关于Java的坑问题来源于占小狼公众号今天发的几个问题

客官°小女子只卖身不卖艺 2021-12-14 03:07 144阅读 0赞

今天看公众号的时候，看到占小狼公众号有篇文章叫Java中的坑，然后就点进去看了，我先罗列一下问题：

第一个坑：

Set<Integer> set = new HashSet<>();
 for (int i = 0; i < 100; i++) {
 set.add(i);
 set.remove(i-1);
 }
 System.out.println(set.size());

第二个坑：

package com.example.demo.chapter3;
 
 import java.util.HashSet;
 import java.util.Set;
 
 public class Test {
 public static void main(String[] args) {
 Set<Short> set = new HashSet<>();
 for (short i = 0; i < 100; i++) {
 set.add(i);
 set.remove(i-1);
 }
 System.out.println(set.size());
 }
 }

上面两个坑，问你输出结果是什么？

第三个坑：

package com.example.demo.chapter3;
    
    public class TestTwo {
        public static void main(String[] args) {
            Object i = 1 == 1? new Integer(3):new Double(1);
            System.out.println(i);
        }
    }

第4个坑：

Map<Long,String> map = new HashMap<>();
 map.put(1L,"1111");
 map.put(2L,"2222");
 map.put(3L,"3333");
 long[] ids = ArrayUtils.toPrimitive((Long[]) map.keySet().toArray());

其中`ArrayUtils.java`是`commons-lang3.jar`中的一个工具类，你只需要在你的`pom.xml`中添加下面的依赖就可以：

<dependency>
 <groupId>org.apache.commons</groupId>
 <artifactId>commons-lang3</artifactId>
 <version>3.9</version>
 </dependency>

结果抛出了异常：

Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.Long;
    	at com.example.demo.chapter3.ExceptionTest.main(ExceptionTest.java:14)

--------------------

第一个坑的答案是:1  
为什么是1，原因在于`remove`方法，`set`的`remove`方法源码如下所示：

/**
 * Removes the specified element from this set if it is present
 * (optional operation). More formally, removes an element <tt>e</tt>
 * such that
 * <tt>(o==null ? e==null : o.equals(e))</tt>, if
 * this set contains such an element. Returns <tt>true</tt> if this set
 * contained the element (or equivalently, if this set changed as a
 * result of the call). (This set will not contain the element once the
 * call returns.)
 *
 * @param o object to be removed from this set, if present
 * @return <tt>true</tt> if this set contained the specified element
 * @throws ClassCastException if the type of the specified element
 * is incompatible with this set
 * (<a href="Collection.html#optional-restrictions">optional</a>)
 * @throws NullPointerException if the specified element is null and this
 * set does not permit null elements
 * (<a href="Collection.html#optional-restrictions">optional</a>)
 * @throws UnsupportedOperationException if the <tt>remove</tt> operation
 * is not supported by this set
 */
 boolean remove(Object o);

*  该方法的参数是一个对象`Object`类型
 *  从set集合中移除某个具体的元素

## 坑1的答案和解析 ##

从第一个坑的代码可以看到，当`i=0`的时候，同时向`set`集合中添加0元素，然后移除-1元素；依次类推，那么当`i=1`的时候，set集合中只有1，0元素会被移除掉，所以最后就只剩下一个元素99。

需要注意的是`remove`的参数，在不同类中存在重载方法，有的参数是下标索引，有的是元素对象，所以调用不同的方法传入不同的参数，得到的结果也是不同的。

## 坑2的答案和解析 ##

第二个与第一个不同之处便是，数据类型是`Short`，但是这个结果就不一样的，最后打印出来的结果是100，为什么会这样子？同一段代码，只是数据类型的改变，导致最后的结果不一样。

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTI5MzQzMjU_size_16_color_FFFFFF_t_70]  
上图是第二个坑的字节码文件，直接使用`javap -c 类名`的方式得到了该文件的字节码文件，它在`set`集合中装入的是`Short`类型的数据，但是`remove`的时候，它却是`remove`的`Integer`对象，因为`Short`和`Integer`类型都重写了`Object`类的`equals`方法，所以在`remove`的时候作比较，最后得到的结果就是`false`，`set`集合中的元素都没有被移除掉。

public boolean equals(Object obj) {
            if (obj instanceof Integer) {
                return value == ((Integer)obj).intValue();
            }
            return false;
        }

public boolean equals(Object obj) {
            if (obj instanceof Short) {
                return value == ((Short)obj).shortValue();
            }
            return false;
        }

## 第三个坑的答案与解析 ##

答案是`3.0`,不是你想的3。  
这个原因我也是通过字节码看出来的，因为前面是一个恒真的条件，然后使用了三目运算符，本来应该返回`new Integer(3)`的结果3，但是实际却返回了3.0，问题的本质在**于三目运算符存在隐式的类型转换**。所以返回了3.0。字节码如下：

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTI5MzQzMjU_size_16_color_FFFFFF_t_70 1]  
需要注意的是里面有个`i2f`，表示将`int`类型转为float类型，关于这些指令是什么意思，可以去字节码指令列表去对照看每个指令的意思[JVM 虚拟机字节码指令表][JVM]

## 第四个坑的答案与解析 ##

/**
 * Returns an array containing all of the elements in this set; the
 * runtime type of the returned array is that of the specified array.
 * If the set fits in the specified array, it is returned therein.
 * Otherwise, a new array is allocated with the runtime type of the
 * specified array and the size of this set.
 *
 * If this set fits in the specified array with room to spare
 * (i.e., the array has more elements than this set), the element in
 * the array immediately following the end of the set is set to
 * <tt>null</tt>. (This is useful in determining the length of this
 * set only if the caller knows that this set does not contain
 * any null elements.)
 *
 * If this set makes any guarantees as to what order its elements
 * are returned by its iterator, this method must return the elements
 * in the same order.
 *
 * Like the {@link #toArray()} method, this method acts as bridge between
 * array-based and collection-based APIs. Further, this method allows
 * precise control over the runtime type of the output array, and may,
 * under certain circumstances, be used to save allocation costs.
 *
 * Suppose <tt>x</tt> is a set known to contain only strings.
 * The following code can be used to dump the set into a newly allocated
 * array of <tt>String</tt>:
 *
 * <pre>
 * String[] y = x.toArray(new String[0]);</pre>
 *
 * Note that <tt>toArray(new Object[0])</tt> is identical in function to
 * <tt>toArray()</tt>.
 *
 * @param a the array into which the elements of this set are to be
 * stored, if it is big enough; otherwise, a new array of the same
 * runtime type is allocated for this purpose.
 * @return an array containing all the elements in this set
 * @throws ArrayStoreException if the runtime type of the specified array
 * is not a supertype of the runtime type of every element in this
 * set
 * @throws NullPointerException if the specified array is null
 */
 <T> T[] toArray(T[] a);

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTI5MzQzMjU_size_16_color_FFFFFF_t_70 2]

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTI5MzQzMjU_size_16_color_FFFFFF_t_70]: /images/20211213/b9179f24f2e54ff4b46ceebbb75f6d7e.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTI5MzQzMjU_size_16_color_FFFFFF_t_70 1]: /images/20211213/6189f5b1816c4d0e93f57e5e66f5dd34.png
[JVM]: https://segmentfault.com/a/1190000008722128?utm_source=tag-newest
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTI5MzQzMjU_size_16_color_FFFFFF_t_70 2]: /images/20211213/714bfe19d37d451bbb757fbe85a18755.png