Default Values and Initialization in Java

Question

Based on my reference, primitive types have default values and Objects are null. I tested a piece of code.

public class Main {
    public static void main(String[] args) {
        int a;
        System.out.println(a);
    }
}

The line System.out.println(a); will be an error pointing at the variable a that says variable a might not have been initialized whereas in the given reference, integer will have 0 as a default value. However, with the given code below, it will actually print 0.

public class Main {
    static int a;
    public static void main(String[] args) {
        System.out.println(a);
    }
}

What could possibly go wrong with the first code? Does class instance variable behaves different from local variables?

Answer 1

There are a few things to keep in mind while declaring primitive type values.

They are:

1. Values declared inside a method will not be assigned a default value.
2. Values declared as instanced variable or a static variable will have default values assigned which is 0.

So in your code:

public class Main {
    int instanceVariable;
    static int staticVariable;
    public static void main(String[] args) {
        Main mainInstance = new Main() 
        int localVariable;
        int localVariableTwo = 2;
        System.out.println(mainInstance.instanceVariable);
        System.out.println(staticVariable);
       // System.out.println(localVariable); //will throw compilation error
        System.out.println(localVariableTwo);

    }
}

Answer 2

Read more carefully your reference:

Default Values

It's not always necessary to assign a value when a field is declared. Fields that are declared but not initialized will be set to a reasonable default by the compiler. Generally speaking, this default will be zero or null, depending on the data type. Relying on such default values, however, is generally considered bad programming style.

The following chart summarizes the default values for the above data types.

. . .

Local variables are slightly different; the compiler never assigns a default value to an uninitialized local variable. If you cannot initialize your local variable where it is declared, make sure to assign it a value before you attempt to use it. Accessing an uninitialized local variable will result in a compile-time error.