Efficiency: switch statements over if statements

Question

`PMD` tells me A switch with less than 3 branches is inefficient, use a if statement instead. Why is that? Why 3? How do they define efficiency?

Accepted Answer

Because a `switch` statement is compiled with two special JVM instructions that are `lookupswitch` and `tableswitch`. They are useful when working with a lot of cases but they cause an overhead when you have just few branches.

An `if/else` statement instead is compiled into typical `je` `jne` ... chains which are faster but require many more comparisons when used in a long chain of branches.

You can see the difference by looking at byte code, in any case I wouldn't worry about these issues, if anything could become a problem then JIT will take care of it.

Practical example:

```
switch (i)
{
  case 1: return "Foo";
  case 2: return "Baz";
  case 3: return "Bar";
  default: return null;
}
```

is compiled into:

```
L0
 LINENUMBER 21 L0
 ILOAD 1
 TABLESWITCH
   1: L1
   2: L2
   3: L3
   default: L4
L1
 LINENUMBER 23 L1
FRAME SAME
 LDC "Foo"
 ARETURN
L2
 LINENUMBER 24 L2
FRAME SAME
 LDC "Baz"
 ARETURN
L3
 LINENUMBER 25 L3
FRAME SAME
 LDC "Bar"
 ARETURN
L4
 LINENUMBER 26 L4
FRAME SAME
 ACONST_NULL
 ARETURN
```

While

```
if (i == 1)
  return "Foo";
else if (i == 2)
  return "Baz";
else if (i == 3)
  return "Bar";
else
  return null;
```

is compiled into

```
L0
 LINENUMBER 21 L0
 ILOAD 1
 ICONST_1
 IF_ICMPNE L1
L2
 LINENUMBER 22 L2
 LDC "Foo"
 ARETURN
L1
 LINENUMBER 23 L1
FRAME SAME
 ILOAD 1
 ICONST_2
 IF_ICMPNE L3
L4
 LINENUMBER 24 L4
 LDC "Baz"
 ARETURN
L3
 LINENUMBER 25 L3
FRAME SAME
 ILOAD 1
 ICONST_3
 IF_ICMPNE L5
L6
 LINENUMBER 26 L6
 LDC "Bar"
 ARETURN
L5
 LINENUMBER 28 L5
FRAME SAME
 ACONST_NULL
 ARETURN
```

Efficiency: switch statements over if statements

Solution

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