List去重问题

如果集合中的数据类型是基本数据类型，可以直接将list集合转换成set，就会自动去除重复的元素，这个就相对比较简单。

public class Test {

    public static void main(String[] args) {
        List list = new ArrayList();
        list.add(11);
        list.add(12);
        list.add(13);
        list.add(14);
        list.add(15);
        list.add(11);
        System.out.println(list);

        Set set = new HashSet();
        List newList = new ArrayList();
        set.addAll(list);
        newList.addAll(set);
        System.out.println(newList);

    }

}

当list集合中存储的类型是对象类型的时候，我们就不能简单的只把list集合转换成set集合。

我们定义一个对象类：

public class People {

    private String name;
    private String phoneNumber;

    public String getName() {
        return name;
    }

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

    public String getPhoneNumber() {
        return phoneNumber;
    }

    public void setPhoneNumber(String phoneNumber) {
        this.phoneNumber = phoneNumber;
    }

    public People(String name, String phoneNumber) {
        super();
        this.name = name;
        this.phoneNumber = phoneNumber;
}

    @Override
    public String toString() {
        return "People{" +
                "name='" + name + ''' +
                ", phoneNumber='" + phoneNumber + ''' +
                '}';
    }

}

当list集合中存储的是对象时，我们需要在对象的实体类中去重写equals()方法和hashCode()方法，如下：

public class People {

    private String name;
    private String phoneNumber;

    public String getName() {
        return name;
    }

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

    public String getPhoneNumber() {
        return phoneNumber;
    }

    public void setPhoneNumber(String phoneNumber) {
        this.phoneNumber = phoneNumber;
    }

    public People(String name, String phoneNumber) {
        super();
        this.name = name;
        this.phoneNumber = phoneNumber;
}

    @Override
    public String toString() {
        return "People{" +
                "name='" + name + ''' +
                ", phoneNumber='" + phoneNumber + ''' +
                '}';
    }

    @Override
    public boolean equals(Object arg0) {
        // TODO Auto-generated method stub
        People p = (People) arg0;
        return name.equals(p.name) && phoneNumber.equals(p.phoneNumber);
    }

    @Override
    public int hashCode() {
        // TODO Auto-generated method stub
        String str = name + phoneNumber;
        return str.hashCode();
    }

}

test:

public static void main(String[] args) {
    List<People> listPeople = new ArrayList<People>();
    listPeople.add(new People("张三", "11111"));
    listPeople.add(new People("张三",  "22222"));
    listPeople.add(new People("李四",  "33333"));
    listPeople.add(new People("张三",  "22222"));

    Set<People> setData = new HashSet<People>();
    setData.addAll(listPeople);
    System.out.println("list：" + listPeople.toString());
    System.out.println("set：" + setData.toString());

}

此时运行上面的test方法，去重就成功了。