Exception in thread “main” java.lang.IllegalStateException: Duplicate key

Let’s say I want to prepare a Map from a List of Strings, I use below code to achieve,

package com.bruiser.issue.solution;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;

public class DuplicateKey {

	public static void main(String[] args) {
		
		List<String> randomeCodes = new ArrayList<>();
		randomeCodes.add("101");
		randomeCodes.add("102");
		randomeCodes.add("103");
		randomeCodes.add("103");
		randomeCodes.add("104");
		
		Map<String, String> randomMap = randomeCodes.stream().collect(Collectors.toMap(Function.identity(), Function.identity()));
		System.out.println(randomMap);
	}

}

Expected result:

{101=101, 102=102, 103=103, 104=104}

Actual result:

Exception in thread "main" java.lang.IllegalStateException: Duplicate key 103 (attempted merging values 103 and 103)
	at java.base/java.util.stream.Collectors.duplicateKeyException(Collectors.java:135)
	at java.base/java.util.stream.Collectors.lambda$uniqKeysMapAccumulator$1(Collectors.java:182)
	at java.base/java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
	at java.base/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1625)
	at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:484)
	at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:474)
	at java.base/java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:913)
	at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
	at java.base/java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:682)
	at com.bruiser.issue.solution.DuplicateKey.main(DuplicateKey.java:20)

Solution

To fix the problem, we need to pass merge function along with KeyMapper and ValueMapper,

package com.bruiser.issue.solution;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;

public class DuplicateKey {

	public static void main(String[] args) {
		
		List<String> randomeCodes = new ArrayList<>();
		randomeCodes.add("101");
		randomeCodes.add("102");
		randomeCodes.add("103");
		randomeCodes.add("103");
		randomeCodes.add("104");
		
		Map<String, String> randomMap = randomeCodes.stream().collect(Collectors.toMap(Function.identity(), Function.identity(), (newCode, oldCode) -> newCode));
		System.out.println(randomMap);
	}

}

And here is the expected and actual result,

{101=101, 102=102, 103=103, 104=104}