我在理解非静态变量的处理方式时遇到了问题。我选择使用数组,以便检索其内存地址。
请考虑以下代码:
public class tryClass
{
int[] v = {0}; // vector v is non-static (and NOT local to any method)
tryClass obj;
public void met ()
{
obj = new tryClass();
obj.v[0] = 30;
v[0]=3;
}
public static void main (String[] args)
{
tryClass obj = new tryClass(); // is this the SAME object as in met() ?
int[] v = new int[1];
obj.v[0] = 40;
obj.met();
}
}
为了知道每个步骤如何处理向量v
,我用一些println
指令填充了代码,我的输出如下:
In main(), BEFORE running met()
obj.v[0] = 40
obj.v = [I@78456a0c
INSIDE method met()
obj.v[0] = 30
v[0] = 3
obj.v = [I@15357784
v = [I@78456a0c
In main(), AFTER running met()
obj.v[0] = 3
obj.v = [I@78456a0c
我对很多事情感到非常困惑,第一个是为什么在静态方法obj.v
中调用main()
时的引用与非静态内部v
的引用相同方法met()
。此外,在没有对象的情况下调用v
究竟是什么(当然是在非静态上下文中)?
我是Java的新手,我真的有无数的问题,我希望答案可以完全解决它们......提前感谢您的帮助。
为了完整起见,完整的代码是
public class tryClass
{
int[] v = {0};
tryClass obj;
public void met ()
{
obj = new tryClass();
obj.v[0] = 30;
v[0]=3;
System.out.println("\nINSIDE method met()");
System.out.println("\tobj.v[0] = "+obj.v[0]);
System.out.println("\tv[0] = "+v[0]);
System.out.println("\tobj.v = "+obj.v);
System.out.println("\tv = "+v);
}
public static void main (String[] args)
{
tryClass obj = new tryClass();
int[] v = new int[1];
obj.v[0] = 40;
System.out.println("In main(), BEFORE running met()");
System.out.println("\tobj.v[0] = "+obj.v[0]);
System.out.println("\tobj.v = "+obj.v);
obj.met();
System.out.println("\nIn main(), AFTER running met()");
System.out.println("\tobj.v[0] = "+obj.v[0]);
System.out.println("\tobj.v = "+obj.v);
}
}
答案 0 :(得分:1)
为什么obj.v的引用在静态方法main()中调用时与非静态方法met()中的v相同?
答案:因为您没有将其重新分配给内存中的其他对象。它仍指向内存中相同的“数组”对象,即使您在内部更改了数组的内容。
查看您的代码,并附上我的评论:
public class tryClass
{
// here, non-static variable v will be instantiated
// as an array with a length of one, holding the value 0 in it's one slot;
// it will be instantiated when an instance of tryClass is created.
int[] v = {0};
// here, this tryClass has another tryClass named "obj" in it as one of its fields.
tryClass obj;
public void met ()
{
// here, the tryClass's tryClass obj is instantiated
// and this second tryClass's "v" is instantiated
// and then it's one slot is set to 30.
obj = new tryClass();
obj.v[0] = 30;
// now, the first tryClass's "v" is set to 3.
v[0]=3;
}
public static void main (String[] args)
{
// creating a new tryClass. This is NOT the same object as in met.
// But it CONTAINS the same object in met.
// You could call it by going obj.obj.
tryClass obj = new tryClass(); // is this the SAME object as in met() ? Answer: No.
// this does nothing, it just creates another int[] v
// that exists only inside the main() method. It is not
// the same as obj.v!
int[] v = new int[1];
// changing the contents of obj.v, but not reassigning obj.v itself.
obj.v[0] = 40;
// calling met, which will change obj.v's contents again, but not reassign it.
obj.met();
}
}
数组是可变的,这意味着即使它在内存中保持相同的对象,它的内容也可以改变。
答案 1 :(得分:1)
您还应该打印出obj
引用的值,当不在静态上下文中时,也会打印this
的值。这表明,您正在打印v
类的不同实例的tryClass
引用的值...
public class tryClass
{
int[] v = {0};
tryClass obj;
public void met ()
{
obj = new tryClass();
obj.v[0] = 30;
v[0]=3;
System.out.println("\nINSIDE method met()");
System.out.println("\tthis = "+this); //add this
System.out.println("\tobj = "+obj); //add this
System.out.println("\tobj.v[0] = "+obj.v[0]);
System.out.println("\tv[0] = "+v[0]);
System.out.println("\tobj.v = "+obj.v);
System.out.println("\tv = "+v);
}
public static void main (String[] args)
{
tryClass obj = new tryClass();
int[] v = new int[1];
obj.v[0] = 40;
System.out.println("In main(), BEFORE running met()");
System.out.println("\tobj = "+obj); //and this
System.out.println("\tobj.v[0] = "+obj.v[0]);
System.out.println("\tobj.v = "+obj.v);
obj.met();
System.out.println("\nIn main(), AFTER running met()");
System.out.println("\tobj = "+obj); //and also this
System.out.println("\tobj.v[0] = "+obj.v[0]);
System.out.println("\tobj.v = "+obj.v);
}
}
您也可以为该类创建构造函数:
public tryClass() {
System.out.println("Creating new instance of tryClass!");
}
看看发生了什么......
(或者,作为一种学习体验,您可以在IDE中查看调试器。这将会派上用场......)
答案 2 :(得分:1)
执行静态main方法时,首先要做的是创建一个tryClass实例,然后打印一个名为v的数组元素的值,该元素是该实例的一个属性。 (obj.v [0])。 然后调用该实例的方法,因此该方法体中的非静态引用引用该实例的属性(您在main方法上创建的属性)。