我有一个多个多维 HashMap() 实例，我使用它们来存储数据库中的分层数据；
HashMap<String, HashMap<String, ArrayList<String>>>
我使用 3 个主要方法添加它们，我们将其称为 addTop()、addMid() 和 addLow() 。这些方法都接受与其数据组和字符串相匹配的参数，每个方法都返回 HashMap() 的下一个维度；
public static HashMap<String, ArrayList<String>> addTop(HashMap<String, HashMap<String, ArrayList<String>>> data, String val) { ... };
public static ArrayList<String> addMid(HashMap<String, ArrayList<String>> data, String val) { ... };
public static String addLow(ArrayList<String> data, String val) { ... };
我通常在几次检查之间按顺序调用这些方法，并在方法内执行其他检查。本质上，这些方法所做的就是将 val 添加到 data，然后返回一个空的 HashMap()；
out = new HashMap();
data.put(val, out);
return out;
当我在循环/数据填充结束时检查时，来自 addMid() 和 addLow() 的所有数据都丢失了。这是为什么？
我认为 Java 在处理复杂对象时按引用工作，例如 HashMap()。
如何确保 addMid() 和 addLow() 更新主 HashMap()？
编辑:包含的代码。它可以编译并运行，但还有其他问题，我已经尽可能地删除了所有内容来演示发生的情况，除了 SQL 内容，它们无法编译，抱歉。启动时运行的方法是 sqlToArray();
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.HashMap;
public class Av2 {
protected class AvailLookup {
private Integer key;
private String value;
public AvailLookup(Integer inKey, String inValue) {
key = inKey;
value = inValue;
}
public void updateName(String name) {
value = name;
}
public Integer getKey() {
return key;
}
public String getValue() {
return value;
}
public String toString() {
return value;
}
}
private static HashMap<AvailLookup, HashMap<AvailLookup, ArrayList<AvailLookup>>> data = new HashMap<AvailLookup, HashMap<AvailLookup, ArrayList<AvailLookup>>>();
private static Sql sql = new Sql("PlantAvail");
public static HashMap<AvailLookup, ArrayList<AvailLookup>> getChannel(HashMap<AvailLookup, HashMap<AvailLookup, ArrayList<AvailLookup>>> inArray, Integer channel) {
HashMap<AvailLookup, ArrayList<AvailLookup>> out = null;
if (inArray != null ) {
for (AvailLookup lookup : inArray.keySet()) {
if (lookup.getKey() == channel) {
out = inArray.get(lookup);
System.out.println("Channel: " + channel + " found");
break;
}
}
if (out == null) {
System.out.println("Channel: " + channel + " not found");
}
}
return out;
}
public static HashMap<AvailLookup, ArrayList<AvailLookup>> getChannel(HashMap<AvailLookup, HashMap<AvailLookup, ArrayList<AvailLookup>>> inArray, String channel) {
HashMap<AvailLookup, ArrayList<AvailLookup>> out = null;
if (inArray != null ) {
for (AvailLookup lookup : inArray.keySet()) {
if (lookup.getValue() != null) {
if (lookup.getValue().equalsIgnoreCase(channel)) {
out = inArray.get(lookup);
System.out.println("Channel: " + channel + " found");
break;
}
}
}
if (out == null) {
System.out.println("Channel: " + channel + " not found");
}
}
return out;
}
public static HashMap<AvailLookup, ArrayList<AvailLookup>> addChannel(HashMap<AvailLookup, HashMap<AvailLookup, ArrayList<AvailLookup>>> inArray, Integer id, String name) {
HashMap<AvailLookup, ArrayList<AvailLookup>> out = null;
if (inArray != null ) {
if (getChannel(inArray, id) == null) {
out = new HashMap<AvailLookup, ArrayList<AvailLookup>>();
inArray.put(new AvailLookup(id, name), new HashMap<AvailLookup, ArrayList<AvailLookup>>());
System.out.println("Channel: added " + id);
} else {
System.out.println("Channel: " + id + " already exists");
}
} else {
System.out.println("Channel: " + id + " already exists");
}
return out;
}
public static void removeChannel(HashMap<AvailLookup, HashMap<AvailLookup, ArrayList<AvailLookup>>> inArray, Integer channel) {
boolean pass = false;
HashMap<AvailLookup, ArrayList<AvailLookup>> channelLookup = getChannel(inArray, channel);
for (AvailLookup lookup : channelLookup.keySet()) {
if (lookup.getKey() == channel) {
inArray.remove(channel);
System.out.println("Channel: " + channel + " removed");
pass = true;
break;
}
}
if (!pass) {
System.out.println("Channel: " + channel + " cannot be removed");
}
}
public static ArrayList<AvailLookup> getDevice(HashMap<AvailLookup, ArrayList<AvailLookup>> channel, Integer device) {
ArrayList<AvailLookup> out = null;
for(AvailLookup lookup : channel.keySet()) {
if (lookup.getKey() == device) {
out = channel.get(device);
System.out.println("Device: " + device + " found");
break;
}
}
if (out == null) {
System.out.println("Device: " + device + " not found");
}
return out;
}
public static ArrayList<AvailLookup> getDevice(HashMap<AvailLookup, ArrayList<AvailLookup>> channel, String device) {
ArrayList<AvailLookup> out = null;
for(AvailLookup lookup : channel.keySet()) {
if (lookup.getValue() == device) {
out = channel.get(device);
System.out.println("Device: " + device + " found");
break;
}
}
if (out == null) {
System.out.println("Device: " + device + " not found");
}
return out;
}
public static ArrayList<AvailLookup> addDevice(HashMap<AvailLookup, ArrayList<AvailLookup>> channel, Integer id, String value) {
ArrayList<AvailLookup> out = null;
if (getDevice(channel, id) == null) {
out = new ArrayList<AvailLookup>();
channel.put(new AvailLookup(id, value), new ArrayList<AvailLookup>());
System.out.println("Device: added " + id);
} else {
System.out.println("Device: " + id + " already exists");
}
return out;
}
public static void removeDevice(HashMap<AvailLookup, ArrayList<AvailLookup>> channel, Integer device) {
boolean pass = false;
ArrayList<AvailLookup> deviceLookup = getDevice(channel,device);
for (AvailLookup lookup : deviceLookup) {
if (lookup.getKey() == device) {
channel.remove(device);
System.out.println("Device: " + device + " removed");
pass = true;
break;
}
}
if (!pass) {
System.out.println("Device: " + device + " cannot be removed");
}
}
public static AvailLookup getHost(ArrayList<AvailLookup> hosts, Integer host) {
AvailLookup out = null;
for (AvailLookup hostLookup : hosts) {
if (hostLookup.getKey() == host) {
out = hostLookup;
System.out.println("Host: " + host + " found");
}
}
if (hosts.contains(host)) {
} else {
System.out.println("Host: " + host + " not found");
}
return out;
}
public static AvailLookup getHost(ArrayList<AvailLookup> hosts, String host) {
AvailLookup out = null;
for (AvailLookup hostLookup : hosts) {
if (hostLookup.getValue() == host) {
out = hostLookup;
System.out.println("Host: " + host + " found");
}
}
if (hosts.contains(host)) {
} else {
System.out.println("Host: " + host + " not found");
}
return out;
}
public static AvailLookup addHost(ArrayList<AvailLookup> hosts, Integer id, String value) {
AvailLookup out = null;
for (AvailLookup hostLookup : hosts) {
if (hostLookup.getKey() == id) {
out = hosts.set(id, new AvailLookup(id, value));
System.out.println("Host: " + id + " found");
break;
}
}
if (out == null) {
System.out.println("Host: " + id + " not found");
}
return out;
}
public static void removeHost(ArrayList<AvailLookup> hosts, Integer host) {
boolean pass = false;
for (AvailLookup hostLookup : hosts) {
if (hostLookup.getKey() == host) {
hosts.remove(hostLookup);
System.out.println("Host: " + host + " removed");
pass = true;
}
}
if (!pass) {
System.out.println("Host: " + host + " cannot be removed");
}
}
public static ArrayList<AvailLookup> otherHosts(ArrayList<AvailLookup> hosts, Integer key, String value) {
ArrayList<AvailLookup> out = null;
for (AvailLookup host : hosts) {
if (host.getKey() != key) {
if (out == null) {
out = new ArrayList<AvailLookup>();
}
out.add(new AvailLookup(key, value));
}
}
if (out != null) {
if (out.size() > 1) {
System.out.println("Host: generated other hosts");
}
}
return out;
}
public static AvailLookup nextHost(ArrayList<AvailLookup> otherHosts) {
AvailLookup out = null;
if (otherHosts != null) {
out = otherHosts.get(0);
System.out.println("Host: getting next host");
} else {
System.out.println("Host: no other host");
}
return out;
}
public static void sqlToArray() {
HashMap<AvailLookup, HashMap<AvailLookup, ArrayList<AvailLookup>>> tempData = new HashMap<AvailLookup, HashMap<AvailLookup, ArrayList<AvailLookup>>>();
Integer iHost = null;
Integer iDevice = null;
Integer iChannel = null;
String sHost = null;
String sDevice = null;
String sChannel = null;
HashMap<AvailLookup, ArrayList<AvailLookup>> channel = null;
ArrayList<AvailLookup> device = null;
Sql obj = new Sql("plantavail");
obj.query("select j_channel.id as channelid, j_channel.name as channelname, j_device.id as deviceid, j_device.name as devicename, j_io.id as hostid, j_io.host as hostname, alias"
+ " from j_io"
+ " left join j_channel on j_io.id = j_channel.iofk"
+ " left join j_device on j_channel.iofk = j_device.id");
try {
while(obj.getResult().next()) {
sChannel = obj.getResult().getString("channelname");
sDevice = obj.getResult().getString("devicename");
sHost = obj.getResult().getString("hostname");
iChannel = obj.getResult().getInt("channelid");
iDevice = obj.getResult().getInt("deviceid");
iHost = obj.getResult().getInt("hostid");
channel = addChannel(tempData, iChannel, sChannel);
if (channel != null) {
device = addDevice(channel, iDevice, sDevice);
if (device != null) {
addHost(device, iHost, sHost);
}
}
}
} catch (SQLException e1) {
e1.printStackTrace();
}
data = tempData;
}
}
最佳答案
小心不要意外覆盖现有的 map 值。如果您使用 java 8，您可以使用:
map.computeIfAbsent("entry", s -> new ArrayList<>());
在 Java 8 之前，您需要检查该值是否为 null:
List<String> list = map.get("entry");
if(list == null){
list = map.put("entry", new ArrayList<String>());
}
您还需要确保正确更新 map :
一个小例子:
Map<String, String> map = new HashMap<>();
String a = "a";
String b = "b";
map.put(a, b);
System.out.println(map.get(a));
b = "c";
System.out.println(map.get(a));
System.out.println(b);
输出为:
b
b
c
所以你看看如果你更新b， map 不会更新。现在， map 中的 map 也是如此:
final String a = "a";
final String b = "b";
Map<String, Map<String, String>> topMap = new HashMap<>();
Map<String, String> middleMap = topMap.getOrDefault(a, new HashMap<>());
middleMap.put(b, "c");
topMap.put("a", middleMap);
System.out.println(topMap.get(a).get(b));
middleMap.replace(b, "d");
System.out.println(topMap.get(a).get(b));
topMap.put("a", middleMap);
System.out.println(topMap.get(a).get(b));
输出为:
c
d
d
但是为什么呢？不应该是“c c d”吗？不!因为Java中的String是不可变的，而Map却不是。如果您考虑到这一点，您应该能够解决您的问题。
关于java - 通过引用传递 HashMap，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/30734053/