goto in Java bytecode

Question

So the other day when I was looking at the wikipedia page for Java bytecode I came across this example:

Consider the following Java code:

  outer:
  for (int i = 2; i < 1000; i++) {
      for (int j = 2; j < i; j++) {
          if (i % j == 0)
          continue outer;
     }
     System.out.println (i);
  }

A Java compiler might translate the Java code above into byte code as follows, assuming the above was put in a method:

  0:   iconst_2
  1:   istore_1
  2:   iload_1
  3:   sipush  1000
  6:   if_icmpge       44
  9:   iconst_2
  10:  istore_2
  11:  iload_2
  12:  iload_1
  13:  if_icmpge       31
  16:  iload_1
  17:  iload_2
  18:  irem
  19:  ifne    25
  22:  goto    38
  25:  iinc    2, 1
  28:  goto    11
  31:  getstatic       #84; //Field java/lang/System.out:Ljava/io/PrintStream;
  34:  iload_1
  35:  invokevirtual   #85; //Method java/io/PrintStream.println:(I)V
  38:  iinc    1, 1
  41:  goto    2
  44:  return

And I notice that little word goto appears a couple of times, which upon checking the JVM specification is valid. My question is why? GOTO is a reserved but unusable keyword in Java, so why when we write and compile java code does it seem to get compiled with goto back into it. I am wondering it this is just the way things have always been done at a lower level of programming, or whether it is because the JVM is trusted to use the goto word more effectively. Ultimately I am curious as to why goto is considered such bad practice that it is prohibited in java code, yet seems to be put straight back into your code when compiled.

Answer 1

Java structured programming features such as loops (for/ while) are implemented at the bytecode level with conditional branch (IF..) and unconditional jump (GOTO) instructions.

break or continue to an outer loop are also considered sufficiently useful & legitimate within structured programming, that the Java language has these features (break/ continue to label).

At the JVM/ bytecode level, these are also implemented with GOTO.

See:

• https://en.wikipedia.org/wiki/Java_bytecode_instruction_listings
• http://www.artima.com/underthehood/flowP.html