自动装箱问题以及字符创的==问题

Java提供自动装箱拆箱的模式，下面例子说明其中的一些细节。

public class AutoBoxing {

	public static void main(String[] args) {
		Integer new3 = new Integer(3);
		Integer int3toInteger = 3; // 将3自动装箱成Integer类型
		Integer int3toInteger2 = 3; // 将3自动装箱成Integer类型
		int int3 = 3;
		System.out.println(new3 == int3toInteger); 
		// false 两个引用没有引用同一对象
		System.out.println(int3toInteger == int3toInteger2); 
		// true 两个引用同在缓冲中
		System.out.println(new3 == int3);
		// true a自动拆箱成int类型再和c比较
		Integer int129 = 129;
		Integer int129_2 = 129;
		System.out.println(int129 == int129_2);
		// false -128~127是默认的int缓存范围
	}
}

当我们给一个Integer对象赋一个int值的时候，会调用Integer类的静态方法valueOf，如果看看valueOf的源代码就知道发生了什么。

    /**
     * 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) {
        assert IntegerCache.high >= 127;
        if (i >= IntegerCache.low && i <= IntegerCache.high)
            return IntegerCache.cache[i + (-IntegerCache.low)];
        return new Integer(i);
    }

问题显然。如果整型字面量的值在-128到127之间，那么不会new新的Integer对象，而是直接引用常量池中的Integer对象。

下面关注字符串的等等问题。

	public static void main(String[] args) {
	    String s1 = "Programming";
        String s2 = new String("Programming");
        String s3 = "Program" + "ming";
        System.out.println(s1 == s2);
        System.out.println(s1 == s3);
        System.out.println(s1 == s3.intern());
        System.out.println(s2 == s2.intern());
    }

输出：

false
true
true
false