int x = 1;
Consumer<Object> f = (i) -> {
int x = 1; // invalid
};
vs.
Consumer<Object> f = (i) -> {
int x = 1;
};
int x = 1; // valid
Imagine those two blocks inside a method. Why is the second block valid?
This is very similar to normal Java scopes:
int i;
{
int i; // invalid
}
vs.
{
int i; // valid
}
int i;
In the first block you are "shadowing" the original x variable. You will not be able to access the first x anymore by creating a new x.
The second block is okay, because the second x is created on a moment when the first x does not longer exists (it is out of scope).
Basically: in the first case, you are trying to have two variables called x at the same time. In the second case, you are creating two x variables after each other: their lifetimes will not overlap.
for loops or if statements.