CODEWITHSUNDEEP
javalambdacomparator
Galadriel
Galadriel

Reputation: 369

java.lang.ClassCastException with stream sorting and custom comparator

I am using java version 1.8, which supports lambda expressions.

I am trying to sort a stream by a custom comparator, but I am getting a ClassCastxception:

public class A {
    
    private String type;
    private String ip;
    private String originSubId;
    
    
    public A(String type, String ip, String originSubId) {
        this.type = type;
        this.ip = ip;
        this.originSubId = originSubId;
    }
    public String getType() {
        return type;
    }
    public void setType(String type) {
        this.type = type;
    }
    public String getIp() {
        return ip;
    }
    public void setIp(String ip) {
        this.ip = ip;
    }
    public String getOriginSubId() {
        return originSubId;
    }
    public void setOriginSubId(String originSubId) {
        this.originSubId = originSubId;
    }
}

    Comparator<A> defaultComparator = Comparator.comparing(A::getType)
                    .thenComparing(A::getIp).thenComparing(A::getOriginSubId);
    
    Set<A> entities = new HashSet<>();  
    entities.stream().map(e -> convertToB(e)).sorted(defaultComparator)
                    .collect(Collectors.toCollection(TreeSet::new));

Error: java.lang.ClassCastException: A cannot be cast to java.lang.Comparable

What I am doing wrong?

Upvotes: 0

Views: 497

Answers (1)

Kayaman
Kayaman

Reputation: 73558

You can't collect to a "plain" TreeSet like that because it requires that the elements are either Comparable or that the TreeSet gets a custom Comparator on creation. You also don't need to sort in the stream, as TreeSet performs sorting on insertion. Your convertToA() method looks suspect too, is it converting A -> A?

You would need something like the following

entities.stream().map(e -> convertToA(e))
                .collect(Collectors.toCollection(() -> new TreeSet(customComparator)));

If your conversion method isn't needed, you can just forget the whole stream and do

Set<A> tree = new TreeSet<>(customComparator);
tree.addAll(entities);

Upvotes: 1

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