详解Java 自动装箱与拆箱的实现原理

  public class Test { 
    public static void main(String[] args) {   
      test(); 
    } 

    public static void test() { 
      int i = 40; 
      int i0 = 40; 
      Integer i1 = 40; 
      Integer i2 = 40; 
      Integer i3 = 0; 
      Integer i4 = new Integer(40); 
      Integer i5 = new Integer(40); 
      Integer i6 = new Integer(0); 
      Double d1=1.0; 
      Double d2=1.0; 

      System.out.println("i=i0\t" + (i == i0)); 
      System.out.println("i1=i2\t" + (i1 == i2)); 
      System.out.println("i1=i2+i3\t" + (i1 == i2 + i3)); 
      System.out.println("i4=i5\t" + (i4 == i5)); 
      System.out.println("i4=i5+i6\t" + (i4 == i5 + i6));   
      System.out.println("d1=d2\t" + (d1==d2));  

      System.out.println();     
    } 
  }

  public static Integer valueOf(int i) {
    //判断i是否在-128和127之间，如果不在此范围，则从IntegerCache中获取包装类的实例。否则new一个新实例
    if (i >= IntegerCache.low && i <= IntegerCache.high)
      return IntegerCache.cache[i + (-IntegerCache.low)];
    return new Integer(i);
  }


  //使用亨元模式，来减少对象的创建（亨元设计模式大家有必要了解一下，我认为是最简单的设计模式，也许大家经常在项目中使用，不知道他的名字而已）
  private static class IntegerCache {
    static final int low = -128;
    static final int high;
    static final Integer cache[];

    //静态方法，类加载的时候进行初始化cache[],静态变量存放在常量池中
    static {
      // high value may be configured by property
      int h = 127;
      String integerCacheHighPropValue =
        sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
      if (integerCacheHighPropValue != null) {
        try {
          int i = parseInt(integerCacheHighPropValue);
          i = Math.max(i, 127);
          // Maximum array size is Integer.MAX_VALUE
          h = Math.min(i, Integer.MAX_VALUE - (-low) -1);
        } catch( NumberFormatException nfe) {
          // If the property cannot be parsed into an int, ignore it.
        }
      }
      high = h;

      cache = new Integer[(high - low) + 1];
      int j = low;
      for(int k = 0; k < cache.length; k++)
        cache[k] = new Integer(j++);

      // range [-128, 127] must be interned (JLS7 5.1.7)
      assert IntegerCache.high >= 127;
    }

    private IntegerCache() {}
  }

  //boolean原生类型自动装箱成Boolean
  public static Boolean valueOf(boolean b) {
    return (b ? TRUE : FALSE);
  }

  //byte原生类型自动装箱成Byte
  public static Byte valueOf(byte b) {
    final int offset = 128;
    return ByteCache.cache[(int)b + offset];
  }

  //byte原生类型自动装箱成Byte
  public static Short valueOf(short s) {
    final int offset = 128;
    int sAsInt = s;
    if (sAsInt >= -128 && sAsInt <= 127) { // must cache
      return ShortCache.cache[sAsInt + offset];
    }
    return new Short(s);
  }

  //char原生类型自动装箱成Character
  public static Character valueOf(char c) {
    if (c <= 127) { // must cache
      return CharacterCache.cache[(int)c];
    }
    return new Character(c);
  }

  //int原生类型自动装箱成Integer
  public static Integer valueOf(int i) {
    if (i >= IntegerCache.low && i <= IntegerCache.high)
      return IntegerCache.cache[i + (-IntegerCache.low)];
    return new Integer(i);
  }

  //int原生类型自动装箱成Long
  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);
  }

  //double原生类型自动装箱成Double
  public static Double valueOf(double d) {
    return new Double(d);
  }

  //float原生类型自动装箱成Float
  public static Float valueOf(float f) {
    return new Float(f);
  }

  //1、这个没解释的就是true
  System.out.println("i=i0\t" + (i == i0)); //true
  //2、int值只要在-128和127之间的自动装箱对象都从缓存中获取的，所以为true
  System.out.println("i1=i2\t" + (i1 == i2)); //true
  //3、涉及到数字的计算，就必须先拆箱成int再做加法运算，所以不管他们的值是否在-128和127之间，只要数字一样就为true
  System.out.println("i1=i2+i3\t" + (i1 == i2 + i3));//true 
  //比较的是对象内存地址，所以为false
  System.out.println("i4=i5\t" + (i4 == i5)); //false
  //5、同第3条解释，拆箱做加法运算，对比的是数字，所以为true
  System.out.println("i4=i5+i6\t" + (i4 == i5 + i6));//true   
  //double的装箱操作没有使用缓存，每次都是new Double，所以false
  System.out.println("d1=d2\t" + (d1==d2));//false