在java上将int转换为Object

有时计算机不会给出以下转换：

int a ... 
Object ans = (int) a;

但只有这种转换：

int a ...
Object ans = (Integer) a;

我明白为什么你可以在Object to Integer之间进行转换,但为什么是原始变量 – 有时你可以,有时候你不能进行转换？

谢谢

我附加编译器不允许我在int变量和object之间进行转换的代码：

/** @return minimum element */
    public Object minimum(){
        return minimum(this.root);
    }
    public Object minimum(BSTNode node){
        if (node.left != null) return minimum(node.left);
        return node.data;
    }
        /** @return maximum element */  
    public Object maximum(){
        return maximum(this.root);
    }
    public Object maximum(BSTNode node){
        if (node.right != null) return maximum(node.right);
        return node.data;
    }

    public Object findNearestSmall(Object elem) {
        int diff;
        diff = (int)maximum() - (int)minimum();
        if (compare(minimum(),elem) == 0) return elem;
        else return findNearestSmall(elem,this.root,diff);
    }   
    public Object findNearestSmall(Object elem,BSTNode node,int mindiff){
           if(node == null) return (int)elem - mindiff;

           int diff = (int)elem - (int)node.data;

           if(diff > 0 && mindiff > diff) mindiff = diff;
           /* Case 2 : Look for in left subtree */
           if(compare(node.data,elem)>-1)
                   return findNearestSmall(elem,node.left,mindiff);
           else
           /* Case 3 : Look for in right subtree */ 
                   return findNearestSmall(elem,node.right,mindiff);
    }