为什么初始化实例变量时我会得到零？

Question

i have a simple java program below. i have declared instance integer variable "x1" and "x2", initialize them from parameters in my class constructor. i have also declared an integer variable "diff" that works out the difference between "x1" and "x2" but the variable returns zero!! i don't know where the problem is and my constructor parameters are not zero!!!. i will appreciate any help

public class Smallest {
int x1;
int x2; 

public Smallest(int a,int b){
 this.x1=a;
 this.x2=b;
}

int diff = x2 - x1;   
public void draw(){
    System.out.println("X1= "+ x1 +"\n X2= "+ x2 +"\n diff= " +diff);
}

public static void main(String[] args) {
Smallest small=new Smallest(10,20);
small.draw();
}
}

here's the output

 X1= 10;
 X2= 20;
 diff= 0;

Answer 1

This is because diff is not inside any method it set during compile time. You should move it to the constructor or any other method to get correct value.
Example :

int diff;
public Smallest(int a,int b){
   this.x1=a;
   this.x2=b;
   this.diff=x2 - x1;   
}

Answer 2

int diff = x2 - x1; should be inside the constructor or inside the method draw() (depending on what you want to do with it).

Remember: By default int variables are initialized to 0. Before you call the constructor:

x1=0; 
x2=0; 
diff=x1-x2
    =0-0
    =0

When you call the constructor you give different values to x1 and x2 but you don't calculate the diff on the newly updated values.

After you call the constructor:

x1=10;
x2=20;
diff=0; // you don't recalculate it, it's not in the constructor or in the draw method.

Answer 3

Member variables are initialized before your constructor is called. Thus diff is equal to 0 - 0 since x1 and x2 have not been set yet.