Passing final variables to anonymous classes

Question

In final variable passed to anonymous class via constructor, Jon Skeet mentioned that variables are passed to the anonymous class instance via an auto-generated constructor. Why would I not be able to see the constructor using reflection in that case:

public static void main(String... args) throws InterruptedException {
final int x = 100;
new Thread() {
    public void run() {
        System.out.println(x);      
        for (Constructor<?> cons : this.getClass()
                .getDeclaredConstructors()) {
            StringBuilder str = new StringBuilder();
            str.append("constructor : ").append(cons.getName())
                    .append("(");
            for (Class<?> param : cons.getParameterTypes()) {
                str.append(param.getSimpleName()).append(", ");
            }
            if (str.charAt(str.length() - 1) == ' ') {
                str.replace(str.length() - 2, str.length(), ")");
            } else
                str.append(')');
            System.out.println(str);
        }
    }

}.start();
Thread.sleep(2000);

}

The output is:

100
constructor : A$1()

score 29 · Answer 1 · answered Sep 19 '11 at 14:10

29

In this case, it's because 100 is a constant. That gets baked into your class.

If you change x to be:

final int x = args.length;

... then you'll see Test$1(int) in the output. (This is despite it not being explicitly declared. And yes, capturing more variables adds parameters to the constructor.)

answered Sep 19 '11 at 14:10

Jon Skeet

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1

@Bohemian: Given that I know the origin of the question, I think it is :) – Jon Skeet Sep 19 '11 at 14:12

score 18 · Accepted Answer · answered Sep 19 '11 at 14:12

Here is what your program prints out on my system:

100
constructor : A$1()

So the constructor is there. However, it is parameterless. From looking at the disassembly, what happens is that the compiler figures out that it doesn't need to pass x to run() since its value is known at compile time.

If I change the code like so:

public class A {

    public static void test(final int x) throws InterruptedException {
        new Thread() {
            public void run() {
                System.out.println(x);
                for (Constructor<?> cons : this.getClass()
                        .getDeclaredConstructors()) {
                    StringBuilder str = new StringBuilder();
                    str.append("constructor : ").append(cons.getName())
                            .append("(");
                    for (Class<?> param : cons.getParameterTypes()) {
                        str.append(param.getSimpleName()).append(", ");
                    }
                    if (str.charAt(str.length() - 1) == ' ') {
                        str.replace(str.length() - 2, str.length(), ")");
                    } else
                        str.append(')');
                    System.out.println(str);
                }
            }

        }.start();
        Thread.sleep(2000);
        }

    public static void main(String[] args) throws InterruptedException {
        test(100);
    }

}

The constructor that gets generated is now:

constructor : A$1(int)

The sole argument is the value of x.

Passing final variables to anonymous classes

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