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The simplest code would be to check n % 3 == 0. The modulo operator uses the JVM opcode irem, which is about as efficient as you can get. On an Intel CPU, irem is probably implemented using the IDIV (signed divide) instruction.

The simplest code would be to check n % 3 == 0. The modulo operator uses the JVM opcode irem, which is about as efficient as you can get.

On an Intel CPU, irem is probably implemented using the IDIV (signed divide) instruction. On most modern x86-compatible processors, IDIV with a 32-bit operand uses 10 to 30 micro-operations and has a latency of 20 to 61 clock cycles . More importantly, the trend on newer and more powerful hardware is for IDIV to take fewer clock cycles. In other words, writing the program as if it were a RISC processor is a counterproductive "optimization" — it would be better to let the hardware perform the division operation to its full potential.

The simplest code would be to check n % 3 == 0. The modulo operator uses the JVM opcode irem, which is about as efficient as you can get.

The simplest code would be to check n % 3 == 0. The modulo operator uses the JVM opcode irem, which is about as efficient as you can get. On an Intel CPU, irem is probably implemented using the IDIV (signed divide) instruction.

For comparison...

javap -c DivThreeEfficiently

public final class DivThreeEfficiently {
 public static boolean isMultipleOfThree(int);
 Code:
 0: iload_0 
 1: ifge 7
 4: iload_0 
 5: ineg 
 6: istore_0 
 7: iconst_0 
 8: istore_1 
 9: iconst_0 
 10: istore_2 
 11: iload_0 
 12: ifeq 46
 15: iload_0 
 16: iconst_1 
 17: iand 
 18: iconst_1 
 19: if_icmpne 25
 22: iinc 2, 1
 25: iload_0 
 26: iconst_1 
 27: ishr 
 28: istore_0 
 29: iload_0 
 30: iconst_1 
 31: iand 
 32: iconst_1 
 33: if_icmpne 39
 36: iinc 1, 1
 39: iload_0 
 40: iconst_1 
 41: ishr 
 42: istore_0 
 43: goto 11
 46: iload_1 
 47: iload_2 
 48: if_icmpne 55
 51: iconst_1 
 52: goto 56
 55: iconst_0 
 56: ireturn 
}

Div3MoreEfficiently.java

public class Div3MoreEfficiently {
 private Div3MoreEfficiently() {}
 public static boolean isMultipleOfThree(int n) {
 return n % 3 == 0;
 }
}

javap -c Div3MoreEfficiently

public class Div3MoreEfficiently {
 public static boolean isMultipleOfThree(int);
 Code:
 0: iload_0 
 1: iconst_3 
 2: irem 
 3: ifne 10
 6: iconst_1 
 7: goto 11
 10: iconst_0 
 11: ireturn 
}