Java compiler does not work when compiling a simple program

Question

I have a problem with the Java compiler. I simplified my code:

package a; public class Base { // compiles if this is made public or an int protected Integer value = 0; } --- package b; // must be in a separate package import a.Base; public class Sub extends Base { public void increment() { System.out.println(super.value); value++; super.value = 1; super.value = super.value + 1; // this line crashes the compiler; the others all work super.value++; } } 

after compilation I get:

 Information:java: An exception has occurred in the compiler (1.8.0_51). Please file a bug at the Java Developer Connection (http://java.sun.com/webapps/bugreport) after checking the Bug Parade for duplicates. Include your program and the following diagnostic in your report. Thank you. Information:java: java.lang.NullPointerException Information:java: at com.sun.tools.javac.code.Symbol$ClassSymbol.isSubClass(Symbol.java:1020) Information:java: at com.sun.tools.javac.comp.Lower.accessClass(Lower.java:1108) Information:java: at com.sun.tools.javac.comp.Lower.accessSymbol(Lower.java:983) Information:java: at com.sun.tools.javac.comp.Lower.access(Lower.java:1220) Information:java: at com.sun.tools.javac.comp.Lower.visitSelect(Lower.java:3855) Information:java: at com.sun.tools.javac.tree.JCTree$JCFieldAccess.accept(JCTree.java:1897) Information:java: at com.sun.tools.javac.tree.TreeTranslator.translate(TreeTranslator.java:58) Information:java: at com.sun.tools.javac.comp.Lower.translate(Lower.java:2371) Information:java: at com.sun.tools.javac.comp.Lower.translate(Lower.java:2382) Information:java: at com.sun.tools.javac.comp.Lower.visitVarDef(Lower.java:3547) Information:java: at com.sun.tools.javac.tree.JCTree$JCVariableDecl.accept(JCTree.java:852) Information:java: at com.sun.tools.javac.tree.TreeTranslator.translateVarDefs(TreeTranslator.java:78) Information:java: at com.sun.tools.javac.comp.Lower.visitLetExpr(Lower.java:3859) Information:java: at com.sun.tools.javac.tree.JCTree$LetExpr.accept(JCTree.java:2426) Information:java: at com.sun.tools.javac.comp.Lower.visitLetExpr(Lower.java:3860) Information:java: at com.sun.tools.javac.comp.Lower.visitUnary(Lower.java:3319) Information:java: at com.sun.tools.javac.tree.JCTree$JCUnary.accept(JCTree.java:1746) Information:java: at com.sun.tools.javac.tree.TreeTranslator.visitExec(TreeTranslator.java:245) Information:java: at com.sun.tools.javac.tree.JCTree$JCExpressionStatement.accept(JCTree.java:1296) Information:java: at com.sun.tools.javac.tree.TreeTranslator.translate(TreeTranslator.java:70) Information:java: at com.sun.tools.javac.tree.TreeTranslator.visitBlock(TreeTranslator.java:162) Information:java: at com.sun.tools.javac.comp.Lower.visitBlock(Lower.java:3561) Information:java: at com.sun.tools.javac.tree.JCTree$JCBlock.accept(JCTree.java:909) Information:java: at com.sun.tools.javac.tree.TreeTranslator.visitMethodDef(TreeTranslator.java:145) Information:java: at com.sun.tools.javac.comp.Lower.visitMethodDefInternal(Lower.java:2828) Information:java: at com.sun.tools.javac.comp.Lower.visitMethodDef(Lower.java:2737) Information:java: at com.sun.tools.javac.tree.JCTree$JCMethodDecl.accept(JCTree.java:778) Information:java: at com.sun.tools.javac.comp.Lower.visitClassDef(Lower.java:2508) Information:java: at com.sun.tools.javac.tree.JCTree$JCClassDecl.accept(JCTree.java:693) Information:java: at com.sun.tools.javac.comp.Lower.translate(Lower.java:2390) Information:java: at com.sun.tools.javac.comp.Lower.translateTopLevelClass(Lower.java:3932) Information:java: at com.sun.tools.javac.main.JavaCompiler.desugar(JavaCompiler.java:1512) Information:java: at com.sun.tools.javac.main.JavaCompiler.desugar(JavaCompiler.java:1356) Information:java: at com.sun.tools.javac.main.JavaCompiler.compile2(JavaCompiler.java:901) Information:java: at com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:860) Information:java: at com.sun.tools.javac.main.Main.compile(Main.java:523) Information:java: at com.sun.tools.javac.api.JavacTaskImpl.doCall(JavacTaskImpl.java:129) Information:java: at com.sun.tools.javac.api.JavacTaskImpl.call(JavacTaskImpl.java:138) Information:java: at org.jetbrains.jps.javac.JavacMain.compile(JavacMain.java:168) Information:java: at org.jetbrains.jps.incremental.java.JavaBuilder.compileJava(JavaBuilder.java:382) Information:java: at org.jetbrains.jps.incremental.java.JavaBuilder.compile(JavaBuilder.java:296) Information:java: at org.jetbrains.jps.incremental.java.JavaBuilder.doBuild(JavaBuilder.java:204) Information:java: at org.jetbrains.jps.incremental.java.JavaBuilder.build(JavaBuilder.java:176) Information:java: at org.jetbrains.jps.incremental.IncProjectBuilder.runModuleLevelBuilders(IncProjectBuilder.java:1202) Information:java: at org.jetbrains.jps.incremental.IncProjectBuilder.runBuildersForChunk(IncProjectBuilder.java:877) Information:java: at org.jetbrains.jps.incremental.IncProjectBuilder.buildTargetsChunk(IncProjectBuilder.java:948) Information:java: at org.jetbrains.jps.incremental.IncProjectBuilder.buildChunkIfAffected(IncProjectBuilder.java:840) Information:java: at org.jetbrains.jps.incremental.IncProjectBuilder.buildChunks(IncProjectBuilder.java:665) Information:java: at org.jetbrains.jps.incremental.IncProjectBuilder.runBuild(IncProjectBuilder.java:372) Information:java: at org.jetbrains.jps.incremental.IncProjectBuilder.build(IncProjectBuilder.java:193) Information:java: at org.jetbrains.jps.cmdline.BuildRunner.runBuild(BuildRunner.java:137) Information:java: at org.jetbrains.jps.cmdline.BuildSession.runBuild(BuildSession.java:293) Information:java: at org.jetbrains.jps.cmdline.BuildSession.run(BuildSession.java:124) Information:java: at org.jetbrains.jps.cmdline.BuildMain$MyMessageHandler$1.run(BuildMain.java:242) Information:java: at org.jetbrains.jps.service.impl.SharedThreadPoolImpl$1.run(SharedThreadPoolImpl.java:41) Information:java: at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) Information:java: at java.util.concurrent.FutureTask.run(FutureTask.java:266) Information:java: at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) Information:java: at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) Information:java: at java.lang.Thread.run(Thread.java:745) Information:java: Errors occurred while compiling module 'testAGH' Information:2015-10-23 14:23 - Compilation completed with 1 error and 0 warnings in 671ms Error:java: Compilation failed: internal java compiler error 

Why can't Java compile this?

Answer 1

This is clearly a compiler error. You should indicate an error as the output indicates, but since you asked why I will try to study this :)

JDK langtools repo contains code for javac . I looked at several different built ones , but could not find a version with line numbers that matched the stack (I don’t how to match the Java version with the build number ), so I’ll leave the latest build, b132 .

As you can see, the main problem is a NullPointerException in com.sun.tools.javac.code.Symbol in Symbol.ClassSymbol.isSubClass() . So the source of the question. Looking higher in the stack trace, we can see access() calls to accessSymbol() , then accessClass() before reaching isSubClass() . Therefore, we can conclude that the compiler is trying to verify that the subclass is allowed to access super.value at the time of its failure.

Not knowing which line NPE occurs on it, it is difficult to pinpoint the exact problem, so, unfortunately, everything here becomes fuzzy. Since this failure occurs only when we try to increase and set the ( ++ or += ) field, and only if we call it super.value , we expect to see some code related to casting, autoboxing, assignment or arithmetic . Alas, nothing in isSubClass() really related to this.

My guess, given that there is nothing incriminating in isSubClass() , is that Lower.accessClass() passes null to isSubClass() . In this method, three parameters are passed to isSubClass() : field currentClass (like c , this is “currently class”), sym.owner (I think sym is our value field, I don't know what its owner , but presumably the class he declared), and types , which always looks set to a nonzero value. So this is either currentClass or sym.owner , and there are links to sym.owner that would previously be Lower.access() in Lower.access() , so it also seems not null.

Therefore, I will be afraid that the culprit of Lower.currentClass for some reason is null. Since currentClass is a private closing field, I'm not going to try to determine if it can be null at this time or not, but it seems possible. Of course, there are several other possible culprits in isSubClass() , so I could be wrong.

Endless, but I hope it was informative! Of course, I will be updated if I learn more.