OBJECTS AND CLASSES

Outline Reference Variables and Reference Types Primitive Types and Reference Types Static Variable, Constants, and Methods Visibility Modifiers Displaying GUI Components [Sample code]

Reference Variables and Reference Types Objects are accessed via object reference variables. 宣告範例 : Car car1; A class is a reference type, which means that a variable of the class type can reference an instance of the class. Create an object and assigns its reference to car1. car1 = new Car(); Combine: Car car1 = new Car(); ( 課本 p.295)

補充 : ( 課本 p.296) Usually you create an object and assign it to a variable. Later you can use the variable to reference the object. new Car(); or System.out.println ( “ 耗油量 : ” + Car().OilConsume(10) ); 有時候你也可以不用宣告 object reference variable 而直接呼叫 class 的方法

Primitive Types and Reference Types Primitive type: int i = 1; i Object type: Car car1; car1 1 referenceCac carName = BMW car1: Car Create using new Car() memory ( 課本 p.297)

Primitive type: i = j Object type: car1 = car jj ii Cac carName = BMW car2: Car Cac carName = TOYOTA car1: Car Cac carName = BMW car2: Car Cac carName = TOYOTA car1: Car car2 car1 X

Static Variable, Constants, and Methods Static( 靜態 ): 靜態的含意是, 程式建立之初, 系統就已經先配置 好一塊 Memory space 給它了.

Static Variable Static variables store values for the variables in a common memory location. Car car1 = new Car(); Car car2 = new Car(); speed position speed position carID drive() stop() printID() drive() stop() printID() static variable car1car2 data field method class 就可以直接做呼叫, 不需要實作出 instance 來呼叫

Static Method Static method can be called without creating an instance of the class. (p.302) EX: Car.printID(); static methodclass name class 就可以直接做呼叫, 不需要實作出 instance 來呼叫

Static Constant Constants in a class are shared by all objects of the class. 必須宣告成 final static EX: final static double PI = ; class 就可以直接做呼叫, 不需要實作出 instance 來呼叫

Visibility Modifiers Package: can be used to organize classes. package p1; public class C1 { public int x; int y; private int z; public void m1() { } void m2() { }; private void m3() { } } package p1; public class C2 { void aMethod() { C1 o = new C1(); can access o.x; can access o.y; cannot access o.z; can invoke o.m1(); can invoke o.m2(); cannot invoke o.m3(); } package p2; public class C3 { void aMethod() { C1 o = new C1(); can access o.x; cannot access o.y; cannot access o.z; can invoke o.m1(); cannot invoke o.m2(); cannot invoke o.m3(); }

If a class is not defined public, it can be accessed only within the same package. package p1; class C1 { } package p1; public class C2 { can access C1; } package p2; public class C3 { cannot access C1; can access C2; }

同 class 同 package 子 class 其他 package publicYYYY protectedYYY privateY

Displaying GUI Components 程式練習 :  p.300 Listing 8.5

程式練習 : ( 參考 p.301)  1. 宣告一個 JButton 2. 宣告一個 JLabel 3. 宣告一個 JPanel 4. 宣告一個 JFrame 5. 把 button 和 label 加進 panel 6. 把 panel 加進 frame 7. 顯示視窗