Uninitialized class members in Java do not issue any compiler errors. local variables however do. Why?

Question

Consider the following code snippet in Java. It won\'t compile.

package temppkg;

final public class Main
{
    private String x;
    private int y;

    pri         


        

Answer 1

Member variables are automatically initialized to their default values when you construct (instantiate) an object. That holds true even when you have manually initialized them, they will be initialized to default values first and then to the values you supplied.

This is a little little lengthy article but it explains it: Object initialization in Java

Whereas for the local variables (ones that are declared inside a method) are not initialized automatically, which means you have to do it manually, even if you want them to have their default values.

You can see what the default values are for variables with different data types here.

The default value for the reference type variables is null. That's why it's throwing NullPointerException on the following:

System.out.println(x.toString()); // Causes a NullPointerException but doesn't issue a compiler error.

In the following case, the compiler is smart enough to know that the variable is not initialized yet (because it's local and you haven't initialized it), that's why compilation issue:

System.out.println(z.toString()); // "Cuases a compile-time error - variable z might not have been initialized.