JAVA类与对象（六）------实例变量与类变量的区别，实例方法和类方法的区别

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);

    }

}

aaarticlea/png;base64,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" alt="" />

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();

    }

}

aaarticlea/png;base64,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" alt="" />