Java基础教程之基本类型数据类型、包装类及自动拆装箱

public static void main(String[] args) {
 Integer a = 0;
 for(int i = 0; i < 100; i++){
 a += i;
 }
}

public static void main(String[] args) {
 Integer a = Integer.valueOf(0);
 for (int i = 0; i < 100; i++) {
 a = Integer.valueOf(a.intValue() + i);
 }
}

/**
 * Returns the value of this {@code Integer} as an
 * {@code int}.
 */
public int intValue() {
 return value;
}

/**
 * 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) {
 if (i >= IntegerCache.low && i <= IntegerCache.high)
 return IntegerCache.cache[i + (-IntegerCache.low)];
 return new Integer(i);
}

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.
 }
}