getDeclaredConstructors() lists 2 constructors but there is only one

Question

Given this little pice of code:

import java.util.Arrays;

public class Sample {
    private final int test;

    private Sample(int test) {
        this.test = test;
    }

    public static void main(String[] args) {
        System.out.println(Arrays.toString(Hello.class.getDeclaredConstructors()));
    }

    public static class Hello {
        private final int i;

        private Hello(int i) {
            this.i = i;
        }

        public int getI() {
            return i;
        }

        public static class Builder {
            private int i;

            private Builder() {

            }

            public static Builder builder() {
                return new Builder();
            }

            public void add(int i) {
                this.i = i;
            }

            public Hello build() {
                return new Hello(i);
            }
        }
    }
}

i don't understand the output that is shown:

[private Sample$Hello(int), Sample$Hello(int,Sample$1)]

what is the second listed constructor in here with the second parameter to the Builder. I thought that the output will only show the private constructor of Hello but not the second one.

I was just about to say that that looks synthetic. A disassembly of the class (and of the `$1` class) might be instructive. — chrylis -cautiouslyoptimistic-, Mar 10 '14 at 08:53
Run a `javap` command, to see how your `Hello` class look like. — Rohit Jain, Mar 10 '14 at 08:57
It's interesting that there are only static nested classes though... — Jon Skeet, Mar 10 '14 at 09:08
@EJP - But this class is `static`! Why synthesize the `this` version when not needed? — OldCurmudgeon, Mar 10 '14 at 09:09
It's synthesized by the compile for some reason, but what Sample$1 refers to is a mystery. Are you sure this is the real code? The whole code? And nothing but the code? — user207421, Mar 10 '14 at 09:26
this is the real code nothing more nothing less, so this is why i am so astonished about the second constructor that is added by the compiler. — BigMichi1, Mar 10 '14 at 09:36
possible duplicate of [Issue with constructors of nested class](http://stackoverflow.com/questions/14266052/issue-with-constructors-of-nested-class) — assylias, Mar 10 '14 at 10:47

score 4 · Accepted Answer · answered Mar 10 '14 at 09:40

4

Builder is a static nested class of Hello, therefore it should be able to access private constructor Sample$Hello(int).

However, JVM doesn't have any special support for nested classes, therefore, from the JVM point of view, Builder cannot access Sample$Hello(int).

In order to solve this problem compiler generates synthetic constructor Sample$Hello(int,Sample$1) with default visibility, and calls it instead of Sample$Hello(int) from Builder.

Sample$1 is an empty class generated by the compiler to make distinguished signature for Sample$Hello(int,Sample$1). Actually, the second argument of this constructor is always null.

answered Mar 10 '14 at 09:40

axtavt

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Makes sense - would like a reference to back your suggestion though. – OldCurmudgeon Mar 10 '14 at 09:58
I cannot find any official proof so far, but you can verify this behavior by doing `javap -c -private Sample$Hello` and `javap -c -private Sample$Hello$Builder`. Also, synthetic constructor disappears if you comment out `new Hello()` in `Builder` or make constuctor of `Hello` `public`. – axtavt Mar 10 '14 at 12:12

getDeclaredConstructors() lists 2 constructors but there is only one

1 Answers1