JAVA 数据结构之 Integer 是否相等

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我是靠谱客的博主潇洒店员，这篇文章主要介绍JAVA 数据结构之 Integer 是否相等，现在分享给大家，希望可以做个参考。

int 常量池中初始化-128~127的范围

eqauls 判断两个对象数值是否相等
== 判断基本数据类型时，只判断数值，判断对象时不仅判断两个对象数值是否相等，而且需要指向同一个地址

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public static void main(String[] args) {
     Integer i = 127;
     Integer j = 127;
     System.out.println(i == j);
 }

的结果为 true，因为当 Integer i = 127 时，127在常量池中，在自动装箱的过程中取的是常量池中的数值，所以指向的同一个地址。

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public static void main(String[] args) {
     Integer i = 128;
     Integer j = 128;
     System.out.println(i == j);
 }

的结果为 false，因为当 Integer i = 128 时，128 在常量池中没有，所以在自动装箱过程中需要 new Integer(128)，所以指向的不是一个地址。

对此可以查看 Integer 的 IntegerCache 内部类

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/**
     * 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() {}
    }

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/**
     * 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() {}
    }