Introduction to Java Programming Lecture 12 Method Benefits, Declaring, and Calling Methods

Methods F Introducing Methods –Methods ( 方法 ) 的宣告 F Passing Parameters – 代入參數 – pass by value ( 代入數值 ) F Overloading Methods –Method 的重載 F Scope of Local Variables F Method Abstraction

Method Structure : Declaring Methods ( 方法的宣告 ) public static int max( int num1, int num2 ) { int result = 0; if (num1 > num2) result = num1; else result = num2; return result ; } Modifiers Return value typeMethod nameParameters Return value Method body

使用 Methods 的好處 寫好之後可以在不同的程式中重複使用. Information hiding -- Hide the implementation from the user (Method 的使用者不需知道這個 method 是怎麼寫 的，只要知道怎麼用就行 : method 的功能、需代入的參數 、 return 的值 ). 減低程式的複雜度 – 程式分隔成數個 methods ，容易整理， 容易維護，也比較容易理解 Squares1.javaSquares1.java Square2.java Square3.javaSquare2.javaSquare3.java

Squares 1 // Squares1: without using methods public class Squares1 { public static void main( String[ ] args ) { for (int index = 1; index <= 10; index ++) { System.out.println("The square of " + index + " is " + ( index*index )); } System.exit(0); } // end of main } // end of class squares

Squares 2 /* Squares2 Using methods with a return value.*/ public class Squares2 { public static void main( String[] args ) { for (int index = 1; index <= 10; index ++) { System.out.println("The square of " + index + " is " + ( square( index ))); System.out.println("The sum of " + index + "+2 squared is " + ( square( index+ 2))); System.out.println("The sum of " + index + " squared+ 2 is " + (( square( index))+2) ); System.out.println(); } System.exit(0); } // end of main

// square method declaration follows: public static int square( int x ) { int y = x * x; return y; } // end of method square } // end of class squares2

Spuares 3 /* Squares3 This program demonstrates the use of methods with a return value and two inputs.*/ public class Squares3 { public static void main( String[] args ) { for (int index = 1; index <= 10; index ++) { System.out.println(index + " The sum of squares of "+ index + " & " + (index+1) + " is " + square( index, index+1 )); } System.exit(0); } // end of main

// square method declaration follows: public static int square( int x, int y ) { int a, b; a = x * x; b = y * y; return (a+b); } // end of method square } // end of class squares3

Pass by Value // TestPassByValue.java: Demonstrate passing values to methods public class TestPassByValue { /** Main method */ public static void main(String[] args) { // Declare and initialize variables int num1 = 1; int num2 = 2; System.out.println("Before invoking the swap method, num1 is " + num1 + " and num2 is " + num2); // Invoke the swap method to attempt to swap two variables swap(num1, num2); System.out.println("After invoking the swap method, num1 is " + num1 + " and num2 is " + num2); }

Pass by Value, Cont. /** Swap two variables */ public static void swap(int n1, int n2) { System.out.println(" Inside the swap method"); System.out.println(" Before swapping n1 is " + n1 + " n2 is " + n2); // Swapping n1 with n2 int temp = n1; n1 = n2; n2 = temp; System.out.println(" After swapping n1 is " + n1 + " n2 is " + n2); }

Method overloading ( 重載 ) // TestMethodOverloading.java: Demonstrate method overloading public class TestMethodOverloading { public static void main(String[] args) { // 代入兩個 int 的 max method System.out.println("The maximum between 3 and 4 is " + max(3, 4)); // 代入兩個 double 的 max method System.out.println("The maximum between 3.0 and 5.4 is " + max(3.0, 5.4)); // 代入三個 double 的 max method System.out.println("The maximum between 3.0, 5.4, and is " + max(3.0, 5.4, 10.14)); }

public static int max(int num1, int num2) { if (num1 > num2) return num1; else return num2; } public static double max(double num1, double num2) { if (num1 > num2) return num1; else return num2; } public static double max(double num1, double num2, double num3) { return max(max(num1, num2), num3); }

Scope of Local Variables A local variable : 宣告在 method 中的變數. Scope : 變數的 scope 指的是該變數在整個程式中可被參照到 的部分 ( 可使用到這個變數的區域 ). 一個 local variable 的 scope 從這個變數的宣告開始，一直到包 含這個變數的 block 結束為止 同一名稱的變數可宣告在不互相包含於的 blocks 中，但不可在 ”nested blocks” 中宣告兩次

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; }

Scope of Local Variables, cont. // With compilation errors public static void incorrectMethod() { int x = 1; int y = 1; for (int i = 1; i < 10; i++) { int x = 0; x += i; }

Method Abstraction You can think of the method body as a black box that contains the detailed implementation for the method.