Java是一個近乎純潔的面向物件程式語言，但是為了程式設計的方便還是引入了基本資料型別，但是為了能夠將這些基本資料型別當成物件操作，Java為每 一個基本資料型別都引入了對應的包裝型別（wrapper class），int的包裝類就是Integer，從Java 5開始引入了自動裝箱/拆箱機制，使得二者可以相互轉換。

class AutoUnboxingTest {
    public static void main(String[] args) {
        Integer a = new Integer(3);
        Integer b = 3;                  // 將3自動裝箱成Integer型別
        int c = 3;
        System.out.println(a == b);     // false 兩個引用沒有引用同一物件
        System.out.println(a == c);     // true a自動拆箱成int型別再和c比較
    }
}
 

public class Test03 {
    public static void main(String[] args) {
        Integer f1 = 100, f2 = 100, f3 = 150, f4 = 150;

        System.out.println(f1 == f2);
        System.out.println(f3 == f4);
    }
}

public static Integer valueOf(int i) {
	if (i >= IntegerCache.low && i <= IntegerCache.high)
		return IntegerCache.cache[i + (-IntegerCache.low)];
	return new Integer(i);
}

/**
 * Cache to support the object identity semantics of autoboxing for values between
 * -128 and 127 (inclusive) as required by JLS.
 *
 * The cache is initialized on first usage.  The size of the cache
 * may be controlled by the {@code -XX:AutoBoxCacheMax=<size>} option.
 * During VM initialization, java.lang.Integer.IntegerCache.high property
 * may be set and saved in the private system properties in the
 * sun.misc.VM class.
 */

private static class IntegerCache {
	static final int low = -128;
	static final int high;
	static final Integer cache[];

	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() {}
}