Initialization of variables in Java

Question

Good night Folks!

I’m learning java, and if you can help me with that question,...

I saw it in a course that I’m doing, in a book that I’m reading, and on websites where I researched this assuming the same statement, which is more or less this: "Primitive type instance variables are initialized by default, variables of byte, char, short, int, long, float and double types are initialized as 0" - Devmedia(https://www.devmedia.com.br/tipos-de-dados-por-valor-e-por-referencia-em-java/25293)

However, this code shows me an error:

int a;
System.out.println("Valor de a: " + a);

In the case of the eclipse, he accuses this error:

The local variable a may not have been initialized

If the default value of numeric primitive types is zero, why do I need to explicitly initialize a variable of this type? In the case of this example:

int a = 0;

Pure javac also accuses this same error. You know why the compiler behaves this way ?

Answer 1

As in the very description you posted, just the instance variables, also called attributes, have default values.

Instance variables would be variables defined at class level, not at the scope of a function. For example:

class Saldo {
    public int reais;    // <-- atributo, possui valor padrão
    public int centavos; // <-- atributo, possui valor padrão
}

If I create an instance of that class var saldo = new Saldo(), i don’t need to manually initialize these attributes reais and centavos, they will have the default value 0.

However for a variable declared in a local scope of a function, it needs to be initialized before being accessed, as in its own example:

public static void main() {
    int i = 0; // <-- variável local, não possui valor padrão.
}