从构造函数创建对象

Question

I am a newbie to Java, just been fiddling with the code for a while.

public class ThreeVector {
private double x,y,z;   // definign local variables 

public ThreeVector(){} // a constructor that has no input

public ThreeVector (double va1,double va2, double va3){va1=x;va2=y;va3=z;};// creatign a constructor , so can be used for calling by a method later
// Takes 3 values 

public double magnitude (){
    double y1= Math.sqrt(x*x+y*y+z*z);
    return y1 ; // finds the magnitude of a vector
}

public ThreeVector unitv(){

    ThreeVector unitv= new ThreeVector ();
unitv.ThreeVector(x/magnitude(),y/magnitude(),z/magnitude());
}

Now here is where I get stuck. I created an object unitV so I could call the ThreeVector constructor, but the compiler keeps saying to create a new method for ThreeVector . Not sure whats going on...

Answer 1

A constructor can be called only using the new keyword. What you're doing here:

unitv.ThreeVector(x/magnitude(),y/magnitude(),z/magnitude());

is calling a method called ThreeVector , so the compiler complains that there's no such method in your ThreeVector class.

To fix this, you must use the ThreeVector constructor with the arguments to create your unitv instance instead:

public ThreeVector unitv(){
    ThreeVector unitv = new ThreeVector(x/magnitude(),y/magnitude(),z/magnitude());
    //and, of course, return this ThreeVector instance
    return unitv;
}

And this code can be shorten to

public ThreeVector unitv() {
    return new ThreeVector(x/magnitude(),y/magnitude(),z/magnitude());
}

But since you can have your x , y and z with 0 value at the same time, it would be better to change the logic in your unitv method to first get the magnitude value and doing an evaluation, this in order to avoid a division by 0:

public ThreeVector unitv() {
    double magnitude = magnitude();
    if (magnitude != 0) {
        return new ThreeVector(x/magnitude, y/magnitude, z/magnitude);
    }
    return new ThreeVector(0, 0, 0);
}

---

By the way, your constructor logic is wrong, you're assigning the fields values to the arguments, it should be the other way around:

public ThreeVector (double va1,double va2, double va3) {
    x = va1;
    y = va2;
    z = va3
}

Answer 2

No you need to directly call the constructor:

ThreeVector unitv= new ThreeVector(x,y,z);

Only after this you can call the magnitude method

Also your constructor does the assignments the wrong way:

Should be: public ThreeVector (double va1,double va2, double va3){x = va1;y = va2;z=va3;}

Answer 3

You need to initialize a new ThreeVector(va1, va2, va3) .

Attention

Your code in the constructor is wrong.

{
    // instance field assigned to argument        
    va1=x;va2=y;va3=z;
}

... should be:

{
    // argument assigned to instance field
    x = va1;
    y = va2;
    z = va3;
}