Java: How does the == operator work when comparing int?

Question

Given this Java code:

int fst = 5;
int snd = 6;

if(fst == snd)
    do something;

I want to know how Java will compare equality for this case. Will it use an XOR operation to check equality?

Answer 1

Are you asking "what native machine code does this turn into?"? If so, the answer is "implementation-depdendent".

However, if you want to know what JVM bytecode is used, just take a look at the resulting .class file (use eg javap to disassemble it).

Answer 2

In case you are asking about the JVM, use the javap program.

public class A {

    public static void main(String[] args) {

        int a = 5;
        System.out.println(5 == a);

    }

}

Here is the disassembly:

public class A {
  public A();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: iconst_5
       1: istore_1
       2: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
       5: iconst_5
       6: iload_1
       7: if_icmpne     14
      10: iconst_1
      11: goto          15
      14: iconst_0
      15: invokevirtual #3                  // Method java/io/PrintStream.println:(Z)V
      18: return
}

In this case it optimized the branching a bit and used if_icmpne . In most cases, it will use if_icmpne or if_icmpeq .

if_icmpeq : if ints are equal, branch to instruction at branchoffset (signed short constructed from unsigned bytes branchbyte1 << 8 + branchbyte2)

if_icmpn : if ints are not equal, branch to instruction at branchoffset (signed short constructed from unsigned bytes branchbyte1 << 8 + branchbyte2)