关于Integer数值比较的问题以及不可变对象

前言

写这篇文章是因为在之前的项目中通过findbugs进行代码优化，爆出的问题。其实我们的代码中暗藏危机，只是没有暴露出来而已

我这里使用jdk7

测试

public static void main(String[] args) {

        Integer a = 10;
        Integer b = 10;
        System.out.println("a == b  " + (a == b));
        System.out.println("a.equals(b)  " + (a.equals(b)));
        System.out.println("a.intValue() == b.intValue()  " + (a.intValue() == b.intValue()));
        System.out.println("a.compareTo(b)  " + (a.compareTo(b)));
        System.out.println("---------");

        a = new Integer(10);
        b = new Integer(10);
        System.out.println("a == b  " + (a == b));
        System.out.println("a.equals(b)  " + (a.equals(b)));
        System.out.println("a.intValue() == b.intValue()  " + (a.intValue() == b.intValue()));
        System.out.println("a.compareTo(b)  " + (a.compareTo(b)));
        System.out.println("---------");

        a = 189;
        b = 189;
        System.out.println("a == b  " + (a == b));
        System.out.println("a.equals(b)  " + (a.equals(b)));
        System.out.println("a.intValue() == b.intValue()  " + (a.intValue() == b.intValue()));
        System.out.println("a.compareTo(b)  " + (a.compareTo(b)));
        System.out.println("---------");

        int m = 189;
        int n = 189;
        System.out.println(m == n);
    }

运行结果

a == b  true
a.equals(b)  true
a.intValue() == b.intValue()  true
a.compareTo(b)  0
---------
a == b  false
a.equals(b)  true
a.intValue() == b.intValue()  true
a.compareTo(b)  0
---------
a == b  false
a.equals(b)  true
a.intValue() == b.intValue()  true
a.compareTo(b)  0
---------
true

奇不奇怪，为什么会酱紫呢？

原因分析

我们都知道，在java中==是比较的两个对象所指向的内存地址是否相等，而equals方法比较的是值。如果按照这个理论，上面的结果中可以看出Integer a = 10;Integer b = 10;这两个对象内存地址是同一个，也就是a和b两个引用指向同一个对象。而Integer a = 189;Integer b = 189;中，a和b两个引用指向了两个不同的对象。真的是这样吗？为什么呢？

我们查看Integer的源码：

/**
     * 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 -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) {
                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);
            }
            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 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) {
        assert IntegerCache.high >= 127;
        if (i >= IntegerCache.low && i <= IntegerCache.high)
            return IntegerCache.cache[i + (-IntegerCache.low)];
        return new Integer(i);
    }

    /**
     * The value of the {@code Integer}.
     *
     * @serial
     */
    private final int value;

    /**
     * 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;
    }

通过这段源码可知：

• Integer是不可变对象，因为里面的value是final的private final int value;
• -128到127之间的数据放到了IntegerCache中，IntegerCache是static的，因此将会放到常量池中作为缓存使用

因此可知，Integer a = 10;Integer b = 10;这两个对象其实是从IntegerCache缓存中取的，是同一个对象，地址肯定是相同的。而Integer a = 189;Integer b = 189;是创建的两个新的对象，因此地址肯定不同啦

结论

永远不要用==比较integer对象，以避免一些偶发不可预测的错误。

其他与此相关的知识点

Object与Integer比较

Object o1 = 10;
Integer o2 = 10;
System.out.println(o1 == o2);

Object o3 = 200;
Integer o4 = 200;
System.out.println(o3 == o4);

结果：

true
false

原因与上面的一样。

jdk8中上面代码编译可是不通过的吆

锁相关

我们都知道用于加锁的对象必须是不可变对象， 永远不要再不可变对象上加锁

因此永远不要再integer对象上加锁，因为其实不可变对象private final int value;。当integer重新赋值或者进行了计算以后，得到的值是新创建的对象。

class Stu{
    private int a;
    public int getA() { return a; }
    public void setA(int a) {
        this.a = a;
    }
}
public class Test {
    public static void main(String[] args) {
        Stu stu = new Stu();
        stu.setA(1);
        increase(stu);
        increase(stu);

        System.out.println(stu.getA());
        System.out.println("---");

        Integer a = 1;
        increase(a);
        increase(a);
        System.out.println(a);
    }
    //我们知道java方法传递对象是传引用的
    public static void increase(Stu stu){
        stu.setA(stu.getA() + 1);
        System.out.println(stu.getA());
    }
    public static void increase(Integer integer){
        integer = integer + 1;
        System.out.println(integer);//这个integer其实是新创建的一个对象，而不是参数传入进来的对象
    }
}

结果

2
3
3
---
2
2
1