06slide.ppt

Chapter 6 Methods Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 1 ...

Chapter 6 Methods Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 1  Opening Problem Find the sum of integers from 1 to 10, from 20 to 30, and from 35 to 45, respectively. Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 2  Problem int sum = 0; for (int i = 1; i num2) result = num1; System.out.println( else "The maximum between " + i + result = num2; " and " + j + " is " + k); } return result; } Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 14 animation Trace Method Invocation i is now 5 public static void main(String[] args) { public static int max(int num1, int num2) { int i = 5; int result; int j = 2; int k = max(i, j); if (num1 > num2) result = num1; System.out.println( else "The maximum between " + i + result = num2; " and " + j + " is " + k); } return result; } Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 15 animation Trace Method Invocation j is now 2 public static void main(String[] args) { public static int max(int num1, int num2) { int i = 5; int result; int j = 2; int k = max(i, j); if (num1 > num2) result = num1; System.out.println( else "The maximum between " + i + result = num2; " and " + j + " is " + k); } return result; } Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 16 animation Trace Method Invocation invoke max(i, j) public static void main(String[] args) { public static int max(int num1, int num2) { int i = 5; int result; int j = 2; int k = max(i, j); if (num1 > num2) result = num1; System.out.println( else "The maximum between " + i + result = num2; " and " + j + " is " + k); } return result; } Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 17 animation Trace Method Invocation invoke max(i, j) Pass the value of i to num1 Pass the value of j to num2 public static void main(String[] args) { public static int max(int num1, int num2) { int i = 5; int result; int j = 2; int k = max(i, j); if (num1 > num2) result = num1; System.out.println( else "The maximum between " + i + result = num2; " and " + j + " is " + k); } return result; } Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 18 animation Trace Method Invocation declare variable result public static void main(String[] args) { public static int max(int num1, int num2) { int i = 5; int result; int j = 2; int k = max(i, j); if (num1 > num2) result = num1; System.out.println( else "The maximum between " + i + result = num2; " and " + j + " is " + k); } return result; } Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 19 animation Trace Method Invocation (num1 > num2) is true since num1 is 5 and num2 is 2 public static void main(String[] args) { public static int max(int num1, int num2) { int i = 5; int result; int j = 2; int k = max(i, j); if (num1 > num2) result = num1; System.out.println( else "The maximum between " + i + result = num2; " and " + j + " is " + k); } return result; } Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 20 animation Trace Method Invocation result is now 5 public static void main(String[] args) { public static int max(int num1, int num2) { int i = 5; int result; int j = 2; int k = max(i, j); if (num1 > num2) result = num1; System.out.println( else "The maximum between " + i + result = num2; " and " + j + " is " + k); } return result; } Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 21 animation Trace Method Invocation return result, which is 5 public static void main(String[] args) { public static int max(int num1, int num2) { int i = 5; int result; int j = 2; int k = max(i, j); if (num1 > num2) result = num1; System.out.println( else "The maximum between " + i + result = num2; " and " + j + " is " + k); } return result; } Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 22 animation Trace Method Invocation return max(i, j) and assign the return value to k public static void main(String[] args) { public static int max(int num1, int num2) { int i = 5; int result; int j = 2; int k = max(i, j); if (num1 > num2) result = num1; System.out.println( else "The maximum between " + i + result = num2; " and " + j + " is " + k); } return result; } Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 23 animation Trace Method Invocation Execute the print statement public static void main(String[] args) { public static int max(int num1, int num2) { int i = 5; int result; int j = 2; int k = max(i, j); if (num1 > num2) result = num1; System.out.println( else "The maximum between " + i + result = num2; " and " + j + " is " + k); } return result; } Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 24  CAUTION A return statement is required for a value-returning method. The method shown below in (a) is logically correct, but it has a compilation error because the Java compiler thinks it possible that this method does not return any value. public static int sign(int n) { public static int sign(int n) { if (n > 0) Should be if (n > 0) return 1; return 1; else if (n == 0) else if (n == 0) return 0; return 0; else if (n < 0) else return –1; return –1; } } (a) (b) To fix this problem, delete if (n < 0) in (a), so that the compiler will see a return statement to be reached regardless of how the if statement is evaluated. Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 25 Reuse Methods from Other Classes NOTE: One of the benefits of methods is for reuse. The max method can be invoked from any class besides TestMax. If you create a new class Test, you can invoke the max method using ClassName.methodName (e.g., TestMax.max). Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 26  Call Stacks Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 27 animation Trace Call Stack i is declared and initialized public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); System.out.println( "The maximum between " + i + " and " + j + " is " + k); } public static int max(int num1, int num2) { int result; i: 5 if (num1 > num2) result = num1; The main method else is invoked. result = num2; return result; } Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 28 animation Trace Call Stack j is declared and initialized public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); System.out.println( "The maximum between " + i + " and " + j + " is " + k); } public static int max(int num1, int num2) { j: 2 int result; i: 5 if (num1 > num2) result = num1; The main method else is invoked. result = num2; return result; } Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 29 animation Trace Call Stack Declare k public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); System.out.println( "The maximum between " + i + " and " + j + " is " + k); } Space required for the main method k: public static int max(int num1, int num2) { j: 2 int result; i: 5 if (num1 > num2) result = num1; The main method else is invoked. result = num2; return result; } Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 30 animation Trace Call Stack Invoke max(i, j) public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); System.out.println( "The maximum between " + i + " and " + j + " is " + k); } Space required for the main method k: public static int max(int num1, int num2) { j: 2 int result; i: 5 if (num1 > num2) result = num1; The main method else is invoked. result = num2; return result; } Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 31 animation Trace Call Stack pass the values of i and j to num1 and num2 public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); System.out.println( "The maximum between " + i + " and " + j + " is " + k); num2: 2 } num1: 5 Space required for the public static int max(int num1, int num2) { int result; main method k: if (num1 > num2) j: 2 result = num1; i: 5 else result = num2; return result; The max method is } invoked. Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 32 animation Trace Call Stack Declare result public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); System.out.println( "The maximum between " + i + result: " and " + j + " is " + k); num2: 2 } num1: 5 Space required for the public static int max(int num1, int num2) { int result; main method k: if (num1 > num2) j: 2 result = num1; i: 5 else result = num2; return result; The max method is } invoked. Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 33 animation Trace Call Stack (num1 > num2) is true public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); System.out.println( "The maximum between " + i + result: " and " + j + " is " + k); num2: 2 } num1: 5 Space required for the public static int max(int num1, int num2) { int result; main method k: if (num1 > num2) j: 2 result = num1; i: 5 else result = num2; return result; The max method is } invoked. Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 34 animation Trace Call Stack Assign num1 to result public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); Space required for the System.out.println( max method "The maximum between " + i + result: 5 " and " + j + " is " + k); num2: 2 } num1: 5 Space required for the public static int max(int num1, int num2) { int result; main method k: if (num1 > num2) j: 2 result = num1; i: 5 else result = num2; return result; The max method is } invoked. Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 35 animation Trace Call Stack Return result and assign it to k public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); Space required for the System.out.println( max method "The maximum between " + i + result: 5 " and " + j + " is " + k); num2: 2 } num1: 5 Space required for the public static int max(int num1, int num2) { int result; main method k:5 if (num1 > num2) j: 2 result = num1; i: 5 else result = num2; return result; The max method is } invoked. Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 36 animation Trace Call Stack Execute print statement public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); System.out.println( "The maximum between " + i + " and " + j + " is " + k); } Space required for the main method k:5 public static int max(int num1, int num2) { j: 2 int result; i: 5 if (num1 > num2) result = num1; The main method else is invoked. result = num2; return result; } Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 37  void Method Example This type of method does not return a value. The method performs some actions. TestVoidMethod TestReturnGradeMethod Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 38  Passing Parameters public static void nPrintln(String message, int n) { for (int i = 0; i < n; i++) System.out.println(message); } Suppose you invoke the method using nPrintln(“Welcome to Java”, 5); What is the output? Suppose you invoke the method using nPrintln(“Computer Science”, 15); What is the output? Can you invoke the method using nPrintln(15, “Computer Science”); Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 39  Pass by Value This program demonstrates passing values to the methods. Increment Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 40  Pass by Value Testing Pass by value This program demonstrates passing values to the methods. TestPassByValue Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 41  Pass by Value, cont. Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 42  Modularizing Code Methods can be used to reduce redundant coding and enable code reuse. Methods can also be used to modularize code and improve the quality of the program. GreatestCommonDivisorMethod PrimeNumberMethod Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 43 Case Study: Converting Hexadecimals to Decimals Write a method that converts a hexadecimal number into a decimal number. ABCD => A*16^3 + B*16^2 + C*16^1+ D*16^0 = ((A*16 + B)*16 + C)*16+D = ((10*16 + 11)*16 + 12)*16+13 = ? Hex2Dec Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 44  Overloading Methods Overloading the max Method public static double max(double num1, double num2) { if (num1 > num2) return num1; else return num2; } TestMethodOverloading Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 45  Ambiguous Invocation Sometimes there may be two or more possible matches for an invocation of a method, but the compiler cannot determine the most specific match. This is referred to as ambiguous invocation. Ambiguous invocation is a compile error. Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 46  Ambiguous Invocation public class AmbiguousOverloading { public static void main(String[] args) { System.out.println(max(1, 2)); } public static double max(int num1, double num2) { if (num1 > num2) return num1; else return num2; } public static double max(double num1, int num2) { if (num1 > num2) return num1; else return num2; } } Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 47  Scope of Local Variables A local variable: a variable defined inside a method. Scope: the part of the program where the variable can be referenced. The scope of a local variable starts from its declaration and continues to the end of the block that contains the variable. A local variable must be declared before it can be used. Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 48  Scope of Local Variables, cont. You can declare a local variable with the same name multiple times in different non- nesting blocks in a method, but you cannot declare a local variable twice in nested blocks. Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 49  Scope of Local Variables, cont. A variable declared in the initial action part of a for loop header has its scope in the entire loop. But a variable declared inside a for loop body has its scope limited in the loop body from its declaration and to the end of the block that contains the variable. public static void method1() {.. for (int i = 1; i < 10; i++) {. The scope of i. int j;. The scope of j.. } } Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 50  Scope of Local Variables, cont. It is fine to declare i in two It is wrong to declare i in non-nesting blocks two nesting blocks public static void method1() { public static void method2() { int x = 1; int y = 1; int i = 1; int sum = 0; for (int i = 1; i < 10; i++) { x += i; for (int i = 1; i < 10; i++) { } sum += i; } for (int i = 1; i < 10; i++) { y += i; } } } Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 51  Scope of Local Variables, cont. // Fine with no errors public static void correctMethod() { int x = 1; int y = 1; // i is declared for (int i = 1; i < 10; i++) { x += i; } // i is declared again for (int i = 1; i < 10; i++) { y += i; } } Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 52  Scope of Local Variables, cont. // With errors public static void incorrectMethod() { int x = 1; int y = 1; for (int i = 1; i < 10; i++) { int x = 0; x += i; } } Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 53  Method Abstraction You can think of the method body as a black box that contains the detailed implementation for the method. Optional arguments Optional return for Input value Method Header Black Box Method body Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 54  Benefits of Methods Write a method once and reuse it anywhere. Information hiding. Hide the implementation from the user. Reduce complexity. Liang, Introduction to Java Programming and Data Structures, Twelfth Edition, (c) 2020 Pearson Education, Inc. All rights reserved. 55  Case Study: Generating Random Characters Computer programs process numerical data and characters. You have seen many examples that involve numerical data. It is also important to understand characters and how to process them. As introduced in Section 4.3, each character has a unique Unicode between 0 and FFFF in hexadecimal (65535 in decimal). To generate a random character is to generate a random integer between 0 and 65535 using the following expression: (note that since 0