Reverse Engineering for Beginners

CHAPTER 54. JAVA CHAPTER 54. JAVA

25: invokevirtual #4 // Method java/io/PrintStream.print:(Ljava/lang/⤦

ÇString;)V

28: iload_0

29: bipush 100

31: if_icmple 42

34: getstatic #2 // Field java/lang/System.out:Ljava/io/⤦

ÇPrintStream;

37: ldc #6 // String >100

39: invokevirtual #4 // Method java/io/PrintStream.print:(Ljava/lang/⤦

ÇString;)V

42: iload_0

43: ifne 54

46: getstatic #2 // Field java/lang/System.out:Ljava/io/⤦

ÇPrintStream;

49: ldc #7 // String ==0

51: invokevirtual #4 // Method java/io/PrintStream.print:(Ljava/lang/⤦

ÇString;)V

54: return

if_icmpgepops two values and compares them. If the second one is larger than the first, a jump to offset 14 is performed.
if_icmpneandif_icmplework just the same, but impement different conditions.

There is also aifneinstruction at offset 43. Its name is misnomer, it would’ve be better to name itifnz(jump if the value
atTOSis not zero). And that is what it does: it jumps to offset 54 if the input value is not zero. If zero,the execution flow
proceeds to offset 46, where the “==0” string is printed.

N.B.:JVMhas no unsigned data types, so the comparison instructions operate only on signed integer values.

54.9 Passing arguments.

Let’s extend ourmin()/max()example:

public class minmax
{
public static int min (int a, int b)
{
if (a>b)
return b;
return a;
}

public static int max (int a, int b)

{

if (a>b)

return a;

return b;

}

public static void main(String[] args)
{
int a=123, b=456;
int max_value=max(a, b);
int min_value=min(a, b);
System.out.println(min_value);
System.out.println(max_value);
}
}

Here ismain()function code:

public static void main(java.lang.String[]);

flags: ACC_PUBLIC, ACC_STATIC

Code:

stack=2, locals=5, args_size=1

0: bipush 123

2: istore_1

3: sipush 456

6: istore_2