Decompiled Java class producing different output

Question

Out of curiosity I just decompiled below code using DJ Java Decompiler as well using CAVAJ Java Decompiler ( Java version is 1.7 ) here is the normal source code :

---

    byte a = 10;
    a = (byte) (a +1);

    byte b = 10;
    b = b++;

    byte c = 10;
    c +=c;

    System.out.printf("a=%d \t b=%d \t c=%d\n",a,b,c);

Which shows output as: a=11 b=10 c=20

---

---

And here is the decompiled one:

    byte a = 10;
    a++;
    byte b = 10;
    b++;
    b = b;
    byte c = 10;
    c += c;
    System.out.printf("a= %d \t b = %d \t c = %d\n", new Object[] {
        Byte.valueOf(a), Byte.valueOf(b), Byte.valueOf(c)
    });

Which when used as source code output as: a=11 b=11 c=20

---

To be more clear it has nothing to do with byte same thing happening for int as well and I even checked above codes in online compiler IDEONE and giving the same output as mine.

So, is the decompiler producing wrong code or is something else ??

Answer 1

I will give you short answer: yes, it seems, that the decompiler is producing wrong code. Because this one:

byte b = 10;
b = b++;

has strongly predicted behavior (b will not change).

UPD: Furthermore, no one of decompilers can give you 100% warranty of the correctness of generated decompiled code.

UPD-2: Are you sure, that you provide us actual version of your code? Because this one:

byte aa = 10;
a = (byte) (a +1);

of course is a mistake. It even will not compile :)

UPD-3 Well, I need to say, that Jad decompiler (Jad 1.5.8g for Windows 9x/NT/2000 on Intel platform) produces the same code:

    byte b = 10;
    b++;
    b = b;

...

    java.lang.System.out.printf("a=%d \t b=%d \t c=%d\n", new java.lang.Object[] {...

But this is not surprising: Cavaj Java decompiler uses Jad as its Java decompiling engine.

Conclusion: Consider this behavior as a feature/bug of Jad decompiler, which is far from a perfection.

Answer 2

This is not about decompiler. When you compile a code using java compiler, in some cases, the compiler changes your code and optimize it to another code that does not change the result. Your code is:

byte a = 10;
a = (byte) (a + 1);

In this case, java compiler knows that there is no need to type cast between byte and byte and because of it, the compiler tries to remove your typecast and in this process, it replaces your code with closest code that it can understand. This is the compiler output:

byte a = 10;
a++;

So, this is not about decompile process. The java compiler changes your code.