java 自动拆箱的陷阱

标签：autobox   自动拆箱   陷阱   

  java  自动拆箱运用非常广泛。但是这里面有一些“陷阱”。废话不多说，来看一段代码：

public static void main(String[] args) {
	Integer a=1;
	Integer b=2;
	Integer c=3;
	Integer d=3;
	System.out.println(c==(a+b));
	System.out.println(c==d);
	System.out.println(c.equals(d));
	Integer f=200;
	Integer e=200;
	System.out.println(e==f);
	System.out.println(e.equals(f));
}

打印结果:

true

true

true

false

true

从以上的代码片段可以发现，原来是调用了Integer.valueOf这个类方法。我们来看一下这个类方法：

  /**
     * Returns a {@code Integer} instance for the specified integer value.
     * <p>
     * If it is not necessary to get a new {@code Integer} instance, it is
     * recommended to use this method instead of the constructor, since it
     * maintains a cache of instances which may result in better performance.
     *
     * @param i
     *            the integer value to store in the instance.
     * @return a {@code Integer} instance containing {@code i}.
     * @since 1.5
     */
    public static Integer valueOf(int i) {
        return  i >= 128 || i < -128 ? new Integer(i) : SMALL_VALUES[i + 128];
    }

    /**
     * A cache of instances used by {@link Integer#valueOf(int)} and auto-boxing
     */
    private static final Integer[] SMALL_VALUES = new Integer[256];

java 自动拆箱的陷阱

标签：autobox   自动拆箱   陷阱   

原文地址：http://blog.csdn.net/asking1233/article/details/47207257