Java - 在没有方法的情况下编辑实例变量

Question

I am quite new to Java, and I am constantly looking for ways to improve my code. But I don't seem to get this, if it is even possible to do.

Let's say I have this code (I edited out the irrelevant parts, so the code might seem weird):

public class NewBody {

public static int distanceScale = 5;

    public int x, y;
    public float xMeter = x * distanceScale;
    public float yMeter = y * distanceScale;

    public NewBody(int x, int y){
        this.x = x;
        this.y = y;
    }

    public void pixToMeter(){
         this.xMeter = distanceScale * this.x;
    }

If I don't call pixToMeter() and just try to use "instance.xMeter" directly, it just returns the vaulue 0, even though I've already set the x variable in the constructor.

So my question is: Is there a way to properly set variables without calling a method to do it? It seems highly unneccessary since I am not even passing a parameter to it.

Sorry for my poor english, I hope you understand what I am trying to say.

Answer 1

The initialisation of xMeter is done when x is still zero.

This is what actually happens:

public NewBody(int x, int y) {
    // All fields are zeroed: 0, null, 0.0.

    super(); // Object constructor, as Object is the parent class.

    // Those fields that are initialized:
    xMeter = this.x * distanceScale; // 0.0f * 5
    yMeter = this.y * distanceScale;

    // The rest of the constructor:
    this.x = x;
    this.y = y;
}

For a depending value:

public final void setX(int x) {
    this.x = x;
    xMeter = this.x * distanceScale;
}

And to apply the DRY principle (Don't Repeat Yourself): one could drop the initialisation of xMeter and call setX(x) in the constructor instead.

When called in the constructor it is important to make setX final, that is: not overridable.

Answer 2

The source of the problem is here:

public float xMeter = x * distanceScale;

The issue is that you're initializing this instance variable outside the constructor. As a result, since x is initialized to 0, the result of your multiplication is also 0.

If you need xMeter and yMeter initialized to a value based on x or y , simply declare them as you did the other fields:

public int xMeter;

And initialize their values in the constructor:

public newBody(int x, int y){
    // initialize x and y ...
    this.xMeter = x * distanceScale;

Answer 3

As others have mentioned, when the xMeter is initialized, the constructor is not called yet and x is still 0 , so the value of xMeter is 0 as well.

To change that, you must update xMeter 's value once x is initialized in the constructor, like so:

public NewBody(int x, int y){
    this.x = x;
    this.y = y;

    // update x and y meter
    xMeter = x * distanceScale;
    yMeter = y * distanceScale;
}

However, you mentioned how you want xMeter to update every time x is changed as well. As it stands with your current code, that will not happen. However, a suggestion of mine would be to create a method to change the value of x (and y as well) and in those methods, also update the values of xMeter and yMeter . That way, whenever you want to change x , call the methods and it will update your other values too.

Try adding these methods and changing your constructor to this:

// called setter methods
public void setX(int x) {
    this.x = x;
    this.xMeter = x * distanceScale;
}
public void setY(int y) {
    this.y = y;
    this.yMeter = y * distanceScale;
}

// constructor
public NewBody(int x, int y){
    setX(x);
    setY(y);
}