fix: Fix Sniper printer failing to put added import statement on separate line

src/test/java/spoon/test/prettyprinter/TestSniperPrinter.java

@@ -12,7 +12,6 @@
 import org.junit.rules.TemporaryFolder;
 import spoon.Launcher;
 import spoon.SpoonException;
-import spoon.processing.Processor;
 import spoon.refactoring.Refactoring;
 import spoon.reflect.CtModel;
 import spoon.reflect.code.CtConstructorCall;
@@ -23,6 +22,7 @@
 import spoon.reflect.code.CtStatement;
 import spoon.reflect.code.CtThrow;
 import spoon.reflect.declaration.CtClass;
+import spoon.reflect.declaration.CtCompilationUnit;
 import spoon.reflect.declaration.CtElement;
 import spoon.reflect.declaration.CtField;
 import spoon.reflect.declaration.CtMethod;
@@ -33,8 +33,6 @@
 import spoon.reflect.reference.CtExecutableReference;
 import spoon.reflect.reference.CtReference;
 import spoon.reflect.reference.CtTypeReference;
-import spoon.reflect.visitor.ImportCleaner;
-import spoon.reflect.visitor.ImportConflictDetector;
 import spoon.reflect.visitor.filter.TypeFilter;
 import spoon.support.modelobs.ChangeCollector;
 import spoon.support.modelobs.SourceFragmentCreator;
@@ -59,6 +57,9 @@
 import java.util.function.BiConsumer;
 import java.util.function.Consumer;
 
+import static org.hamcrest.CoreMatchers.containsString;
+import static org.hamcrest.CoreMatchers.anyOf;
+import static org.hamcrest.MatcherAssert.assertThat;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;
@@ -369,6 +370,25 @@ public void testPrintTypesThrowsWhenPassedTypesFromMultipleCompilationUnits() {
 		}
 	}
 
+	@Test
+	public void testAddedImportStatementPlacedOnSeparateLine() {
+		// contract: newline must be inserted between import statements when a new one is added
+
+		Consumer<CtType<?>> addArrayListImport = type -> {
+			Factory factory = type.getFactory();
+			CtCompilationUnit cu = factory.CompilationUnit().getOrCreate(type);
+			CtTypeReference<?> arrayListRef = factory.Type().get(java.util.ArrayList.class).getReference();
+			cu.getImports().add(factory.createImport(arrayListRef));
+		};
+		BiConsumer<CtType<?>, String> assertImportsPrintedCorrectly = (type, result) -> {
+			assertThat(result, anyOf(
+					containsString("import java.util.Set;\nimport java.util.ArrayList;\n"),
+					containsString("import java.util.ArrayList;\nimport java.util.Set;\n")));
+		};
+
+		testSniper("ClassWithSingleImport", addArrayListImport, assertImportsPrintedCorrectly);
+	}
+
 	/**
 	 * 1) Runs spoon using sniper mode,
 	 * 2) runs `typeChanger` to modify the code,
@@ -398,16 +418,7 @@ private void testSniper(String testClass, Consumer<CtType<?>> transformation, Bi
 	private static Launcher createLauncherWithSniperPrinter() {
 		Launcher launcher = new Launcher();
 		launcher.getEnvironment().setPrettyPrinterCreator(() -> {
-			SniperJavaPrettyPrinter printer = new SniperJavaPrettyPrinter(launcher.getEnvironment());
-			printer.setPreprocessors(Collections.unmodifiableList(Arrays.<Processor<CtElement>>asList(
-					//remove unused imports first. Do not add new imports at time when conflicts are not resolved
-					new ImportCleaner().setCanAddImports(false),
-					//solve conflicts, the current imports are relevant too
-					new ImportConflictDetector(),
-					//compute final imports
-					new ImportCleaner()
-			)));
-			return printer;
+			return new SniperJavaPrettyPrinter(launcher.getEnvironment());
 		});
 		return launcher;
 	}

src/test/resources/ClassWithSingleImport.java

@@ -0,0 +1,4 @@
+import java.util.Set;
+
+public class ClassWithSingleImport {
+}