技術標籤：javajava

java8 stream原始碼中stream.collect(Collectors.toList())去重

**
利用 java8 stream對list或者set中帶有(“aaa”,“88888”)等複雜欄位去重。如若失敗可能是equals方法原因，需要重寫hashCode()， equals(Object obj)方法。
測試發現如果hashCode()的欄位為Long型別，即不是String的話equals(Object obj)方法中的this.id==other.getId()應將換成this.id.equals(other.getId()，測試即可成功。

package zzh;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
import java.util.stream.Collectors;
public class Test {
    public static void main(String[] args) {
        EqualsObj obj1 =new EqualsObj("1","xxx"
 
);
        EqualsObj obj2 =new EqualsObj("1","88888");
        EqualsObj obj3 =new EqualsObj("2","xxx");
        List<EqualsObj> list = new ArrayList<>();
        list.add(obj1);
        list.add(obj2);
        list.add(obj3);
        list =  list.
 
stream().distinct().collect(Collectors.toList());
        Set<EqualsObj> set = new HashSet<>();
        set.add(obj1);
        set.add(obj2);
        set.add(obj3);
    }
    static class EqualsObj{
        private String id;
        private String name;
        public EqualsObj(String id,String name){
            this.id = id;
            this.name = name;
        }
        public String getId() {
            return id;
        }
        public void setId(String id) {
            this.id = id;
        }
        public String getName() {
            return name;
        }
        public void setName(String name) {
            this.name = name;
        }
        @Override
        public int hashCode() {
            int result = 17;
            result = 31 * result + (id == null ? 0 : id.hashCode());
            return result;
        }
        @Override
        public boolean equals(Object obj) {
            //地址相等
            if(this == obj){
                return true;
            }
            if(obj == null){
                //非空性：對於任意非空引用x，x.equals(null)應該返回false。
                return false;
            }
            if(obj instanceof EqualsObj){
                EqualsObj other = (EqualsObj) obj;
                //需要比較的欄位相等，則這兩個物件相等
                if(this.id==other.getId()){
                    return true;
                }
            }
            return false;
        }
    }
}