Why Java initializing only class variables by default but not local variables?

Question

I am learning linkedlist in Java and I have three files Main.java, List.java and Node.java. When I am doing this, I got a question why should I initialize a local variable which is declared in method and but not class variable which is declared in class.

In the first pic, I declared head as class variable, it doesn't throw any error.

But in the second pic I initialized head as local variable.Now, it throws a error to initialize local variable.

What makes the difference when declared as class variable?

Beginner in Java.

Update: I know how to fix this but I am not clear why Java initializing only class variables by default but not local variables.

Answer 1

Static/Non-static fields that are not primitives, like your Node, are initialized at null by default. Static/Non-static fields that are primitive gets their default values.

There's also another case where some variables are initialized with default: when you instantiate an array. Each cell represents has default value, regarding the type:

• 0 for int
• null for Integer
• etc.

However, in a local method, compiler does not assign default value to local variables.
That's why your IDE warns about: "may not be initialized!".

To understand why, you may be interested in this post.

Answer 2

This is explained rather well by the Java Language Specification (specifically, §4.12.5):

Every variable in a program must have a value before its value is used:

• Each class variable, instance variable, or array component is initialized with a default value when it is created (§15.9, §15.10):
• For all reference types (§4.3), the default value is null.
• A local variable (§14.4, §14.14) must be explicitly given a value before it is used, by either initialization (§14.4) or assignment (§15.26), in a way that can be verified using the rules for definite assignment (§16).

To expand a bit, §16 goes into the rules for definite assignment, which is at the crux of the reason:

Each local variable (§14.4) and every blank final field (§4.12.4, §8.3.1.2) must have a definitely assigned value when any access of its value occurs.

For every access of a local variable or blank final field x, x must be definitely assigned before the access, or a compile-time error occurs.

Simply put: Java will assign default values to class/instance variables, but will not assign a default value to a local variable. Local variables must be definitely assigned in this manner (either by initialization or assignment), or a compile time error will occur (as you observe).

If you think of it from another angle, when you initialize a class that contains specific fields, you may not want those initialized to a value at first (think JavaBeans). If you're in a code block and you declare a variable, the expectation instead is on the developer to control that object's life cycle right there in the block.

It doesn't make sense to simply declare a variable and attempt to do something with it without assigning a value to it, as the variable doesn't have a value.