Java hates TABs!

Today I Learned one more thing for “tabs versus spaces” debate: Java hates tabs.

Turns out Java compiler counts each tab symbol as 8 spaces while parsing the source code. And when it reports errors it uses the 8-space version of column number.

You don’t get this normally when using javac from the command line, but all IDEs call Java compiler programmatically to access diagnostic information. So here is a sample code that compiles “Hello world” program containing a bit more tabs than usual

import javax.tools.*;
import javax.tools.JavaCompiler.CompilationTask;
import java.net.URI;
import java.util.Collections;

public class Javac {
  static class MemorySource extends SimpleJavaFileObject {
    final String code;
    MemorySource(String name, String code) {
      super(URI.create("memory:///" + name + Kind.SOURCE.extension), Kind.SOURCE);
      this.code = code;
    }
    @Override
    public CharSequence getCharContent(boolean ignoreEncodingErrors) {
      return code;
    }
  }

  public static void main(String[] args) {
    JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
    DiagnosticCollector<JavaFileObject> diagnostics = new DiagnosticCollector<>();

    MemorySource file = new MemorySource("Hello",
                      "public class Hello {\n" +
                      "\t\t\tpublic static void main(String[] args) {\n" +
                      "\t\t\t\t\t\tSyste.out.println(\"Hello world\");\n " +
                      "\t\t\t}\n" +
                      "}\n");
    CompilationTask task = compiler.getTask(null, null, diagnostics, null, null, Collections.singletonList(file));
    task.call();
    for (Diagnostic diagnostic : diagnostics.getDiagnostics()) {
      System.out.println(diagnostic.getKind() + " on line:" + diagnostic.getLineNumber() + " column:" + diagnostic.getColumnNumber() + " - " + diagnostic.getMessage(null));
      int n = 1;
      for (String line : file.code.split("\n")) {
        System.out.println("Line " + (n++) + " length " + line.length());
      }
    }
  }
}

And the output it produces says that error is around column 54 while the longest line of code is just 43 characters long.

ERROR on line:3 column:54 - package Syste does not exist
Line 1 length 20
Line 2 length 43
Line 3 length 39
Line 4 length 5
Line 5 length 1

One more point in favor of spaces!

Comments

[Vincent A. Cicirello]

What error do you get with this same example from javac at the command line? I think my brain filters out any column numbers in errors.