package 內(nèi)存管理;

/**
 盡管Java提供了自動(dòng)垃圾回收，有時(shí)也想知道對(duì)象堆的大小以及它還剩下多少。可以
 利用這些信息檢驗(yàn)?zāi)愕拇a的效率，或估計(jì)對(duì)某些類型，有多少對(duì)象可以被實(shí)例化。為了
 獲得這些值，可以使用totalMemory( )和freeMemory( )方法。
 正如我們?cè)诘?部分提及的，Java的垃圾回收器周期性地運(yùn)行將不再使用的對(duì)象放入回
 收站。然而有時(shí)想在收集器的下一個(gè)指定循環(huán)之前收集被丟棄的對(duì)象。可以通過調(diào)用gc( )
 方法按照要求運(yùn)行垃圾回收器。一個(gè)好的嘗試是調(diào)用gc( )方法，然后再調(diào)用freeMemory( )
 方法以獲得內(nèi)存使用的底線。接著執(zhí)行你的程序，并再一次調(diào)用freeMemory( )方法看分配
 了多少內(nèi)存。下面的例子說明了這個(gè)思想。#
 #Total memory is: 2031616
Initial free memory: 1730584
Free memory after garbage collection: 1867616
Free memory after allocation: 1851032
Memory used by allocation: 16584
Free memory after collecting discarded Integers: 1867616

 */

// Demonstrate totalMemory(), freeMemory() and gc().

class MemoryDemo {
  public static void main(String args[]) {
    Runtime r = Runtime.getRuntime();
    long mem1, mem2;
    Integer someints[] = new Integer[1000];

    System.out.println("Total memory is: " +
                       r.totalMemory());

    mem1 = r.freeMemory();
    System.out.println("Initial free memory: " + mem1);
    r.gc();
    mem1 = r.freeMemory();
    System.out.println("Free memory after garbage collection: "
                       + mem1);

    for (int i = 0; i < 1000; i++) {
      someints[i] = new Integer(i); // allocate integers

    }
    mem2 = r.freeMemory();
    System.out.println("Free memory after allocation: "
                       + mem2);
    System.out.println("Memory used by allocation: "
                       + (mem1 - mem2));

    // discard Integers
    for (int i = 0; i < 1000; i++) {
      someints[i] = null;

    }
    r.gc(); // request garbage collection

    mem2 = r.freeMemory();
    System.out.println("Free memory after collecting" +
                       " discarded Integers: " + mem2);

  }
}