According to Official Documentation, always close the realm instance when you done with them.

Best Practise Close Instance

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try {
 Realm realm = Realm.getDefaultInstance();
 //Use the realm instance
} catch(Exception e) {
  //handle exceptions
} finally {
    realm.close();
}

if you useminSdkVersion >= 19andJava >= 7, you won’t close it manually.

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try (Realm realm = Realm.getDefaultInstance()) {
// No need to close the Realm instance manually
}

Getting the above answer from the this stackoverflow answer

Kotlin

The try (Realm realm = Realm.getDefaultInstance()) in Java is called try-with-resource. This is how we do in Kotlin:

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Realm.getDefaultInstance().use {
  // no need to close realm with .use keyword
	it.where(...).equalTo(...).findAll()
}

Note: You have to use minSdkVersion >= 19andJava >= 7 in order to not close it manually.