Java@Ch8. OBJECTS AND CLASSES

2010.12.10

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

reference Cac

carName = BMW

car1: Car

Create using new Car()

memory

( 課本 p.297)

• Primitive type:

i = j

• Object type:

car1 = car2

1

2

2

2j j

i i

Cac

carName = BMW

car2: Car Cac

carName = TOYOTA

car1: Car Cac

carName = BMW

car2: Car Cac

carName = TOYOTA

car1: Car

1

1

1

1car2 car2

car1 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

car1 car2

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 = 3.14159265;

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

public Y Y Y Y

protected Y Y Y

private Y

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