng-Liang Lu Java Programming 189 / 248

6 More Observations There are alternatives to the max() method in the previous page: 1 public static int max(int x, int y) { 2 if (x > y) { 3 return x; 4 } else { 5 return y; 6 } 7 } 1 public static int max(int x, int y) { 2 return x > y? x : y; 3 } Zheng-Liang Lu Java Programming 190 / 248

7 return Statements A method returns to the code that invoked it if one of the following situations occurs: the method completes all the statements, reaches a return statement, or throws an exception (covered later). The return statements are the end of the method. Note that the return statement is not necessarily at the bottom of the method. 7 7 Thanks to a lively discussion on November 22, Zheng-Liang Lu Java Programming 191 / 248

8 However, your method should make sure that the return statement are available for all conditions. For example, 1 static int run1() { 2 while (true); 3 return 0; //unreachable code 4 } 5 6 static int run2(int x) { 7 if (x > 0) { 8 return x; 9 } 10 // what if x < 0? 11 } Once you define the return type, you should return a value or an object depending the return type. Zheng-Liang Lu Java Programming 192 / 248

9 Invoking A Method Invoking a method is to execute the code defined in the method. When a program invokes a method, the program control is transferred to the called method. Each time a method is invoked, the system creates an activation frame that stores parameters and variables for the method and places it in the call stack. A called method transfers the program control to the caller when its return statement is executed or when its end brace is reached. Zheng-Liang Lu Java Programming 193 / 248

10 Note that the parameters are new declared variables within the method. When calling a method, you need to provide arguments, which must match the parameters in order, number, and compatible type, as defined in the method signature. This mechanism is so-called pass-by-value. Zheng-Liang Lu Java Programming 194 / 248

11 Zheng-Liang Lu Java Programming 195 / 248

12 Variable Scope The variable scope is the range of the program where the variable can be referenced. Variables can be declared in class level, method level, and loop level. In general, a pair of curly brackets defines a particular scope. Moreover, one can declare variables with the same name in different levels of scopes. Yet, one cannot declare the variables with the same name in the same scope. Zheng-Liang Lu Java Programming 196 / 248

13 Example 1 public class incrementmain { 2 3 static int i = 1; // Aka class member. 4 5 public static void main(string[] args) { 6 System.out.printf("%d\n", i); // Where is i? 7 int i = 2; 8 i++; 9 System.out.printf("%d\n", i); 10 p(); 11 System.out.printf("%d\n", i); 12 } static void p() { 15 i = i + 1; 16 System.out.printf("%d\n", i); 17 } 18 } Zheng-Liang Lu Java Programming 197 / 248

14 Example Write a program which allows the user to enter the math grades in order (-1 to exit), and makes a histogram. Zheng-Liang Lu Java Programming 198 / 248

15 int[] hist = new int[5]; 3 // hist[0]: the number of students who get Scanner in = new Scanner(System.in); 6 int inputvalue; 7 do { 8 do { 9 System.out.println("Enter x? ( 1 to exit)"); 10 inputvalue = in.nextint(); 11 } while (inputvalue!= 1 && (inputvalue < 0 inputvalue > 100)); 12 if (inputvalue >= 90) ++hist[0]; 13 else if (inputvalue >= 80) ++hist[1]; 14 else if (inputvalue >= 70) ++hist[2]; 15 else if (inputvalue >= 60) ++hist[3]; 16 else if (inputvalue >= 0) ++hist[4]; 17 else System.out.println("End of input."); 18 } while (inputvalue!= 1); 19 in.close(); int number = 0; 22 for (int i = 0; i < hist.length; i++) { 23 number += hist[i]; Zheng-Liang Lu Java Programming 199 / 248

16 24 } 25 System.out.printf("Total: %d\n", number); int[] tick = {101, 90, 80, 70, 60, 0}; 28 for (int i = 0; i < 5; i++) { 29 System.out.printf("%3d %3d: ", tick[i], tick[i + 1] 1); 30 for (int j = 0; j < hist[i]; j++) { 31 System.out.printf(" "); 32 } 33 System.out.printf("\n"); 34 } Shall we rearrange the codes in order to reduce the complexity of the main method? Zheng-Liang Lu Java Programming 200 / 248

17 static int[] inputmethod() { 3 int[] cnt = new int[5]; 4 Scanner in = new Scanner(System.in); 5 int inputvalue; 6 do { 7 do { 8 System.out.println("Enter x? ( 1 to exit)"); 9 inputvalue = in.nextint(); 10 } while (inputvalue!= 1 && (inputvalue < 0 inputvalue > 100)); 11 if (inputvalue >= 90) ++cnt[0]; 12 else if (inputvalue >= 80) ++cnt[1]; 13 else if (inputvalue >= 70) ++cnt[2]; 14 else if (inputvalue >= 60) ++cnt[3]; 15 else if (inputvalue >= 0) ++cnt[4]; 16 else System.out.println("End of input."); 17 } while (inputvalue!= 1); 18 in.close(); 19 return cnt; 20 } Zheng-Liang Lu Java Programming 201 / 248

18 24 static void displayhistogram(int[] cnt) { 25 int number = 0; 26 for (int i = 0; i < cnt.length; i++) { 27 number += cnt[i]; 28 } 29 System.out.printf("Total: %d\n", number); int[] tick = {101, 90, 80, 70, 60, 0}; 32 for (int i = 0; i < 5; i++) { 33 System.out.printf("%3d %3d: ", tick[i], tick[i + 1] 1); 34 for (int j = 0; j < cnt[i]; j++) { 35 System.out.printf(" "); 36 } 37 System.out.printf("\n"); 38 } 39 } Zheng-Liang Lu Java Programming 202 / 248

19 public static void main(string[] args) { 3 int[] hist = inputmethod(); 4 displayhistogram(hist); 5 } 6... Zheng-Liang Lu Java Programming 203 / 248

20 Math Class The Math class contains the methods needed to perform basic mathematical functions. The Math class is public. All methods of Math class are public and static with two global constants Math.PI 8 and Math.E 9. The Math class provides common methods like: max, min, round, ceil, floor, abs, pow, exp, sqrt, cbrt, log, log10, sin, cos, asin, acos, and random. Full document of Math class can be found here. 8 π is a mathematical constant, the ratio of a circle s circumference to its diameter, commonly approximated as e is the base of the natural logarithm. It is approximately equal to Zheng-Liang Lu Java Programming 204 / 248

21 Example Password generator Write a program which generates ten characters as a password. There may be lower-case letters, upper-case letters, and digital characters in the character sequence. Recall that a character is encoded using an integer. How to generate these characters randomly? Zheng-Liang Lu Java Programming 205 / 248

22 static char getrandomuppercaseletter() { 3 return (char)(math.random() ( Z A + 1) + A ); 4 } 5 6 static char getrandomlowercaseletter() { 7 return (char)(math.random() ( z a + 1) + a ); 8 } 9 10 static char getrandomdigitalcharacter() { 11 return (char)(math.random() ( ) + 0 ); 12 } Zheng-Liang Lu Java Programming 206 / 248

23 public static void main (String[] args) { 3 // def: 0 > upper case, 1 > lower case, 2 > numbers 4 for (int i = 1; i <= 10; ++i){ 5 int type = (int) (Math.random() 3); 6 switch (type) { 7 case 0: 8 System.out.printf("%c", getrandomuppercaseletter()); 9 break; 10 case 1: 11 System.out.printf("%c", getrandomlowercaseletter()); 12 break; 13 case 2: 14 System.out.printf("%c", getrandomdigitalcharacter()); 15 break; 16 } 17 } 18 } Zheng-Liang Lu Java Programming 207 / 248

24 Overloading Methods Overloading methods enables you to define the methods with the same name as long as their signatures are different. Overloading methods can make programs clearer and more readable. Note that overloaded methods must have different parameter lists. You cannot overload methods based on different modifiers or return types. Zheng-Liang Lu Java Programming 208 / 248

25 Example (Revisit) Printing stars Write a program which prints a certain number of lines with some symbol determined by the user. The program repeats itself until the input number is negative. In addition, use * as the default symbol if the input is left blank. Zheng-Liang Lu Java Programming 209 / 248

26 static void print(int n) { 3 for (int i = 1; i <= n; i++) { 4 for (int j = 1; j <= i; j++) { 5 System.out.printf(" "); 6 } 7 System.out.println(); 8 } 9 } static void print(int n, String m) { 12 for (int i = 1; i <= n; i++) { 13 for (int j = 1; j <= i; j++) { 14 System.out.printf(m); 15 } 16 System.out.println(); 17 } 18 } Zheng-Liang Lu Java Programming 210 / 248

27 while (true) { 3 System.out.println("Enter n =? "); 4 n = Integer.parseInt(input.nextLine()); 5 if (n > 0) { 6 System.out.println("Enter a symbol? "); 7 m = input.nextline(); 8 if (m.equals("")) print(n); 9 else print(n, m); 10 } else { 11 System.out.println("Bye."); 12 break; 13 } 14 } The method Integer.parseInt() converts the input string to an integer. The method nextline() returns a string from the keyboard. The method equals() is a String method used to compare if the two strings are identical. Zheng-Liang Lu Java Programming 211 / 248

28 Divide and Conquer When developing a program, you can use the divide-andconquer strategy, aka stepwise refinement, to decompose the original problem into subproblems. The subproblems can be further decomposed into smaller, more manageable problems. Pros: easier to write, reuse, debug, test, modify, maintain, and better facilitating teamwork For most problems, do not write the entire program at once. 10 Be aware that sitting in front of the computer is the last thing when programming. 10 So you need more skills such that system analysis, project management, and software engineering. Zheng-Liang Lu Java Programming 212 / 248

29 Recursion 11 Recursion is the process of defining something in terms of itself. A method that calls itself is said to be recursive. Recursion is an alternative form of program control. It is essentially repetition without a loop. 11 Recursion is a commom pattern in nature. Zheng-Liang Lu Java Programming 213 / 248

30 Try Fractal. Zheng-Liang Lu Java Programming 214 / 248

31 Example The factorial of a non-negative integer n, denoted by n!, is the product of all positive integers less than and equal to n. Note that 0! = 1. For example, 5! = = 5 4! = 120. Can you find the pattern? Zheng-Liang Lu Java Programming 215 / 248

32 Write a program which determines n! static int factorial(int n) { 3 if (n!= 0) { 4 return n factorial(n 1); 5 } else { 6 return 1; // base condition 7 } 8 } 9... Note that there must be a base case in recursion. Can you implement the same method by using a loop? Zheng-Liang Lu Java Programming 216 / 248

33 Zheng-Liang Lu Java Programming 217 / 248

34 Equivalence: Loop Version int y = 1; 3 for (int i = 1; i < = 10; i++) { 4 y = i; 5 } 6... One intriguing question is, Can we always turn a recursive method into the loop version of that? Yes, theoretically The Church-Turing Thesis proves it if the memory serves. Zheng-Liang Lu Java Programming 218 / 248

35 In Practice Recursion bears substantial overhead. So the recursive algorithm may execute a bit more slowly than the iterative equivalent. Additionally, a deeply recursive method depletes the call stack, which is limited, and causes stack overflow fast. Zheng-Liang Lu Java Programming 219 / 248

36 Exercise Fibonacci numbers Write a program which determines F n, the (n + 1)-th Fibonacci number. The sequence of Fibonacci numbers is defined by the recurrence relation F n = F n 1 + F n 2, where F 0 = 0, F 1 = 1, and n 2. The first 10 Fibonacci numbers are 0, 1, 1, 2, 3, 5, 8, 13, 21, and 34. Zheng-Liang Lu Java Programming 220 / 248

37 Zheng-Liang Lu Java Programming 221 / 248

38 static int fibrecursive(int n) { 3 if (n == 0) return 0; 4 else if (n == 1) return 1; 5 else return fibrecursive(n 1) + fibrecursive(n 2); 6 } 7... This recursive implementation is straightforward. Yet, this algorithm isn t efficient since it requires more time and memory fibrecursive is an exponential-time algorithm while factorial is a linear-time algorithm. (Why?) Zheng-Liang Lu Java Programming 222 / 248

39 You can try an iterative approach for Fibonacci numbers static double fibiter(int n) { 3 int x = 0, y = 1; 4 if (n < 2) { 5 return (n == 1)? y : x; 6 } else { 7 for (int i = 0; i < n; ++i){ 8 int tmp = x + y; 9 x = y; 10 y = tmp; 11 } 12 return y; 13 } 14 } Can you find a linear recursion for Fibonacci numbers? Zheng-Liang Lu Java Programming 223 / 248

40 Abstraction Abstraction is a technique for managing complexity of computer systems. Higher level of abstraction hides the detail at lower levels, and provides the interfaces only. The programmer works with the well-defined interface and can add additional levels of functionality that would otherwise be too complex to handle. For example, no need to know how System.out.println() works. Zheng-Liang Lu Java Programming 224 / 248

41 Example: Abstraction of Computer System Zheng-Liang Lu Java Programming 225 / 248

42 Example: Methods as Control Abstraction Zheng-Liang Lu Java Programming 226 / 248

43 Abstraction (Concluded) Control abstraction is the abstraction of actions while data abstraction is that of data structures. One can view the notion of an object as a way to combine abstractions of data and code. It is encapsulation Recall that object-oriented programming languages fulfill encapsulation, inheritance, and polymorphism. Zheng-Liang Lu Java Programming 227 / 248

44 1 class Lecture7 { 2 3 // Object oriented Programming 4 5 } 6 7 // Reserved words: 8 class, new, this, extends, super, interface, implements, abstract, static, protected, null, default 9 // References: Ch.5 6 in Learning Java Zheng-Liang Lu Java Programming 228 / 248

45 Real Objects Look around. We can easily find many examples for real-world objects. For example, students and an instructor. Real-world objects all have states and behaviors. Identifying these states and behaviors for real-world objects is a great way to begin thinking in object-oriented programming. Zheng-Liang Lu Java Programming 229 / 248

46 Observations For each object, raise two questions: What possible states can the object be in? What possible behaviors can the object perform? Moreover, you may also notice that there are many objects under consideration, which may be the same type or not, These real-world observations all helps translate what you care about into the world of object-oriented programming. Zheng-Liang Lu Java Programming 230 / 248

47 Software Objects From now, we call the objects for short. An object stores its states in fields and exposes its behaviors through methods. Methods operate on an object s internal state and serve as the primary mechanism for object-to-object communication. Hiding internal state and requiring all interaction to be performed through an object s methods is known as encapsulation. Zheng-Liang Lu Java Programming 231 / 248

48 Classes In the real world, you ll often find many individual objects all of the same kind. For example, each bicycle was built from the same set of blueprints and therefore contains the same components. In object-oriented terms, we say that your bicycle is an instance of the class of objects known as Bicycle. A class is the blueprint to create class instances which are runtime objects. Classes are the building blocks of Java applications. Zheng-Liang Lu Java Programming 232 / 248

49 Example: Points in 2D Cartesian Coordinate 1 class Point { 2 int x, y; // data member, aka fields 3 } 1 public class PointDemo { 2 public static void main(string[] args) { 3 // create a new instance of Point 4 Point p1 = new Point(); 5 p1.x = 1; 6 p1.y = 2; 7 System.out.println(p1.x + p1.y); 8 9 // create another new instance of Point 10 Point p2 = new Point(); 11 p2.x = 3; 12 p2.y = 4; 13 System.out.println(p2.x + p2.y); 14 } 15 } Zheng-Liang Lu Java Programming 233 / 248

50 Class Declaration Give a class name, with the initial letter capitalized by convention. The class body, surrounded by balanced braces {}, contains all the code that provides for the life cycle of the objects. More explicitly, we declare the fields that provide the state of the class and its objects. Also we implement the methods for the behaviors of the class and its objects. May have a modifier such as public, private, and a number of others that you will encounter later. Access modifiers realize encapsulation! Zheng-Liang Lu Java Programming 234 / 248

51 Declaring Member Variables It is known that there are several kinds of variables: Local variables (method level) Parameters: variables in method declarations Fields (class level): members of a class The first (left-most) modifier lets you control what other classes have access to a member field. public: accessible from all classes. private: accessible only within its own class. In the spirit of encapsulation, it is common to make fields private. We still need access to these values, however. Zheng-Liang Lu Java Programming 235 / 248

52 Defining Methods So we need accessors and mutators. Accessors: to return some property of an object Mutators: to change some property of an object For example, getpoint() and setpoint(int, int) in the class Point. Zheng-Liang Lu Java Programming 236 / 248

53 Example: Point (Revisited) 1 class Point { 2 private int x; 3 private int y; 4 5 String getpoint() { 6 return "(" + x + ", " + y + ")"; 7 } 8 9 void setpoint(int a, int b) { 10 x = a; 11 y = b; 12 } 13 } Zheng-Liang Lu Java Programming 237 / 248

54 Unified Modeling Language The Unified Modeling Language (UML) is a language for specifying, visualizing, constructing, and documenting the artifacts of software systems, as well as for business modeling and other non-software systems. References: IntroToUML.pdf Free software: (available for all platforms) Zheng-Liang Lu Java Programming 238 / 248

55 Class Diagram Class diagrams are the most popular UML diagrams used for construction of software applications. The class diagram describes the fields and methods of a class and also the constraints imposed on the system. Zheng-Liang Lu Java Programming 239 / 248

56 Example: Point (Revisited) Modifiers are placed before the states and the methods: + for public for private Zheng-Liang Lu Java Programming 240 / 248

57 Constructors Constructors are like special methods that are called implicitly as soon as an object is instantiated. Constructor declarations look like method declarations, except that they use the name of the class and have no return type. A constructor is called by the new operator. Note that the constructors are not the part of the instances. As with methods, the constructors can be overloaded. If you don t define an explicit constructor, Java assumes a default constructor for your class. Adding any explicit constructor disables the default constructor. Zheng-Liang Lu Java Programming 241 / 248

58 Parameterized Constructors You can provide any information to the parameterized constructor for a specific object. For example, 1 class Point { Point() {} // default constructor 5 6 Point(int a, int b) { 7 x = a; 8 y = b; 9 } } Zheng-Liang Lu Java Programming 242 / 248

59 Example: Point (Revisited) 1 public class PointDemo { 2 public static void main(string[] args) { 3 Point p1 = new Point(1, 2); 4 System.out.println(p1.getPoint()); // output (1, 2) 5 6 Point p2 = new Point(); 7 System.out.println(p2.getPoint()); // output (0, 0) 8 p2.setpoint(3, 4); 9 System.out.println(p2.getPoint());// output (3, 4) 10 } 11 } Zheng-Liang Lu Java Programming 243 / 248

60 Self-reference Within an instance method or a constructor, the key word this is a reference to the current object. You can refer to any member of the current object from within an instance method or a constructor by using this. The most common reason for using the this keyword is because a field is shadowed by a method or constructor parameter. From within a constructor, you can also use the this keyword to call another constructor in the same class. Zheng-Liang Lu Java Programming 244 / 248

61 Example: Point (Revisited) 1 class Point { Point(int x, int y) { 5 this.x = x; 6 this.y = y; 7 } } Note that the this operator is referenced to instance members only but not class members. Zheng-Liang Lu Java Programming 245 / 248

62 static Members When we declare a member of a class as static, it means no matter how many objects of the class are created, there is only one copy of the static members. The static keyword can be applied to variables (aka class variables) methods (aka class methods) initial blocks nested classes We will see nested classes later. Zheng-Liang Lu Java Programming 246 / 248

63 The static members do not belong to the instances. These members exist when the class is loaded. So they can be invoked directly by the class name without creating an instance. For example, Math.random(). A static method can access static data member and can change its value. However, static methods cannot access to instance members directly. (Why?) Zheng-Liang Lu Java Programming 247 / 248

64 Example: Count of Points 1 class Point { private static int cnt = 0; 4 5 Point() { 6 cnt++; 7 } 8 9 Point(int x, int y) { 10 Point(); 11 this.x = x; 12 this.y = y; 13 } } Zheng-Liang Lu Java Programming 248 / 248