refactor: throw informative exception when sniper printing cannot be done

src/main/java/spoon/support/sniper/SniperJavaPrettyPrinter.java

@@ -9,9 +9,13 @@
 
 import java.util.ArrayDeque;
 import java.util.Arrays;
+import java.util.Collection;
 import java.util.Collections;
 import java.util.Deque;
+import java.util.IdentityHashMap;
 import java.util.List;
+import java.util.Set;
+import java.util.stream.Collectors;
 
 import org.apache.commons.lang3.tuple.Pair;
 import spoon.OutputType;
@@ -130,6 +134,8 @@ private static CtCompilationUnit getUnambiguousCompilationUnit(CtType<?>[] type)
 
 	@Override
 	public void calculate(CtCompilationUnit compilationUnit, List<CtType<?>> types) {
+		checkGivenTypesMatchDeclaredTypes(compilationUnit, types);
+
 		sourceCompilationUnit = compilationUnit;
 
 		//use line separator of origin source file
@@ -149,6 +155,27 @@ public void calculate(CtCompilationUnit compilationUnit, List<CtType<?>> types)
 		});
 	}
 
+	/** Throws an {@link IllegalArgumentException} if the given types do not exactly match the types of the CU. */
+	private static void checkGivenTypesMatchDeclaredTypes(CtCompilationUnit cu, List<CtType<?>> types) {
+		Set<CtType<?>> givenTypes = toIdentityHashSet(types);
+		Set<CtType<?>> declaredTypes = toIdentityHashSet(cu.getDeclaredTypes());
+		if (!givenTypes.equals(declaredTypes)) {
+			throw new IllegalArgumentException(
+					"Can only sniper print exactly all declared types of the compilation unit. Given types: "
+							+ toNameList(givenTypes) + ". Declared types: " + toNameList(declaredTypes));
+		}
+	}
+
+	private static List<String> toNameList(Collection<CtType<?>> types) {
+		return types.stream().map(CtType::getQualifiedName).collect(Collectors.toList());
+	}
+
+	private static <T> Set<T> toIdentityHashSet(Collection<T> items) {
+		Set<T> idHashSet = Collections.newSetFromMap(new IdentityHashMap<>());
+		idHashSet.addAll(items);
+		return idHashSet;
+	}
+
 	private static final String CR = "\r";
 	private static final String CRLF = "\r\n";
 	private static final String LF = "\n";

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

@@ -28,6 +28,7 @@
 import spoon.reflect.declaration.CtField;
 import spoon.reflect.declaration.CtImport;
 import spoon.reflect.declaration.CtMethod;
+import spoon.reflect.declaration.CtModifiable;
 import spoon.reflect.declaration.CtPackage;
 import spoon.reflect.declaration.CtType;
 import spoon.reflect.declaration.ModifierKind;
@@ -67,6 +68,7 @@
 import static org.hamcrest.MatcherAssert.assertThat;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertThrows;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;
 import static spoon.test.SpoonTestHelpers.assumeNotWindows;
@@ -630,6 +632,23 @@ public void testPrintTypeWithMethodImportBelowMethodDefinition() {
 		assertThat(output, containsString("import static methodimport.ClassWithStaticMethod.staticMethod;"));
 	}
 
+	@Test
+	public void testThrowsWhenTryingToPrintSubsetOfCompilationUnitTypes() {
+		// contract: Printing a subset of a compilation unit's types is a hassle to implement at the time of writing
+		// this, as a) DJPP will replace the compilation unit with a clone, and b) it makes it more difficult to
+		// match source code fragments. For now, we're lazy and simply don't allow it.
+
+		Launcher launcher = createLauncherWithSniperPrinter();
+		launcher.addInputResource(getResourcePath("MultipleTopLevelTypes"));
+
+		CtModel model = launcher.buildModel();
+		CtType<?> primaryType = model.getAllTypes().stream().filter(CtModifiable::isPublic).findFirst().get();
+		CtCompilationUnit cu = primaryType.getFactory().CompilationUnit().getOrCreate(primaryType);
+		SniperJavaPrettyPrinter sniper = (SniperJavaPrettyPrinter) launcher.getEnvironment().createPrettyPrinter();
+
+		assertThrows(IllegalArgumentException.class, () -> sniper.calculate(cu, Collections.singletonList(primaryType)));
+	}
+
 	/**
 	 * 1) Runs spoon using sniper mode,
 	 * 2) runs `typeChanger` to modify the code,

src/test/resources/MultipleTopLevelTypes.java

@@ -0,0 +1,12 @@
+public class MultipleTopLevelTypes {
+    private int x;
+}
+
+class OtherTopLevelType {
+    private long z;
+}
+
+interface TopLevelInterface {
+    int getValue();
+}
+