Find Null or Empty Strings in a Java List Example

1. Introduction

Finding Null or empty Strings in a Java List is a common operation because a list can contain null or empty (“”) values. The null or empty can cause bugs or unexpected behavior if not handled properly. In this example, I will demonstrate three approaches to find null or empty string in a list:

• via the List.add method in a For loop
• via the ArrayList.removeIf method for a mutable list.
• via Stream APIs filter and collect methods.

2. Setup

In this step, I will create a simple Java project in Eclipse with JDK 21 with the following Java source files.

Java Source Files

C:\Users\zzhen\eclipse-workspace\FindFromList\src\org\jcg\zheng\demo>dir
 Volume in drive C is OS
 Volume Serial Number is FE59-88F4

 Directory of C:\Users\zzhen\eclipse-workspace\FindFromList\src\org\jcg\zheng\demo

09/10/2025  09:02 PM              .
09/08/2025  07:39 AM              ..
09/10/2025  12:17 PM               177 FindNullorEmptyService.java
09/10/2025  09:33 PM               992 ForLoopAddImpl.java
09/10/2025  09:32 PM               926 ListTestData.java
09/10/2025  09:08 PM               988 RemoveIfImpl.java
09/10/2025  09:32 PM             1,028 StreamImmutableImpl.java
09/10/2025  09:33 PM             1,037 StreamMutableImpl.java
               6 File(s)          5,148 bytes
               2 Dir(s)  886,891,036,672 bytes free

C:\Users\zzhen\eclipse-workspace\FindFromList\src\org\jcg\zheng\demo>

3. Test List Data

A list is an ordered, index-based collection that can contain duplicate elements. A List can be both mutable and immutable. In this step, I will create a ListTestData.java that constructs three test data lists:

• fixed-size list – A list created by Arrays.asList is a fixed-size list. You can replace the content of list, but cannot add nor remove elements.
• immutable list – A list is unmodificable. A list created by List.of is immutable.
• mutable list – A list created by new ArrayList, new LinkedList, or new Vector is mutable.

ListTestData

package org.jcg.zheng.demo;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class ListTestData {

	public static List<String> dummyMutableList() {
		List<String> stringList = new ArrayList<>();
		stringList.add(null);
		stringList.add("");
		stringList.add(" ");
		stringList.add("Mary");
		return stringList;
	}

	public static List<String> dummyFixedSizeList() {
		return Arrays.asList(null, "", "", "Mary");
	}

	public static List<String> dummyImmutableList() {
		return List.of("Mary");
	}

	public static void demoFind(FindNullorEmptyService serivce, List<String> inputList) {
		List<String> emptyItemList = serivce.findNullOrEmptyItems(inputList);
		System.out.println("Null or Empty count: " + emptyItemList.size());

	}

	public static void main(String[] args) {
		System.out.println("can edit the element of fixed_size list");
		List<String> dummyFixedSizeList = dummyFixedSizeList();
		dummyFixedSizeList.set(0, "z");

		List<String> immutableList = dummyImmutableList();
		System.out.println("cannot edit the element of immutable list");
		immutableList.set(0, null);
	}
}

• Line 10: The ArrayList is a resizable-array implementation of the List interface.
• Line 19: The Arrays.asList() returns a fixed-size list and allows null value in the list. The list elements can be replaced, but the add, remove operations are not allowed.
• Line 23: The List.of static factory methods provide a convenient way to create unmodifiable lists and not allow null value in the list.
• Line 27: invoke the findNullOrEmptyItems method to find the null or empty items.
• Line 39: an immutable list throws exception on the set method.

Run the ListTestData as a Java application and capture the output here.

ListTestData Output

can edit the element of fixed_size list
cannot edit the element of immutable list
Exception in thread "main" java.lang.UnsupportedOperationException
	at java.base/java.util.ImmutableCollections.uoe(ImmutableCollections.java:142)
	at java.base/java.util.ImmutableCollections$AbstractImmutableList.set(ImmutableCollections.java:262)
	at org.jcg.zheng.demo.ListTestData.main(ListTestData.java:39)

• Line 3, 6: confirmed that the UnsupportedOperationException is thrown on the set method for an immutable list.

4. Find Null or Empty From List Interface

In this step, I will create a FindNullorEmptyService.java interface that finds the null or empty from a list of String objects.

FindNullorEmptyService.java

package org.jcg.zheng.demo;

import java.util.List;

public interface FindNullorEmptyService {
	
	public List<String> findNullOrEmptyItems(List<String> stringList);

}

5. Find Null or Empty from List via For Loop

In this step, I will create a ForLoopAddImpl.java that implements the FindNullorEmptyService interface. Note: it loops through the elements and adds null or empty elements to a mutable list if the element is null or isEmpty.

ForLoopAddImpl.java

package org.jcg.zheng.demo;

import java.util.ArrayList;
import java.util.List;

public class ForLoopAddImpl implements FindNullorEmptyService {

	@Override
	public List<String> findNullOrEmptyItems(final List<String> stringList) {
		List<String> nullOrEmptyList = new ArrayList<>();
		for (String str : stringList) {
			if (str == null || str.isEmpty()) {
				nullOrEmptyList.add(str);
			}
		}
		return nullOrEmptyList;
	}

	public static void main(String[] args) {

		FindNullorEmptyService service = new ForLoopAddImpl();

		ListTestData.demoFind(service, ListTestData.dummyMutableList());
		ListTestData.demoFind(service, ListTestData.dummyFixedSizeList());
		ListTestData.demoFind(service, ListTestData.dummyImmutableList());
	}

}

• Line 10: the returned list is mutable as it’s created via new ArrayList();
• Line 11-13: adds to a mutable list if the element is null or isEmpty inside the For loop. Note: if the blank space (” “) is considered as empty, then change the isEmpty to isBlank.
• Line 23-25: verify the returned list contains null or empty.

Run ForLoopAddImpl.java and capture output.

ForLoopAddImpl Output

Null or Empty count: 2
Null or Empty count: 3
Null or Empty count: 0

6. Remove Not Null nor Not Empty via RemoveIf

In this step, I will create a RemoveIfImpl.java that implements the FindNullorEmptyService interface. Note: it removes any element in the list that is not null and not empty. It requires a mutable list argument.

RemoveIfImpl.java

package org.jcg.zheng.demo;

import java.util.List;

public class RemoveIfImpl implements FindNullorEmptyService {

	@Override
	public List<String> findNullOrEmptyItems(final List<String> stringList) {
		stringList.removeIf(str -> str != null && str.isEmpty());
		return stringList;
	}

	public static void main(String[] args) {
		FindNullorEmptyService service = new RemoveIfImpl();

		ListTestData.demoFind(service, ListTestData.dummyMutableList());	

		System.out.println("the remove method throws UnsupportedOperationException as List.of is Immutable ");
		service.findNullOrEmptyItems(ListTestData.dummyImmutableList());
		System.out.println("the remove method throws UnsupportedOperationException as Arrays.asList is Immutable"
				+ service.findNullOrEmptyItems(ListTestData.dummyFixedSizeList()));
	}

}

• Line 9: the argument list must be mutable.
• Line 16: it works for a mutable list with the removeIf method.
• Line 18, 20: UnsupportedOperationException is thrown for fixed-size or immutable list on the removeIf method.

Run RemoveIfImpl.java and capture output

RemoveIfImpl Output

Null or Empty count: 3
the remove method throws UnsupportedOperationException as List.of is Immutable 
Exception in thread "main" java.lang.UnsupportedOperationException
	at java.base/java.util.ImmutableCollections.uoe(ImmutableCollections.java:142)
	at java.base/java.util.ImmutableCollections$AbstractImmutableCollection.removeIf(ImmutableCollections.java:152)
	at org.jcg.zheng.demo.RemoveIfImpl.findNullOrEmptyItems(RemoveIfImpl.java:9)
	at org.jcg.zheng.demo.RemoveIfImpl.main(RemoveIfImpl.java:19)

• Line 3,6: confirmed that the UnsupportedOperationException is thrown on the set method for an immutable list.

7. Find Null or Empty From List via Stream API

7.1 Collect as a Mutable List

In this step, I will create a StreamMutableImpl.java that implements the FindNullorEmptyService interface. The returned list is a mutable list.

StreamMutableImpl.java

package org.jcg.zheng.demo;

import java.util.List;
import java.util.stream.Collectors;

public class StreamMutableImpl implements FindNullorEmptyService {

	@Override
	public List<String> findNullOrEmptyItems(List<String> stringList) {
		return stringList.stream().filter(s -> s == null || s.isEmpty()).collect(Collectors.toList());
	}

	public static void main(String[] args) {
		FindNullorEmptyService service = new StreamMutableImpl();

		ListTestData.demoFind(service, ListTestData.dummyMutableList());
		ListTestData.demoFind(service, ListTestData.dummyFixedSizeList());
		ListTestData.demoFind(service, ListTestData.dummyImmutableList());
	}

}

• Line 10: the Collectors.toList() returns a mutable list.

Run the StreamMutableImpl.java and capture the output:

StreamMutableImpl Output

Null or Empty count: 2
Null or Empty count: 3
Null or Empty count: 0

7.2 Collect as an Immutable List

In this step, I will create a StreamImmutableImpl.java that implements the FindNullorEmptyServic interface. The returned list is an immutable list.

StreamImmutableImpl.java

package org.jcg.zheng.demo;

import java.util.List;

public class StreamImmutableImpl implements FindNullorEmptyService {

	@Override
	public List<String> findNullOrEmptyItems(List<String> stringList) {
		return stringList.stream().filter(s -> s == null || s.isEmpty()).toList();
	}

	public static void main(String[] args) {

		FindNullorEmptyService service = new StreamImmutableImpl();

		ListTestData.demoFind(service, ListTestData.dummyMutableList());
		ListTestData.demoFind(service, ListTestData.dummyFixedSizeList());
		ListTestData.demoFind(service, ListTestData.dummyImmutableList());

		System.out.println("the add method throws UnsupportedOperationException as toList is Imutable");
		service.findNullOrEmptyItems(ListTestData.dummyFixedSizeList()).add("test");
	}
}

• Line 9: the Stream.toList() returns an immutable list.

Run the StreamImmutableImpl.java and capture the output:

StreamImmutableImpl Output

Null or Empty count: 2
Null or Empty count: 3
Null or Empty count: 0
the add method throws UnsupportedOperationException as toList is Imutable
Exception in thread "main" java.lang.UnsupportedOperationException
	at java.base/java.util.ImmutableCollections.uoe(ImmutableCollections.java:142)
	at java.base/java.util.ImmutableCollections$AbstractImmutableCollection.add(ImmutableCollections.java:147)
	at org.jcg.zheng.demo.StreamImmutableImpl.main(StreamImmutableImpl.java:21)

8. Conclusion

In this example, I created a simple Java program to find null or empty(“”) elements from a list via For Loop, removeIf, and Stream API. The For loop is the simplest and most explicit way, great for readability and beginners. The removeIf works only for a mutable list. The Stream API provides a concise and functional style, especially useful when you want to filter, map, or collect results in a single chain.

9. Download

This was an example of a Java project which found null or empty elements from a list.

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