java - Lists>添加到 HashMap

Question

所以，在请求编译代码之后，它就在这里。问题是，在添加第二个名为“B”的哈希元素后，输出变得困惑。

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
public class test {
    public static void main(String[] args) {
        Map<String, List<List<Double>>> alphabet = new HashMap<String,List<List<Double>>>();
        List<List<Double>> something = new ArrayList<List<Double>>();
        List<Double> stuffList = new ArrayList<Double>();
        stuffList.add(3.1);
        stuffList.add(3.2);
        stuffList.add(3.3);
        something.add(stuffList);
        alphabet.put("A", something);
        System.out.println(something);
        System.out.println(alphabet);
        stuffList.clear();
        something.clear();
        stuffList.add(3.4);
        something.add(stuffList);
        alphabet.put("B", something);
        System.out.println(something);
        System.out.println(alphabet);
    }
}

输出为:

[[3.1, 3.2, 3.3]]
{A=[[3.1, 3.2, 3.3]]}
[[3.4]]
{A=[[3.4]], B=[[3.4]]}

在我看来和需要，应该是:

[[3.1, 3.2, 3.3]]
{A=[[3.1, 3.2, 3.3]]}
[[3.4]]
{A=[[3.1, 3.2, 3.3]], B=[[3.4]]}

Answer 1

您仍在引用列表的旧实例。这应该按预期工作:

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
public class test {
    public static void main(String[] args) {
        Map<String, List<List<Double>>> alphabet = new HashMap<String,List<List<Double>>>();
        List<List<Double>> something = new ArrayList<List<Double>>();
        List<Double> stuffList = new ArrayList<Double>();
        stuffList.add(3.1);
        stuffList.add(3.2);
        stuffList.add(3.3);
        something.add(stuffList);
        alphabet.put("A", something);
        System.out.println(something);
        System.out.println(alphabet);
        // Create new instances:
        something = new ArrayList<List<Double>>();
        stuffList = new ArrayList<Double>();
        stuffList.add(3.4);
        something.add(stuffList);
        alphabet.put("B", something);
        System.out.println(something);
        System.out.println(alphabet);
    }
}