如何理解kotlin数字装箱

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我是靠谱客的博主愤怒向日葵，最近开发中收集的这篇文章主要介绍如何理解kotlin数字装箱，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

- 疑惑

怎么理解kotlin官方文档一下一段话

在 Java 平台数字是物理存储为 JVM 的原生类型，除非我们需要一个可空的引用（如 Int?）或泛型。后者情况下会把数字装箱。

注意数字装箱不一定保留同一性:

val a: Int = 100
val boxedA: Int? = a
val anotherBoxedA: Int? = a

val b: Int = 10000
val boxedB: Int? = b
val anotherBoxedB: Int? = b

println(boxedA === anotherBoxedA) // true
println(boxedB === anotherBoxedB) // false

从代码中，我们关注到只是数字不一样，为啥有两种结果？

- 探索过程

1.按一下步骤打开Kotlin Bytecode
在这里插入图片描述
2.从kotlin翻译成字节码中，我们可以看出kotlin对数字装箱

3. 不管数值是100还是10000，都调用了Integer.valueOf ()

INVOKESTATIC java/lang/Integer.valueOf (I)Ljava/lang/Integer;

4.我们走进Integer.valueOf()源码看究竟

 /**
     * 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);
    }

    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() {}
    }

从源码看出当i在[-128,127]区间,会从IntegerCache.cache中获取，在该区间之外会new Integer()对象，从而导致引用不一致。