9_（JAVA基础300集最全教程 学习）自动装箱和拆箱

在JDK5.0中新增了自动装箱和拆箱，为开发人员解决了很多开发中的效率问题，下面看看是怎么个流程：

1.

     Integer c=1;
     int d=c;
     System.out.println(d);

结果为：

引用

1

2.

 Integer a=null;
 int b=a;
 System.out.println(b);

结果为：

引用

Exception in thread "main" java.lang.NullPointerException
at ArrayListTest.main(ArrayListTest.java:21)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)

3.缓存机制：默认情况下-XX:AutoBoxCacheMax设置的大小即为缓存的大小，当未设置时即为默认大小-128到127之间的数据全部装箱缓存，在需要时直接获取即可。

/**
     * 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. During VM initialization the
     * getAndRemoveCacheProperties method may be used to get and remove any system
     * properites that configure the cache size. At this time, the size of the
     * cache may be controlled by the vm option -XX:AutoBoxCacheMax=<size>.
     */

    // value of java.lang.Integer.IntegerCache.high property (obtained during VM init)
    private static String integerCacheHighPropValue;

    static void getAndRemoveCacheProperties() {
        if (!sun.misc.VM.isBooted()) {
            Properties props = System.getProperties();
            integerCacheHighPropValue =
                (String)props.remove("java.lang.Integer.IntegerCache.high");
            if (integerCacheHighPropValue != null)
                System.setProperties(props);  // remove from system props
        }
    }

    private static class IntegerCache {
        static final int high;
        static final Integer cache[];

        static {
            final int low = -128;

            // high value may be configured by property
            int h = 127;
            if (integerCacheHighPropValue != null) {
                // Use Long.decode here to avoid invoking methods that
                // require Integer's autoboxing cache to be initialized
                int i = Long.decode(integerCacheHighPropValue).intValue();
                i = Math.max(i, 127);
                // Maximum array size is Integer.MAX_VALUE
                h = Math.min(i, Integer.MAX_VALUE - -low);
            }
            high = h;

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

        private IntegerCache() {}
    }

    /**
     * Returns a <tt>Integer</tt> instance representing the specified
     * <tt>int</tt> value.
     * If a new <tt>Integer</tt> 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.
     *
     * @param  i an <code>int</code> value.
     * @return a <tt>Integer</tt> instance representing <tt>i</tt>.
     * @since  1.5
     */
    public static Integer valueOf(int i) {
        if(i >= -128 && i <= IntegerCache.high)
            return IntegerCache.cache[i + 128];
        else
            return new Integer(i);
    }

可以通过以下方式验证：

        Integer e=1234111;
        Integer f=1234111;
        System.out.println(e==f);
        System.out.println(e.equals(f));

        Integer g=100;
        Integer h=100;
        System.out.println(g==h);
        System.out.println(g.equals(h));

结果为：

引用

false
true
true
true