Integer对象两种创建方式的比较

Integer对象两种创建方式的比较

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先看一段代码：

public class Main {
    public static void main(String[] args) {
        Integer i1 = 6;
        Integer i2 = 6;
        Integer i3 = new Integer(6);
        Integer i4 = new Integer(6);

        System.out.println(i1 == i2);//true
        System.out.println(i3 == i4);//false
    }
}

反编译以上代码：

/*
 * Decompiled with CFR 0.149.
 */
package com.learn.java;

public class Main {
    public static void main(String[] args) {
        Integer i1 = Integer.valueOf((int)6);
        Integer i2 = Integer.valueOf((int)6);
        Integer i3 = new Integer((int)6);
        Integer i4 = new Integer((int)6);
        System.out.println((i1 == i2 ? 1 : 0) != 0);
        System.out.println((i3 == i4 ? 1 : 0) != 0);
    }
}

从反编译结果可以看到，i1 和 i2以自动装箱的方式（valueOf()方法）创建，i3 和 i4以构造方法的方式创建。
我们知道，“==”比较的是两个对象的引用，上面的代码的执行结果表明，i1 和 i2指向同一个对象，i3 和 i4 指向不同的对象。

下面是Integer类中的部分源码：

public final class Integer extends Number implements Comparable<Integer> {
    /**
     * Constructs a newly allocated {@code Integer} object that
     * represents the specified {@code int} value.
     *
     * @param   value   the value to be represented by the
     *                  {@code Integer} object.
     */
    public Integer(int value) {
        this.value = value;
    }
    
    /**
     * 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);
    }
}

IntegerCache是Integer类中的一个内部类，其源码如下：

    /**
     * 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() {}
    }

上面的源码表明：

1. Integer类的构造方法返回新的Integer实例；
2. 如果不要求返回新的Integer实例，应该使用valueOf方法；
3. valueOf方法第一次被调用时，会创建值在-128到127之间的Integer实例并且添加到缓存，后续调用valueOf方法时，如果值在-127到127之间，将返回缓存中的实例；
4. 第一次调用valueOf方法，需要消耗额外的时间；
5. 缓存的范围可以调整，但只能调整缓存的上限，默认上限为127。