You declared them, but you didn’t initialize them. Initializing them is setting them equal to a value:
int a; // This is a declaration
a = 0; // This is an initialization
int b = 1; // This is a declaration and initialization
You get the error because you haven’t initialized the variables, but you increment them (e.g., a++) in the for loop.
Java primitives have default values but as one user commented below
Their default value is zero when declared as class members. Local variables don’t have default values
