关于Integer数值比较的问题以及不可变对象

喜欢ヅ旅行 2022-07-11 07:25 237阅读 0赞

# 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\**

![QQ_20161214104324.png][]

## 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

[QQ_20161214104324.png]: http://liubenlong.github.io/images/QQ截图20161214104324.png