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

时间：2022-04-19 09:28:32

实例变量

  1. 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；
  2. 当一个对象被实例化之后，每个实例变量的值就跟着确定；
  3. 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；
  4. 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；
  5. 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；
  6. 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；
  7. 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）

  1. 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。
  2. 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。
  3. 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。
  4. 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。
  5. 静态变量在程序开始时创建，在程序结束时销毁。
  6. 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。
  7. 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。
  8. 静态变量可以通过：ClassName.VariableName的方式访问。
  9. 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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