inject dependencies methods constructor?

dependency injection seems good thing. general, should dependencies injected during methods need them, should injected contructor class?

see samples next denote twin ways inject same dependency.

//inject dependency methods need importantclass
class something {

    open something()
    {
         //empty
    }

    open vacant a() 
    {
         //do something but x
    }

    open vacant b(importantclass x)
    {
         //do something x
    }

    open vacant c(importantclass x)
    {
         //do something x
    }
}

//inject dependency constructor once
class something {
    private importantclass _x
    open something(importantclass x)
    {
         this._x = x;
    }

    open vacant a() 
    {
         //do something but x
    }

    open vacant b()
    {
         //do something this._x
    }

    open vacant c()
    {
         //do something this._x
    }

}