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Exceptiion while executing build 
FAILURE: Build failed with an exception.
* What went wrong:
Execution failed for task ':app:transformClassesWithDexBuilderForDebug'.
> com.android.build.api.transform.TransformException: java.lang.IllegalStateException: Dex archives: setting .DEX extension only for .CLASS files

Sloution :
You need to look at the build.gradle. There may be something wrong with your configuration.
You may have misspelt something or may not have configured the plugins correctly.

My issues were with the annotationProcessor  not being configured properly.

private static String removeBetween(String string, String startChars, String endChars) {
		if(string.contains(startChars)&& string.contains(endChars)) {
			String substringAfter = StringUtils.substringAfterLast(string, startChars);
			String substringBefore = StringUtils.substringBefore(substringAfter, endChars);
			String finalStr = StringUtils.remove(string ,startChars + substringBefore+ endChars);
			string =removeBetween(finalStr, startChars, endChars);
		}
		return string;
	}

// Exception
Exception in thread "main" java.lang.NoClassDefFoundError: org/bouncycastle/jce/provider/BouncyCastleProvider
	at org.apache.pdfbox.pdmodel.PDDocument.protect(PDDocument.java:1318)
---
Caused by: java.lang.ClassNotFoundException: org.bouncycastle.jce.provider.BouncyCastleProvider
	at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
	... 2 more

// Solution
// add dependency in your pom.xml
<dependency>
    <groupId>org.bouncycastle</groupId>
    <artifactId>bcprov-jdk15on</artifactId>
    <version>1.58</version>
</dependency>

public static void main(String[] args) {

		List<String> listwithnulls = new ArrayList<String>();
		listwithnulls.add("apple");
		listwithnulls.add(null);
		listwithnulls.add("mango");
		listwithnulls.add(null);
		listwithnulls.add("banana");
		System.out.println("Let's now Filter Null Elements - ");
		listwithnulls.stream().filter(value -> value != null).forEach(System.out::println);

	}