前戲：

公司有個流程是這樣的，有四條流程，一條流程四個節點。在每個流程第四個節點操作完後，複製當前節點的“疑點”資料至下一流程的節點。本來這是個很簡單的功能，但是當上一條流程的節點資料更改後要同步到下一流程的首節點。原來的做法是刪除下一節點的所以資料，重新插入上一節點的全部資料。當資料很大時就會相當耗費資料資源，所以引出了這篇文章。

注:A與B分別代表整個

思路：listA（老的資料） 與 listB（新的資料），listA-listB的差集就是需要刪除的資料。listB-listA的差集就是需要新增資料庫的資料。

/**
	 * @param args
	 */
	public static void main(String[] args) {
		List<User> listA = new ArrayList<User>();   // 原來的list
		List <User>listB = new ArrayList<User>();        // 新的list
		listA.removeAll(listB);
		
 

// 製造測試資料
		for(int i=0;i<6;i++){
			User u1 = new User(i,"張三"+i,String.valueOf(i));
			listA.add(u1);
			
			if(i<5){
				User u2 = new User("張三"+i,String.valueOf(i));
				listB.add(u2);
			}
			
		}

// 輸出原始資料
		System.out.println("-----------------------listA-------------------------");
		for(User u: listA){
			System.out.println(u.toString());
		}
		System.out.println("-----------------------listB-------------------------");
		for(User u: listB){
			System.out.println(u.toString());
		}
		
	
		
		// 新增資料庫的資料
		List<User> newList = new ArrayList<User>(); 
		newList.addAll(listB);
		newList.removeAll(listA);
		
 

// 刪除資料庫的資料
		List<User> delList = new ArrayList<User>(); 
		delList.addAll(listA);
		delList.removeAll(listB);
		
		System.out.println("-----------------------newlist-------------------------");
		for(User u: newList){
			System.out.println(u.toString());
		}
		System.out.println("-----------------------dellist-------------------------");
		for(User u: delList){
			System.out.println(u.toString());
		}
	}
 

}

java.util
類 AbstractCollection<E>中的removeAll（）方法中使用的contains（）的問題。

    /**
     * {@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) {
	boolean modified = false;
	Iterator<?> e = iterator();
	while (e.hasNext()) {
	    if (c.contains(e.next())) {
		e.remove();
		modified = true;
	    }
	}
	return modified;
    }

  /**
     * {@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> e = iterator();
	if (o==null) {
	    while (e.hasNext())
		if (e.next()==null)
		    return true;
	} else {
	    while (e.hasNext())
		if (o.equals(e.next()))
		    return true;
	}
	return false;
    }

package com.bean;

public class User {

	private int id;
	private String name;
	private String age;
	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 String getAge() {
		return age;
	}
	public void setAge(String age) {
		this.age = age;
	}
	public User(int id, String name, String age) {
		super();
		this.id = id;
		this.name = name;
		this.age = age;
	}
	public User(String name, String age) {
		super();
		this.name = name;
		this.age = age;
	}
	

	
    public boolean equals(Object obj) { 
        if((obj instanceof User) && obj!=null) { 
        	User u = (User)obj; 
        	//System.out.println("u.name:"+u.name+"this.name  "+this.name);
        	//System.out.println("u.age:"+u.age+"this.age  "+this.age);
            if((u.name.equals(this.name)) && (u.age.equals(this.age))) { 
            
                return true; 
            } 
        } 
        return false; 
    } 
	@Override
	public String toString() {
		return "User [age=" + age + ", id=" + id + ", name=" + name + "]";
	}
}