Short Circuiting vs Multiple if's

Question

What are the differences between this:

if(a && b)
{
     //code
}

and this:

if(a)
{
     if(b)
     {
          //code
     }
}

From what I know b will only get evaluated in the first code block if a is true, and the second code block would be the same thing.

Are there any benefits of using one over the other? Code execution time? memory? etc.

Answer 1

They get compiled to the same bytecode. No performance difference.

Readability is the only difference. As a huge generalization, short-circuiting looks better but nesting is slightly clearer. It really boils down to the specific use case. I'd typically short-circuit.

---

I tried this out. Here's the code:

public class Test {

    public static void main(String[] args) {
        boolean a = 1>0;
        boolean b = 0>1;

        if (a && b)
            System.out.println(5);

        if (a)
            if (b)
                System.out.println(5);
    }
}

This compiles to:

  0: iconst_1
  1: istore_1
  2: iconst_0
  3: istore_2
  4: iload_1
  5: ifeq          19
  8: iload_2
  9: ifeq          19
 12: getstatic     #2
 15: iconst_5
 16: invokevirtual #3
 19: iload_1
 20: ifeq          34
 23: iload_2
 24: ifeq          34
 27: getstatic     #2
 30: iconst_5
 31: invokevirtual #3
 34: return

Note how this block repeats twice:

  4: iload_1
  5: ifeq          19
  8: iload_2
  9: ifeq          19
 12: getstatic     #2
 15: iconst_5
 16: invokevirtual #3

Same bytecode both times.

Answer 2

It makes a difference if you have an else associated with each if.

if(a && b)
{
     //do something if both a and b evaluate to true
} else {
    //do something if either of a or b is false
}

and this:

if(a)
{
     if(b)
     {
          //do something if both a and b are true
     } else {
          //do something if only a is true
     }
} else {
     if(b)
     {
          //do something if only b is true
     } else {
          //do something if both a and b are false
     }
}