java 集合removeAll() 方法

问题

通过removeAll方法移除list中和list1一样的元素，定义如下函数：

package com.study;

import java.util.ArrayList;
import java.util.List;

class User {

    private int id;
    private String name;

    public User() {
    }

    public User(int id, String name) {
        super();
        this.id = id;
        this.name = name;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

public class TestRemoveAll {
    public static void main(String[] args) {
        List<User> list = new ArrayList<User>();
        list.add(new User(1, "1"));
        list.add(new User(2, "2"));
        list.add(new User(3, "3"));
        list.add(new User(4, "4"));
        List<User> list1 = new ArrayList<User>();
        list1.add(new User(1, "1"));
        list1.add(new User(2, "2"));
        list1.add(new User(3, "3"));
        list1.add(new User(5, "5"));
        list.removeAll(list1);
        System.out.println("list remove list1 以后的大小为 " + list.size());
    }
}

结果为：

 

 

 显然，这不是想要的结果。那么，为什么会出现这种结果呢？查看jdk源码：

## AbstractCollection<E> ##

 /**
     * {@inheritDoc}
     *
     * <p>This implementation iterates over this collection, checking each
     * element returned by the iterator in turn to see if it's contained
     * in the specified collection.  If it's so contained, it's removed from
     * this collection with the iterator's <tt>remove</tt> method.
     *
     * <p>Note that this implementation will throw an
     * <tt>UnsupportedOperationException</tt> if the iterator returned by the
     * <tt>iterator</tt> method does not implement the <tt>remove</tt> method
     * and this collection contains one or more elements in common with the
     * specified collection.
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     *
     * @see #remove(Object)
     * @see #contains(Object)
     */
    public boolean removeAll(Collection<?> c) {
        Objects.requireNonNull(c);
        boolean modified = false;
        Iterator<?> it = iterator();
        while (it.hasNext()) {
            if (c.contains(it.next())) {
                it.remove();
                modified = true;
            }
        }
        return modified;
    }

上边removeAll方法是通过迭代器去判断c集合中是否包含，包含即调用迭代器的remove方法。在判断包含的时候（c.contains(Object obj)），这里contains方法的源码如下：

/**
     * {@inheritDoc}
     *
     * <p>This implementation iterates over the elements in the collection,
     * checking each element in turn for equality with the specified element.
     *
     * @throws ClassCastException   {@inheritDoc}
     * @throws NullPointerException {@inheritDoc}
     */
    public boolean contains(Object o) {
        Iterator<E> it = iterator();
        if (o==null) {
            while (it.hasNext())
                if (it.next()==null)
                    return true;
        } else {
            while (it.hasNext())
                if (o.equals(it.next()))
                    return true;
        }
        return false;
    }

这里当对象o不为空时，迭代判断用到了Object的equals方法，而Object的equals方法指的是两个对象的引用是否相等，所以才形成了最初的问题。如果我们要判断两个对象的内容相等，这里就需要重写equals方法。

代码如下：

package com.study;

import java.util.ArrayList;
import java.util.List;
import java.util.Objects;

class User {

    private int id;
    private String name;
    
    // 省略

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        User user = (User) o;
        return id == user.id && Objects.equals(name, user.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(id, name);
    }
}

public class TestRemoveAll {
    public static void main(String[] args) {
        List<User> list = new ArrayList<User>();
        list.add(new User(1, "1"));
        list.add(new User(2, "2"));
        list.add(new User(3, "3"));
        list.add(new User(4, "4"));
        List<User> list1 = new ArrayList<User>();
        list1.add(new User(1, "1"));
        list1.add(new User(2, "2"));
        list1.add(new User(3, "3"));
        list1.add(new User(5, "5"));
        list.removeAll(list1);
        System.out.println("list remove list1 以后的大小为 " + list.size());
    }
}

这里重写equals方法推荐使用idea自动生成的，然后执行了下main方法，得到的结果如下：

 

 

 

注意：

1、要想是removeAll方法生效，只需重写equals方法即可。　　

　　为何这里还要重写hashcode方法，重写hashcode方法为了将数据存入HashSet/HashMap/Hashtable类时进行比较。

2、看完removeAll方法的源码后，我们看到的是双层迭代，这种方式在数据量大的情况下，势必会影响效率。所以谨慎使用removeAll()函数