Unexpected error using lambdas in Java 8

Question

I'm using Java 8 Update 20 32 bits, Maven 3.2.3, Eclipse Luna Build id: 20140612-0600 32 bits.

After starting using lambdas, some classes in my projects started to report compilation errors in maven (mvn compile).

These errors appears only when I use lambdas. If I switch back to anonymous classes, the errors are gone.

I can reproduce the error with a simple test case:

package br;

import java.awt.Button;
import java.awt.Panel;

public class Test {

    private final Button button;
    private final Panel panel;

    public Test() {
        button = new Button();
        button.addActionListener(event -> {
            System.out.println(panel);
        });
        panel = new Panel();
    }
}

I compile it this way:

mvn clean;mvn compile

And I get this error:

[ERROR] /C:/Users/fabiano/workspace-luna/Test/src/main/java/br/Test.java:[14,44] variable panel might not have been initialized

Although the error message is pretty clear about what is happening (the compiler thinks the final variable panel is being called before it is instantiated), the variable will not be called until the button generates an action, and how we can't say when the action will happen, the code should compile. Indeed, it compiles as it should if I don't use lambdas:

package br;

import java.awt.Button;
import java.awt.Panel;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class Test {

    private final Button button;
    private final Panel panel;

    public Test() {
        button = new Button();
        button.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                System.out.println(panel);
            }
        });
        panel = new Panel();
    }
}

I noticed two other strange things related to this problem:

1. Eclipse don't report this error when it auto-compile the class. Eclipse is using the same JDK as maven to compile the class.
2. If I use maven to compile the class using anonymous classes, then I change the class to use lambdas and compile it using maven again, it doesn't report the error. In this case it just reports the error again if I use mvn clean followed by mvn compile.

Can someone help me to fix this problem? Or try to reproduce this problem?

Answer 1

Although the error message is pretty clear about what is happening (the compiler thinks the final variable "panel" is being called before it is instantiated), the variable will not be called until the button generates an action, and how we can´t say when the action will happen, the code should compile.

You should consider the fact that compilers follow formal rules and don’t have your knowledge. Especially, a compiler can’t know that the method addActionListener does not invoke the actionPerformed method immediately. It also has no idea about the visibility of the button which determines when actionPerformed might be called.

The formal behavior has a specification. There you find the following points:

Chapter 16. Definite Assignment

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.

An access to its value consists of the simple name of the variable (or, for a field, the simple name of the field qualified by this) occurring anywhere in an expression except as the left-hand operand of the simple assignment operator = (§15.26.1).

and

15.27.2. Lambda Body

…

Unlike code appearing in anonymous class declarations, the meaning of names and the this and super keywords appearing in a lambda body, along with the accessibility of referenced declarations, are the same as in the surrounding context (except that lambda parameters introduce new names).

In your code, the meaning of names within the lambda body, read of panel, is the same as in the surrounding context which is the constructor. In that context, the rule that “every blank final field must have a definitely assigned value when any access of its value occurs” applies.

And, yes, that’s different from inner class definitions. The specification explicitly states that.