Observando JIT em ação com primitivos

TwoLoopsWithObjects.java

public class TwoLoopsWithObjects {
  public static void main(String[] args) {
    StringBuilder sb = new StringBuilder();

    Integer i = new Integer(0);
    Integer max = new Integer(100000);

    while (i.intValue() < 100000) {
      Integer j = i;
      long start = System.nanoTime();
      while (j.intValue() < 100000) {
        j = new Integer(j.intValue() + 1);
      }
      long estimated = System.nanoTime() - start;
      sb.append("It #" + i + " duration " + (estimated / 1000.0d) + "μ\n");
      i = new Integer(i.intValue() + 1);
    }
    System.out.println(sb);
  }
}

TwoLoopsWithObjects.log

# java TwoLoopsWithObjects | head -500
It #0 duration 6600.113μ
It #1 duration 1910.801μ
It #2 duration 2099.174μ
It #3 duration 1913.568μ
It #4 duration 961.353μ
It #5 duration 927.057μ
It #6 duration 848.371μ
It #7 duration 876.483μ
It #8 duration 907.67μ
It #9 duration 868.635μ
It #10 duration 864.433μ
It #11 duration 882.534μ
It #12 duration 868.155μ
It #13 duration 895.743μ
It #14 duration 934.011μ
It #15 duration 1077.41μ
It #16 duration 1043.858μ
It #17 duration 976.768μ
It #18 duration 871.334μ
It #19 duration 862.534μ
It #20 duration 1608.364μ
It #21 duration 450.312μ
It #22 duration 256.206μ
It #23 duration 258.675μ
It #24 duration 257.043μ
It #25 duration 256.346μ
It #26 duration 257.324μ
It #27 duration 258.783μ
It #28 duration 262.372μ
It #29 duration 412.343μ
It #30 duration 283.267μ
It #31 duration 280.472μ
It #32 duration 281.052μ
It #33 duration 281.563μ
It #34 duration 259.117μ
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It #36 duration 261.271μ
It #37 duration 258.312μ
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It #40 duration 256.185μ
It #41 duration 1207.147μ
It #42 duration 286.986μ
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It #45 duration 285.192μ
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It #52 duration 283.436μ
It #53 duration 283.8μ
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It #60 duration 259.512μ
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It #65 duration 273.126μ
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It #72 duration 260.847μ
It #73 duration 275.937μ
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It #100 duration 269.426μ
It #101 duration 256.012μ
It #102 duration 489.191μ
It #103 duration 293.187μ
It #104 duration 305.228μ
It #105 duration 822.587μ
It #106 duration 963.581μ
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It #111 duration 921.699μ
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It #115 duration 1329.776μ
It #116 duration 1275.257μ
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It #119 duration 935.603μ
It #120 duration 922.868μ
It #121 duration 940.122μ
It #122 duration 907.721μ
It #123 duration 922.605μ
It #124 duration 897.023μ
It #125 duration 941.802μ
It #126 duration 1476.401μ
It #127 duration 324.984μ
It #128 duration 266.003μ
It #129 duration 255.706μ
It #130 duration 255.609μ
It #131 duration 315.531μ
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It #179 duration 270.922μ
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It #212 duration 1120.2μ
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It #220 duration 906.813μ
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It #230 duration 998.431μ
It #231 duration 955.672μ
It #232 duration 880.082μ
It #233 duration 982.826μ
It #234 duration 931.27μ
It #235 duration 862.06μ
It #236 duration 834.456μ
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It #238 duration 835.111μ
It #239 duration 1031.864μ
It #240 duration 1007.196μ
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It #245 duration 910.463μ
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It #254 duration 1450.729μ
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TwoLoopsWithPrimitives.java

public class TwoLoopsWithPrimitives {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder();

        int i = 0, j;
        long start, estimated;

        while (i < 100000) {
            j = i;

            start = System.nanoTime();

            while (j < 100000) {
                j++;
            }

            estimated = System.nanoTime() - start;
            sb.append("It #" + i + " duration " + (estimated / 1000.0d) + "μ\n");

            i++;
        }
        System.out.println(sb);
    }
}

TwoLoopsWithPrimitives.log

# java TwoLoopsWithPrimitives | head -50

It #0 duration 1147.059μ
It #1 duration 256.438μ
It #2 duration 228.121μ
It #3 duration 242.152μ
It #4 duration 278.272μ
It #5 duration 213.799μ
It #6 duration 213.827μ
It #7 duration 213.472μ
It #8 duration 231.829μ
It #9 duration 270.146μ
It #10 duration 231.122μ
It #11 duration 213.666μ
It #12 duration 213.735μ
It #13 duration 213.731μ
It #14 duration 213.933μ
It #15 duration 213.911μ
It #16 duration 213.558μ
It #17 duration 214.07μ
It #18 duration 213.776μ
It #19 duration 299.457μ
It #20 duration 266.236μ
It #21 duration 56.386μ
It #22 duration 0.046μ
It #23 duration 0.038μ
It #24 duration 0.046μ
It #25 duration 0.04μ
It #26 duration 0.04μ
It #27 duration 0.04μ
It #28 duration 0.039μ
It #29 duration 0.039μ
It #30 duration 0.039μ
It #31 duration 0.039μ
It #32 duration 0.039μ
It #33 duration 0.039μ
It #34 duration 0.039μ
It #35 duration 0.04μ
It #36 duration 0.041μ
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It #39 duration 0.039μ
It #40 duration 0.039μ
It #41 duration 0.039μ
It #42 duration 0.04μ
It #43 duration 0.04μ
It #44 duration 0.04μ
It #45 duration 0.04μ
It #46 duration 0.039μ
It #47 duration 0.039μ
It #48 duration 0.04μ
It #49 duration 0.039μ

Na classe TwoLoopsWithPrimitives as iterações são feitas usando primitivos e na TwoLoopsWithObjects são feitas as iterações com conversões para objetos conforme o exemplo do @akitaonrails . Incluí uma medição do tempo de execução do laço mais interno nas duas versões. Vejam os arquivos com os resultado.

Notem a redução de tempo na execução do TwoLoopsWithPrimitives a partir da 20a. iteração, momento que deve ocorrer uma otimização. No caso do TwoLoopsWithObjects essa otimização já não é possível e o tempo segue o padrão esperado pelo menos até a iteração 500.