实例变量
- 实例变量声明在一个类中，但在方法、构造方法和语句块之外；
- 当一个对象被实例化之后，每个实例变量的值就跟着确定；
- 实例变量在对象创建的时候创建，在对象被销毁的时候销毁；
- 实例变量的值应该至少被一个方法、构造方法或者语句块引用，使得外部能够通过这些方式获取实例变量信息；
- 实例变量对于类中的方法、构造方法或者语句块是可见的。一般情况下应该把实例变量设为私有。通过使用访问修饰符可以使实例变量对子类可见；
- 实例变量具有默认值。数值型变量的默认值是0，布尔型变量的默认值是false，引用类型变量的默认值是null。变量的值可以在声明时指定，也可以在构造方法中指定；
- 实例变量可以直接通过变量名访问。但在静态方法以及其他类中，就应该使用完全限定名：ObejectReference.VariableName。
类变量（静态变量）
- 类变量也称为静态变量，在类中以static关键字声明，但必须在方法构造方法和语句块之外。
- 无论一个类创建了多少个对象，类只拥有类变量的一份拷贝。
- 静态变量除了被声明为常量外很少使用。常量是指声明为public/private，final和static类型的变量。常量初始化后不可改变。
- 静态变量储存在静态存储区。经常被声明为常量，很少单独使用static声明变量。
- 静态变量在程序开始时创建，在程序结束时销毁。
- 与实例变量具有相似的可见性。但为了对类的使用者可见，大多数静态变量声明为public类型。
- 默认值和实例变量相似。数值型变量默认值是0，布尔型默认值是false，引用类型默认值是null。变量的值可以在声明的时候指定，也可以在构造方法中指定。此外，静态变量还可以在静态语句块中初始化。
- 静态变量可以通过：ClassName.VariableName的方式访问。
- 类变量被声明为public static final类型时，类变量名称必须使用大写字母。如果静态变量不是public和final类型，其命名方式与实例变量以及局部变量的命名方式一致。
class TiXing{
float up,height;
static float down;
TiXing(float x,float y,float z){
up=x;
height=y;
down=z;
}
}
public class ep3_9{
public static void main(String args[]){
TiXing one=new TiXing(1,2,3);
System.out.println("one's down is:"+one.down);
TiXing two=new TiXing(4,5,6);
System.out.println("one's down is:"+one.down);
System.out.println("two's down is:"+two.down);
System.out.println("TiXing's down is:"+TiXing.down);
}
}
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实例方法和类方法对实例变量和类变量的访问
实例方法可以对当前对象的实例变量进行操作，也可以对类变量进行操作。实例方法由实例对象调用。
类方法不能访问实例变量，只能访问类变量。类方法由类名或者实例对象调用。类方法中不能出现this或者super关键字
class TiXing{
private float up,height;
private static float down;
TiXing(float x,float y,float z){
up=x;
height=y;
down=z;
}
public void display(){
System.out.println("up is:"+up+"height is:"+height+"down is:"+down);
}
public static void change(float number){
down=number;
//System.out.println("height:"+height);//出错
}
}
public class ep3_9{
public static void main(String args[]){
TiXing one=new TiXing(1,2,3);
one.display();
TiXing two=new TiXing(4,5,6);
one.display();
two.display();
//TiXing.display();//出错
one.change(101);
one.display();
two.change(102);
two.display();
}
}
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