why I have to anim.getComponent in update() function when I had done in Start () function

Hi , I am trying to make a simple 2D game and I am using Animation State machine for making animations. I am having 4 animation states-
Idle
Walk
Jet
Wait

Layer default state is "Idle"

now I want to make transition from idle--> walk when moving and from walk --> Idle when not moving.

So what I did-
I had made animation parameter as "AnimState"
and set conditions as follows-
-AnimState EQUALS 1 (for transition Idle-->walk)
-AnimState EQUALS 2 (for transition walk-->Idle)

The thing is it is working in one condition and not working in another everything described below-

Case 1-
I had declared "anim" as a variable of type Animation, get its component in start function and then use it in update as described below in simplified version of code.
In this case it's not working.
[code=CSharp]private Animator anim;

void Start()
{
anim=getComponent();
}
void Update()
{
if (movement conditions)
{
movement code....................
..............................................
anim.setInteger("AnimState",1);
}
else(.......)
{
.................
}
}[/code]

case 2-
I had declared "anim" as a variable of type Animation, get its component in start function and then use it in update as described below in simplified version of code.

[code=CSharp]private Animator anim;

void Start()
{
anim=getComponent();
}
void Update()
{
if (movement conditions)
{
movement code....................
..............................................
anim.getComponent().setInteger("AnimState",1); //this is working
}
else(.......)
{
.................
}
}[/code]

this is working.