how to parse xml to hashmap?

Use JAXB to read from xml and save it to a custom object.

Custom object class:

import java.util.List;

import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlElementWrapper;
import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.XmlType;

@XmlRootElement(name = "Details")
@XmlType(propOrder = { "detailA", "detailB" })
public class Details {
    private List<String> detailA;
    private List<String> detailB;

    public void setDetailA(List<String> detailA) {
        this.detailA = detailA;
    }

    @XmlElementWrapper(name = "detail-a")
    @XmlElement(name = "detail")
    public List<String> getDetailA() {
        return detailA;
    }

    public void setDetailB(List<String> detailB) {
        this.detailB = detailB;
    }

    @XmlElementWrapper(name = "detail-b")
    @XmlElement(name = "detail")
    public List<String> getDetailB() {
        return detailB;
    }
}

Extract the data from your xml into the object, then add contents to a map as desired:

public static void main(String[] args) throws JAXBException, FileNotFoundException {
    System.out.println("Output from our XML File: ");
    JAXBContext context = JAXBContext.newInstance(Details.class);
    Unmarshaller um = context.createUnmarshaller();
    Details details = (Details)um.unmarshal(new FileReader("details.xml"));
    List<String> detailA = details.getDetailA();
    List<String> detailB = details.getDetailB();

    Map<String, String[]> map = new HashMap<String, String[]>();
    map.put("detail-a", detailA.toArray(new String[detailA.size()]));
    map.put("detail-b", detailB.toArray(new String[detailB.size()]));


    for (Map.Entry<String, String[]> entry : map.entrySet()) {
        //key "detail a" value={"attribute 1 of detail a","attribute 2 of detail a","attribute 3 of detail a"}
        System.out.print("Key \"" +entry.getKey()+"\" value={");
        for(int i=0;i<entry.getValue().length;i++){
            if(i!=entry.getValue().length-1){
                System.out.print("\""+entry.getValue()[i]+"\",");
            }
            else{
                System.out.print("\""+entry.getValue()[i]+"\"}");
            }
        }
        System.out.println();
    }
}

Output will be:

Output from our XML File: 
Key "detail-a" value={"attribute 1 of detail a","attribute 2 of detail a","attribute 3 of detail a"}
Key "detail-b" value={"attribute 1 of detail b","attribute 2 of detail b"}

As a note: this will work only for the xml you provided as input in your question, if you need to add more details like detail-c and so on you must define them in your custom object as well.

XML used:

<?xml version="1.0" encoding="utf-8"?>
<Details>
    <detail-a>
        <detail>attribute 1 of detail a</detail>
        <detail>attribute 2 of detail a</detail>
        <detail>attribute 3 of detail a</detail>
    </detail-a>
    <detail-b>
        <detail>attribute 1 of detail b</detail>
        <detail>attribute 2 of detail b</detail>
    </detail-b>
</Details>

I can not resist to present a much shorter solution using XMLBeam that works with any number of "detail-x" subelements.

public class Tetst {

@XBDocURL("resource://test.xml")
public interface Projection {
    @XBRead("name()")
    String getName();

    @XBRead("./detail")
    List<String> getDetailStrings();

    @XBRead("/Details/*")
    List<Projection> getDetails();
}

@Test
public void xml2Hashmap() throws IOException {
    HashMap<String, List<String>> hashmap = new HashMap<String, List<String>>();
    for (Projection p : new XBProjector().io().fromURLAnnotation(Projection.class).getDetails()) {
        System.out.println(p.getName() + ": " + p.getDetailStrings());
        hashmap.put(p.getName(), p.getDetailStrings());
    }
}
}

This prints out

detail-a: [ attribute 1 of detail a ,  attribute 2 of detail a ,  attribute 3 of detail a ]
detail-b: [ attribute 1 of detail b ,  attribute 2 of detail b ]

for your example test.xml and fills a Hashmap.

There is underscore-java library. I am the maintainer of the project. Live example

import com.github.underscore.lodash.U;
import java.util.Map;

public class Main {

  @SuppressWarnings("unchecked")
  public static void main(String[] args) {

    Map<String, Object> map = U.fromXmlMap(
        "<Details>\r\n" + 
        "    <detail-a>\r\n" + 
        "\r\n" + 
        "        <detail> attribute 1 of detail a </detail>\r\n" + 
        "        <detail> attribute 2 of detail a </detail>\r\n" + 
        "        <detail> attribute 3 of detail a </detail>\r\n" + 
        "\r\n" + 
        "    </detail-a>\r\n" + 
        "\r\n" + 
        "    <detail-b>\r\n" + 
        "        <detail> attribute 1 of detail b </detail>\r\n" + 
        "        <detail> attribute 2 of detail b </detail>\r\n" + 
        "\r\n" + 
        "    </detail-b>\r\n" + 
        "\r\n" + 
        "\r\n" + 
        "</Details>");
    
    System.out.println(map);
    // {Details={detail-a={detail=[ attribute 1 of detail a ,  attribute 2 of detail a ,  attribute 3 of detail a ]},
    // detail-b={detail=[ attribute 1 of detail b ,  attribute 2 of detail b ]}}, #omit-xml-declaration=yes}
  }
}