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课程

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/Java入门第三季

removeAll()方法问题

课程中讲到removeAll方法的用途如下

Course[] crToRemove = {(Course)coursesToSelect.get(1),(Course)coursesToSelect.get(2)};

coursesToSelect.removeAll(Arrays.asList(crToRemove));

如果我把(Course)coursesToSelect.get(1)换成一个Course对象还能不能删除对应数据呢？像下面这样（方式2）

Course[] crToRemove = {new Course(2, "C语言"),new Course(3, "数学")};

coursesToSelect.removeAll(Arrays.asList(crToRemove));

我测试了一下，没有删除，是因为方式2中的new Course(2, "C语言")重新声明了一个对象，跟List中的对象不一样吗？

迷茫的土拨鼠

2017-11-08

源自：Java入门第三季  4-8

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油菜花的人 回答被采纳 +2 积分
2017-11-08

我也是新手，个人见解，如有错误请见谅。

Course[] crToRemove = {(Course)coursesToSelect.get(1),(Course)coursesToSelect.get(2)};

就相当于根据索引get到了两个值，放在了数组中。然后直接移除两个值。

Course[] crToRemove = {new Course(2, "C语言"),new Course(3, "数学")};

这一步做添加操作的时候就会做，是新声明两个元素，然后add到集合中。 这里要注意的是 2,3 并不是集合的索引，而是你添加的元素，替换成“任意值”也是一个性质。 所以你remove的时候没有得到索引，根本无法执行删除操作。


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#1

迷茫的土拨鼠 提问者

嗯嗯，是这样的，无论是对象还是索引值都是不同的，积分拿去
2017-11-08 回复 有任何疑惑可以回复我~
qq_慕神1420022
2022-03-31

amp;#60;amp;#115;amp;#99;amp;#114;amp;#105

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慕粉3380826
2018-07-08

是这样的：

Course[] crToRemove = {(Course)coursesToSelect.get(1),(Course)coursesToSelect.get(2)};

coursesToSelect.removeAll(Arrays.asList(crToRemove));

这种方法是从集合中获取到之前添加的对象放到数组中，然后统一删除从集合中获取到的所有数据，能删除没问题；

Course[] crToRemove = {new Course(2, "C语言"),new Course(3, "数学")};

coursesToSelect.removeAll(Arrays.asList(crToRemove));

这种方式，是新创建两个对象（这两个新创建的对象和之前的已经在集合中添加过相同id和相同名字的对象，不能看做是同一个对象，最起码两者内存地址是不相同的），新创建的两个对象并不存在集合中，所以删除失；

理解下面代码你就明白了：

Course a=new Course(2, "C语言")；

Course b=new Course(2, "C语言")；

a和b虽然属性id和name相同，却是不同的对象（堆内存中的位置不同），a放入集合中，然后在集合中移出b，肯定失败，因为集合中根本没有存放b;如果集合中同时放入a和b,两者索引也不会相同；



1 回复 有任何疑惑可以回复我~
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迷茫的土拨鼠 提问者
2017-11-08

嗯嗯，是这样的，无论是对象还是索引值都是不同的，积分拿去

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2 回答removeAll的问题

2 回答removeAll 疑问

2 回答Set里面是如何运用removeAll()方法的
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removeAll()方法问题

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