Collecting data from map to list using Java 8 - java-8

Question

Please help me with the Java 8 map - filter - sort - collect code.
Info.java
public class Info {
private final String name;
private final String desc;
private String version = null;
#Override
public boolean equals(Object that) {
if (that == null) {
return false;
}
if (that instanceof Info) {
Info other = (Info) that;
return Objects.equals(this.name, other.name) &&
Objects.equals(this.desc, other.desc) &&
Objects.equals(this.version, other.version);
} else {
return false;
}
}
public boolean equalsWithoutVersion(Object that) {
if (that == null) {
return false;
}
if (that instanceof Info) {
Info other = (Info) that;
return Objects.equals(this.name, other.name) &&
Objects.equals(this.desc, other.desc);
} else {
return false;
}
}
#Override
public int hashCode() {
int hash = 13;
hash = (hash * 7) + name.hashCode();
hash = (hash * 7) + desc.hashCode();
if (version != null)
hash = (hash * 7) + version.hashCode();
return hash;
}
#Override
public String toString() {
String versionString = version == null ? "latest" : version;
return String.format("Name: %s Desc: %s Key Type: %s Version: %s", this.name, this.desc, this.keyType.name(), versionString);
}
}
Value.java
public class Value implements Comparable<Value> {
private String data;
private String version;
public Value(String version, String data) {
this.version = version;
this.data = data;
}
#Override
public int compareTo(Value o) {
return (Integer.parseInt(this.version) > Integer.parseInt(o.version)) ? -1
: (Integer.parseInt(this.version) < Integer.parseInt(o.version)) ? 1
: 0;
}
}
Cache.java
public class Cache {
private final Map<Info, Value> dataMap = new HashMap<>();
...
private Value getlatestVersionFromCache(Info info) {
List<Value> values = dataMap.entrySet().stream()
.filter(p -> p.getKey().equalsWithoutVersion(info))
.sorted(Map.Entry.comparingByValue())
.map(x::getValue))
.collect(Collectors.toList());
return values.isEmpty() ? null : values.get(0);
}
}
The goal is to obtain the latest version of a record from the map. So first I am filtering entries from the map by comparing fields of Info without version. Then I am sorting the map by value based on version. Then I am collecting the values to a list. Then I can get the first element to get the latest version.
But I am getting the following compilation error on the filter statement:
Syntax error on token ")", ElidedSemicolonAndRightBrace expected

Answer 1

Well I doubt about your solution. I think you can do it in simple way. so first change version type to Integer in Value class(in compareTo() method you converted it to Integer). and also change method signature to Optional<Value> in getlatestVersionFromCache() method.
also I think you don't need to sort dataMap.
private Optional<Value> getlatestVersionFromCache(Info info) {
Map<Value,Integer> result = dataMap.entrySet()
.stream()
.filter(p -> p.getKey().equalsWithoutVersion(info))
.collect(Collectors.toMap(Map.Entry::getValue, entry -> entry.getValue().getVersion(), Integer::min));
Optional<Value> latestValue = result.keySet()
.stream()
.min(Comparator.comparingInt(key -> key.getVersion()));
return latestValue;
}
Better solution is something like this:
dataMap.entrySet()
.stream()
.filter(p -> p.getKey().equalsWithoutVersion(info))
.min(Comparator.comparingInt(entry -> entry.getValue().getVersion()))
.map(Map.Entry::getValue)
.orElse(null);

Answer 2

You have several omissions and errors in the code you posted, but the filter statement was actually fine.
The following passes compilation:
List<Value> values = dataMap.entrySet()
.stream()
.filter(p -> p.getKey().equalsWithoutVersion(info))
.sorted(Map.Entry.comparingByValue())
.map(Map.Entry::getValue) // was .map(x::getValue)) - x is not defined anywhere, so
// I assumed you meant Map.Entry::getValue
.collect(Collectors.toList());

Answer 3

class Info {
public static final BiPredicate<Info, Info> IS_EQUAL_IGNORE_VERSION =
(one, two) -> one.getName().equals(two.getName()) && one.getDesc().equals(two.getDesc());
private final String name;
private final String desc;
private String version;
}
private final Map<Info, Value> dataMap = new HashMap<>();
public Value getlatestVersionFromCache(Info info) {
Value value = null;
for (Map.Entry<Info, Value> entry : dataMap.entrySet())
if (Info.IS_EQUAL_IGNORE_VERSION.test(entry.getKey(), info))
if (value == null || Integer.parseInt(entry.getValue().getVersion()) > Integer.parseInt(value.getVersion()))
value = entry.getValue();
return value;
}
Notes:
I think that use a comparator to check that two objects are queal is not correct. You can use e.g. Predicate. And define it as a static method in the target class.
Pay attention on version. It seems that it should be an integer instead of String.
In your approach, compare two string is not correct, because it is used instead of equals method.
I think that using Java8 streams only because this is a stream is alow not correct.