L EC. 06: C LASS D ETAILS (2/2) 0

2015 S PRING C ONTENT  Class method [review]  Access control  Passing arguments  Method overloading  Variable-length Arguments [ 不測驗，可列補充教材 ]  Recursion  Using the keyword static  Nested classes  Shadowing 1

P ASSING ARGUMENTS  There are several ways to implement passing arguments from a caller to a invoked method, such as call-by-value and call-by-reference.  Java only supports call-by-value.  In Java, the precise effect differs between whether the argument type is a primitive type or a reference/class type. 2

C ALL - BY - VALUE APPROACH  The call-by-value approach copies the value of an argument into the space of corresponding formal parameter.  The argument is evaluated prior to the execution of invoked method.  The invoked method does not know where arguments are.  Changes made to the parameter of the method have no effect on the argument in the call. 3

E XAMPLE class Test { void noChange(int i, int j) { i = i + j; j = -j; } class CallByValue { public static void main(String args[]) { Test ob = new Test(); int a = 15, b = 20; System.out.println("a and b before call: " + a + " " + b); ob.noChange(a, b); System.out.println("a and b after call: " + a + " " + b); } 4 a and b before call: a and b after call: ob noChange(int i, int j)

E XERCISE 1 5  Create a stack class called Stack. Calls methods push() and pop() to access the stack. Keep all members of the Stack class private top [0] [1] [2] 21 4

 Hint: class stack { private int stack_data[]; private int stack_top; stack(int size) { … } public int push(int data) { … } public int pop() { … } 6 Handle abnormal situation class Ex1 { public static void main(String[] args) { stack s1 = new stack(100); s1.push(4); s1.push(21); System.out.println(s1.pop()); }

P ASSING REFERENCE TYPE ARGUMENTS IN J AVA  Since the content of a reference variable is the reference of an object, passing a reference type argument copies the reference of the object pointed by the argument to the memory of the corresponding parameter.  Remember that the content of a reference variable is the reference of an object, instead of the actual object.  The invoked method can access the object pointed by the argument through the parameter.  Passing reference type arguments has similar effect to call-by-reference. 7

E XAMPLE class Test { int a, b; Test(int i, int j) { a = i; b = j; } void change(Test obj) { a = obj.a + obj.b; b = -obj.b; } class PassObjRef { public static void main(String args[]) { Test ob = new Test(15, 20); System.out.println("ob.a and ob.b before call: " + ob.a + " " + ob.b); ob.change(ob); System.out.println("ob.a and ob.b after call: " + ob.a + " " + ob.b); } 8 ob.a and ob.b before call: ob.a and ob.b after call: ob a b change() obj

M ETHOD OVERLOADING  In Java, two or more methods within the same class can share the same name, as long as their parameter declarations are different.  When this is the case, the methods are said to be overloaded, and the process is referred to as method overloading.  When an overloaded method is called, the version of the method whose parameters match the arguments is executed.  Numerical promotion and boxing may apply, but the automatic conversions apply only if there is no direct match between a parameter and an argument. 9

E XAMPLE class Overload { void ovlDemo() { System.out.println("No parameters"); } void ovlDemo(int a) { System.out.println("One parameter: " + a); } int ovlDemo(int a, int b) { System.out.println("Two parameters: " + a + " " + b); return a + b; } double ovlDemo(double a, double b) { System.out.println("Two double parameters: " + a + " "+ b); return a + b; } 10 No parameters One parameter: 2 Two parameters: 4 6 Result of ob.ovlDemo(4, 6): 10 Two double parameters: Result of ob.ovlDemo(1.1, 2.2): 3.42

class OverloadDemo { public static void main(String args[]) { Overload ob = new Overload(); int resI; double resD; // call all versions of ovlDemo() ob.ovlDemo(); ob.ovlDemo(2); resI = ob.ovlDemo(4, 6); System.out.println("Result of ob.ovlDemo(4, 6): " + resI); resD = ob.ovlDemo(1.1, 2.32); System.out.println("Result of ob.ovlDemo(1.1, 2.2): " + resD); } 11

E XAMPLE class Overload2 { void f(int x) { System.out.println("Inside f(int): " + x); } void f(double x) { System.out.println("Inside f(double): " + x); } 12 Inside f(int): 10 Inside f(double): 10.1 Inside f(int): 99 Inside f(int): 10 Inside f(double): 11.5

class TypeConv { public static void main(String args[]) { Overload2 ob = new Overload2(); int i = 10; double d = 10.1; byte b = 99; short s = 10; float f = 11.5F; ob.f(i); // calls ob.f(int) ob.f(d); // calls ob.f(double) ob.f(b); // calls ob.f(int)  type conversion ob.f(s); // calls ob.f(int)  type conversion ob.f(f); // calls ob.f(double)  type conversion } 13 byteshortintfloatdouble

E XAMPLE class MyClass { int x; MyClass() { System.out.println("Inside MyClass()."); x = 0; } MyClass(int i) { System.out.println("Inside MyClass(int)."); x = i; } MyClass(double d) { System.out.println("Inside MyClass(double)."); x = (int) d; } MyClass(int i, int j) { System.out.println("Inside MyClass(int, int)."); x = i * j; } 14 Inside MyClass(). Inside MyClass(int). Inside MyClass(double). Inside MyClass(int, int). t1.x: 0 t2.x: 88 t3.x: 17 t4.x: 8

class OverloadConsDemo { public static void main(String args[]) { MyClass t1 = new MyClass(); MyClass t2 = new MyClass(88); MyClass t3 = new MyClass(17.23); MyClass t4 = new MyClass(2, 4); System.out.println("t1.x: " + t1.x); System.out.println("t2.x: " + t2.x); System.out.println("t3.x: " + t3.x); System.out.println("t4.x: " + t4.x); } 15

B INDING FOR METHOD OVERLOADING  Binding is the process for creating the association between data/code with an identifier.  Static binding means that binding is done during compilation.  Dynamic binding means that binding is done during execution.  Binding for overloaded methods is static binding. 16

E XAMPLE class OverloadA { public static void main(String args[]) { OverloadA obj = new OverloadA(); obj.toDo(1); // C0 obj.toDo(2, 3); // C1 obj.toDo('a', 3); // C2 obj.toDo(2, 3, 4); // C3 obj.toDo(2, 3.2f); // C4 } void toDo(int i, int j) { // executed by C1 and C2 System.out.print("A"); } void toDo(int i, int... j) { // executed by C0 and C3 System.out.print("B"); } void toDo(double i, double j){ // executed by C4 System.out.print("C"); } 17 BAABC promotion

E XAMPLE class OverloadB { public static void main(String args[]) { OverloadB obj = new OverloadB(); obj.toDo(2, 3, 4); // ambiguous obj.toDo(3.2f); // cause error } void toDo(int i, int j, int... k) { System.out.print("A"); } void toDo(int i, int... j) { System.out.print("B"); } void toDo(Double i) { System.out.print("C"); } } 18

19 OverloadB.java:5: error: no suitable method found for toDo(float) obj.toDo(3.2f); ^ method OverloadB.toDo(int,int,int...) is not applicable (argument mismatch; possible lossy conversion from float to int) method OverloadB.toDo(int,int...) is not applicable (argument mismatch; possible lossy conversion from float to int) method OverloadB.toDo(Double) is not applicable (argument mismatch; float cannot be converted to Double) Note. It will not perform numerical promotion followed by boxing.

裝箱 ( BOXING ) 、拆箱 ( UNBOXING ) 20  把 primitive 轉成物件，稱之為裝箱  把物件轉乘 primitive ，稱之為拆箱  Ex. Integer A = new Integer(10); // 裝箱 int b = A.intValue(); // 拆箱 b = A; // 自動拆箱 A = 20; // 自動裝箱 A 10

class VarArgs { static void vaTest(int... v) { System.out.println("Number of args: " + v.length); for(int i=0; i < v.length; i++) System.out.println(v[i]); System.out.println(); } public static void main(String args[]) { vaTest(10); vaTest(1, 2, 3) vaTest(); } 21 Number of args: 1 10 Number of args: Number of args: 0 V ARIABLE - LENGTH A RGUMENTS

E XAMPLE class VarArgs2 { static void vaTest(int msg, int... v) { System.out.println("-->" + v.length); for(int i=0; i < v.length; i++) System.out.println(v[i]); System.out.println(); } public static void main(String args[]) { vaTest(100); vaTest(100, 1, 2, 3); } 22 -->0 --> Need at least one parameter 2 hr