fix: don't remove empty default constructor if other constructors exists (#460)

jadx-core/src/main/java/jadx/core/dex/visitors/ClassModifier.java

@@ -295,15 +295,34 @@ public class ClassModifier extends AbstractVisitor {
 
 	private static void removeEmptyMethods(MethodNode mth) {
 		AccessInfo af = mth.getAccessFlags();
-		// remove public empty constructors
+		// remove public empty constructors (static or default)
 		if (af.isConstructor()
 				&& (af.isPublic() || af.isStatic())
-				&& mth.getArguments(false).isEmpty()
-				&& !mth.contains(AType.JADX_ERROR)) {
+				&& mth.getArguments(false).isEmpty()) {
 			List<BlockNode> bb = mth.getBasicBlocks();
 			if (bb == null || bb.isEmpty() || BlockUtils.isAllBlocksEmpty(bb)) {
+				if (af.isStatic() && mth.getMethodInfo().isClassInit()) {
 					mth.add(AFlag.DONT_GENERATE);
+				} else {
+					// don't remove default constructor if other constructors exists
+					if (mth.isDefaultConstructor() && !isNonDefaultConstructorExists(mth)) {
+						mth.add(AFlag.DONT_GENERATE);
+					}
+				}
+			}
 		}
 	}
+
+	private static boolean isNonDefaultConstructorExists(MethodNode defCtor) {
+		ClassNode parentClass = defCtor.getParentClass();
+		for (MethodNode mth : parentClass.getMethods()) {
+			if (mth != defCtor
+					&& mth.getAccessFlags().isConstructor()
+					&& mth.getMethodInfo().isConstructor()
+					&& !mth.isDefaultConstructor()) {
+				return true;
+			}
+		}
+		return false;
 	}
 }

jadx-core/src/test/java/jadx/tests/integration/others/TestDefConstructorNotRemoved.java

+package jadx.tests.integration.others;
+
+import org.junit.Test;
+
+import jadx.core.dex.nodes.ClassNode;
+import jadx.tests.api.IntegrationTest;
+
+import static jadx.tests.api.utils.JadxMatchers.containsOne;
+import static org.hamcrest.Matchers.containsString;
+import static org.hamcrest.Matchers.not;
+import static org.junit.Assert.assertThat;
+
+public class TestDefConstructorNotRemoved extends IntegrationTest {
+
+	public static class TestCls {
+
+		static {
+			// empty
+		}
+
+		public static class A {
+			private final String s;
+
+			public A() {
+				s = "a";
+			}
+
+			public A(String str) {
+				s = str;
+			}
+		}
+
+		public static class B extends A {
+			public B() {
+				super();
+			}
+
+			public B(String s) {
+				super(s);
+			}
+		}
+
+		public void check() {
+			new A();
+			new A("a");
+			new B();
+			new B("b");
+		}
+	}
+
+	@Test
+	public void test() {
+		ClassNode cls = getClassNode(TestCls.class);
+		String code = cls.getCode().toString();
+
+		assertThat(code, not(containsString("super();")));
+		assertThat(code, not(containsString("static {")));
+		assertThat(code, containsOne("public B() {"));
+	}
+}