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POIssue with constructors of nested class
 

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  1. POIssue with constructors of nested class
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    <hr> <blockquote> <p>This question is about interesting behavior of Java: it produces additional (not default) constructor for nested classes in some situations.</p> <p>This question is also about strange anonymous class, which Java produces with that strange constructor.</p> </blockquote> <hr> <p>Consider the following code:</p> <pre><code>package a; import java.lang.reflect.Constructor; public class TestNested { class A { A() { } A(int a) { } } public static void main(String[] args) { Class&lt;A&gt; aClass = A.class; for (Constructor c : aClass.getDeclaredConstructors()) { System.out.println(c); } } } </code></pre> <p>This will prints:</p> <pre><code>a.TestNested$A(a.TestNested) a.TestNested$A(a.TestNested,int) </code></pre> <p>Ok. Next, lets make constructor <code>A(int a)</code> private:</p> <pre><code> private A(int a) { } </code></pre> <p>Run program again. Receive:</p> <pre><code>a.TestNested$A(a.TestNested) private a.TestNested$A(a.TestNested,int) </code></pre> <p>It is also ok. But now, lets modify <code>main()</code> method in such way (addition of new instance of class <code>A</code> creation):</p> <pre><code>public static void main(String[] args) { Class&lt;A&gt; aClass = A.class; for (Constructor c : aClass.getDeclaredConstructors()) { System.out.println(c); } A a = new TestNested().new A(123); // new line of code } </code></pre> <p>Then input becomes:</p> <pre><code>a.TestNested$A(a.TestNested) private a.TestNested$A(a.TestNested,int) a.TestNested$A(a.TestNested,int,a.TestNested$1) </code></pre> <p>What is it: <strong>a.TestNested$A(a.TestNested,int,a.TestNested$1)</strong> &lt;&lt;&lt;---??</p> <p>Ok, lets again make constructor <code>A(int a)</code> package local:</p> <pre><code> A(int a) { } </code></pre> <p>Rerun program again (we <strong>don't remove</strong> line with instance of <code>A</code> creation!), output is as in the first time:</p> <pre><code>a.TestNested$A(a.TestNested) a.TestNested$A(a.TestNested,int) </code></pre> <hr> <p><strong>Questions:</strong></p> <p><strong>1)</strong> How this could be explained?</p> <p><strong>2)</strong> What is this third strange constructor?</p> <hr> <p><strong>UPDATE:</strong> Investigation shown following.</p> <p>1) Lets try to call this strange constructor using reflection from other class. We will not able to do this, because there isn't any way to create instance of that strange <code>TestNested$1</code> class.</p> <p>2) Ok. Lets do the trick. Lets add to the class <code>TestNested</code> such static field:</p> <pre><code>public static Object object = new Object() { public void print() { System.out.println("sss"); } }; </code></pre> <p>Well? Ok, now we could call this third strange constructor from another class:</p> <pre><code> TestNested tn = new TestNested(); TestNested.A a = (TestNested.A)TestNested.A.class.getDeclaredConstructors()[2].newInstance(tn, 123, TestNested.object); </code></pre> <p>Sorry, but I absolutely don't understand it.</p> <hr> <p><strong>UPDATE-2:</strong> Further questions are:</p> <p><strong>3)</strong> Why Java use special anonymous inner class for an argument type for this third synthetic constructor? Why not just <code>Object</code> type, of constructor with special name?</p> <p><strong>4)</strong> What Java could use already defined anonymous inner class for those purposes? Isn't this some kind of violation of security?</p>
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    1. COThat's an anonymous inner class of A that's being passed to the third constructor. The question is why it's there or what's inside. Care to decompile?
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    2. COPerhaps it's just a trick to make the private constructor accessible to the inner classes?
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    3. CO@JanDvorak, yes, and where this constructor is actually invoked? I suspect, then when creating instance of `A` passing int value in it. But, really, why?
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      1. POIssue with constructors of nested class
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