Unreachable statement: while true vs if true [duplicate]

Question

How should I understand this Java compiler behaviour?

while (true) return;
System.out.println("I love Java");
// Err: unreachable statement

if (true) return;
System.out.println("I hate Java");
// OK.

Thanks.

EDIT:

I find out the point after a few minutes:

In the first case compiler throws error because of infinite loop. In both cases compiler does not think about the code inside the statement consequent.

EDIT II:

What impress me on javac now is:

    if (true) return; // Correct
}
    while (true) return; // Correct
}

It looks like javac knows what is inside both loop and if consequent, but when you write another command (as in the first example) you get non-equivalent behaviour (which looks like javac forgot what is inside loop/if).

public static final EDIT III: As the result of this answer I may remark (hopefully correct): Expressions as if (arg) { ...; return;} and while (arg) { ...; return;} are equivalent both semantically and syntactically (in bytecode) for Java iff argv is non-constant (or effectively final type) expression. If argv is constant expression bytecode (and behaviour) may differs.

Disclaimer This question is not on unreachable statements but different handling of logically equivalent expressions such as while true return and if true return.

Answer 1

If you change your code slightly (remove the constant expression), so it doesnt trigger javac reachability it will actually produce identical bytecode for both.

static boolean flag = true;

static void twhile(){
    while (flag) return;
    System.out.println("Java");
}
static void tif(){
    if (flag) return;
    System.out.println("Java");
}

The resulting bytecode:

  static void twhile();
    descriptor: ()V
    flags: ACC_STATIC
    Code:
      stack=2, locals=0, args_size=0
      StackMap locals:
      StackMap stack:
     0: getstatic     #10                 // Field flag:Z
     3: ifeq          7
     6: return
      StackMap locals:
      StackMap stack:
     7: getstatic     #20                 // Field java/lang/System.out:Ljava/io/PrintStream;
    10: ldc           #26                 // String Java
    12: invokevirtual #28                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
    15: return
      LineNumberTable:
    line 8: 0
    line 9: 7
    line 10: 15
      LocalVariableTable:
    Start  Length  Slot  Name   Signature
      StackMapTable: number_of_entries = 1
    frame_type = 7 /* same */

  static void tif();
    descriptor: ()V
    flags: ACC_STATIC
    Code:
      stack=2, locals=0, args_size=0
      StackMap locals:
      StackMap stack:
     0: getstatic     #10                 // Field flag:Z
     3: ifeq          7
     6: return
      StackMap locals:
      StackMap stack:
     7: getstatic     #20                 // Field java/lang/System.out:Ljava/io/PrintStream;
    10: ldc           #26                 // String Java
    12: invokevirtual #28                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
    15: return
      LineNumberTable:
    line 12: 0
    line 13: 7
    line 14: 15
      LocalVariableTable:
    Start  Length  Slot  Name   Signature
      StackMapTable: number_of_entries = 1
    frame_type = 7 /* same */

Answer 2

According to the docs:

Except for the special treatment of while, do, and for statements whose condition expression has the constant value true, the values of expressions are not taken into account in the flow analysis.