What is the difference between assigning an instance variable before and after parameter?

What is the difference between assigning a parameter to an instance variable? Why is it wrong when you write the parameter before the instance variable?

int variable; 

void set(int parameter)
{
    variable = parameter;
    parameter = variable; 
}

Case 1:

int variable;

void set(int parameter)
{
    variable = parameter;
}

Case 2:

int variable;

void set(int parameter)
{
    parameter = variable;
}

Both cases are correct by Java syntax, but case 2 has very little logical value...

The case 2 method parameter has a value and we need to use it. but before using this value, we change this by assigning variable, so we lost the previous value.