local variable vs instance variable (跟 Multi-thread 有關)

class A{       

}
class B{
 /*  
 * B declare 'a' ---is Instance Variable and its Reference Type is A,
 *    
 */ 
 A a;   

 public void setLocalVariable(int i){
    int a=0;//a is Local Variables   
    a=i;   
    System.out.println(a);//印出來會是i的值不是null   
 }

}