Question

我正在窥探Java API并在Integer类中找到它

for (;;) {
    q = (i * 52429) >>> (16+3);
    r = i - ((q << 3) + (q << 1));  // r = i-(q*10) ...
    buf [--charPos] = digits [r];
    i = q;
    if (i == 0) break;
}

我想知道为什么开发人员采用这种方法，而不是像

那样

do {
    q = (i * 52429) >>> (16+3);
    r = i - ((q << 3) + (q << 1));  // r = i-(q*10) ...
    buf [--charPos] = digits [r];
    i = q;
} while (i != 0);

为什么有人会赞成使用循环的break语句？这有点效率吗？

Answer 1

更有可能是开发商青睐或首先想到的模式。字节代码和性能不太可能有很大不同。

Answer 2

除了风格之外，两者没有区别。它们都分解成相同的字节码。

package SO37060005;

public class LoopStyle {
  int q, i, r, charPos, digits[], buf[];

  public void loop1() {
    for (;;) {
      q = (i * 52429) >>> (16 + 3);
      r = i - ((q << 3) + (q << 1));  // r = i-(q*10) ...
      buf[--charPos] = digits[r];
      i = q;
      if (i == 0) {
        break;
      }
    }
  }

  public void loop2() {
    do {
      q = (i * 52429) >>> (16 + 3);
      r = i - ((q << 3) + (q << 1));  // r = i-(q*10) ...
      buf[--charPos] = digits[r];
      i = q;
    } while (i != 0);
  }
}

$ javap -c LoopStyle.class 
Compiled from "LoopStyle.java"
public class SO37060005.LoopStyle {
  int q;

  int i;

  int r;

  int charPos;

  int[] digits;

  int[] buf;

  public SO37060005.LoopStyle();
    Code:
       0: aload_0
       1: invokespecial #16                 // Method java/lang/Object."<init>":()V
       4: return

  public void loop1();
    Code:
       0: aload_0
       1: aload_0
       2: getfield      #23                 // Field i:I
       5: ldc           #25                 // int 52429
       7: imul
       8: bipush        19
      10: iushr
      11: putfield      #26                 // Field q:I
      14: aload_0
      15: aload_0
      16: getfield      #23                 // Field i:I
      19: aload_0
      20: getfield      #26                 // Field q:I
      23: iconst_3
      24: ishl
      25: aload_0
      26: getfield      #26                 // Field q:I
      29: iconst_1
      30: ishl
      31: iadd
      32: isub
      33: putfield      #28                 // Field r:I
      36: aload_0
      37: getfield      #30                 // Field buf:[I
      40: aload_0
      41: dup
      42: getfield      #32                 // Field charPos:I
      45: iconst_1
      46: isub
      47: dup_x1
      48: putfield      #32                 // Field charPos:I
      51: aload_0
      52: getfield      #34                 // Field digits:[I
      55: aload_0
      56: getfield      #28                 // Field r:I
      59: iaload
      60: iastore
      61: aload_0
      62: aload_0
      63: getfield      #26                 // Field q:I
      66: putfield      #23                 // Field i:I
      69: aload_0
      70: getfield      #23                 // Field i:I
      73: ifne          0
      76: return

  public void loop2();
    Code:
       0: aload_0
       1: aload_0
       2: getfield      #23                 // Field i:I
       5: ldc           #25                 // int 52429
       7: imul
       8: bipush        19
      10: iushr
      11: putfield      #26                 // Field q:I
      14: aload_0
      15: aload_0
      16: getfield      #23                 // Field i:I
      19: aload_0
      20: getfield      #26                 // Field q:I
      23: iconst_3
      24: ishl
      25: aload_0
      26: getfield      #26                 // Field q:I
      29: iconst_1
      30: ishl
      31: iadd
      32: isub
      33: putfield      #28                 // Field r:I
      36: aload_0
      37: getfield      #30                 // Field buf:[I
      40: aload_0
      41: dup
      42: getfield      #32                 // Field charPos:I
      45: iconst_1
      46: isub
      47: dup_x1
      48: putfield      #32                 // Field charPos:I
      51: aload_0
      52: getfield      #34                 // Field digits:[I
      55: aload_0
      56: getfield      #28                 // Field r:I
      59: iaload
      60: iastore
      61: aload_0
      62: aload_0
      63: getfield      #26                 // Field q:I
      66: putfield      #23                 // Field i:I
      69: aload_0
      70: getfield      #23                 // Field i:I
      73: ifne          0
      76: return
}

中断比检查Java中的循环条件更有效吗？

2 个答案: