[Java] Java中自动装箱、拆箱引起的耗时详解

private static long getcounterresult() {
 long sum = 0l;
 final int length = integer.max_value;
 for (int i = 0; i < length; i++) {
 sum += i;
 }
 return sum;
}
public static void main(string[] args) {
 long startcounttime = system.currenttimemillis();
 long result = getcounterresult();
 long endcounttime = system.currenttimemillis();
 system.out.println("result = " + result + ", and take up time : " + (endcounttime - startcounttime) / 1000 + "s");
}

result = 2305843005992468481, and take up time : 12s

自从 jdk1.5 之后就有了自动装箱(autoboxing)和自动拆箱(autounboxing)。

int a = 3;
integer b = a;

integer b = new integer(7);
int a = b;

/**
 * returns an {@code integer} instance representing the specified
 * {@code int} value. if a new {@code integer} instance is not
 * required, this method should generally be used in preference to
 * the constructor {@link #integer(int)}, as this method is likely
 * to yield significantly better space and time performance by
 * caching frequently requested values.
 *
 * this method will always cache values in the range -128 to 127,
 * inclusive, and may cache other values outside of this range.
 *
 * @param i an {@code int} value.
 * @return an {@code integer} instance representing {@code i}.
 * @since 1.5
 */
public static integer valueof(int i) {
 // 如果i的值大于-128小于127则返回一个缓冲区中的一个integer对象
 if (i >= integercache.low && i <= integercache.high)
 return integercache.cache[i + (-integercache.low)];
 
 // 否则返回 new 一个integer 对象
 return new integer(i);
}

integer i = 100;

integer i = integer.valueof(100);

public static void main(string[] args) {
 // ⓵
 integer a = new integer(121);
 integer b = new integer(121);
 system.out.println(a == b);
 
 // ⓶
 integer c = 121;
 integer d = 121;
 system.out.println(c == d);
 
 // ⓷
 integer e = 129;
 integer f = 129;
 system.out.println(e == f);
 
 // ⓸
 int g = 50;
 integer h = new integer(50);
 system.out.println(g == h);
}

public static long valueof(long l) {
 final int offset = 128;
 if (l >= -128 && l <= 127) { // will cache
  return longcache.cache[(int)l + offset];
 }
 return new long(l);
}

long sum = 0l;

long sum = long.valueof(0);

private static long getcounterresult() {
 // 修改为普通的基本类型数据
 long sum = 0l;
 final int length = integer.max_value;
 for (int i = 0; i < length; i++) {
  sum += i;
 }
 return sum;
}
public static void main(string[] args) {
 long startcounttime = system.currenttimemillis();
 long result = getcounterresult();
 long endcounttime = system.currenttimemillis();
 system.out.println("result = " + result + ", and take up time : " + (endcounttime - startcounttime) / 1000 + "s");
}