diff --git a/app/src/main/java/com/termux/app/terminal/TermuxTerminalSessionClient.java b/app/src/main/java/com/termux/app/terminal/TermuxTerminalSessionClient.java
index 93689de2..244c6278 100644
--- a/app/src/main/java/com/termux/app/terminal/TermuxTerminalSessionClient.java
+++ b/app/src/main/java/com/termux/app/terminal/TermuxTerminalSessionClient.java
@@ -12,6 +12,9 @@ import android.media.SoundPool;
 import android.text.TextUtils;
 import android.widget.ListView;
 
+import androidx.annotation.NonNull;
+import androidx.annotation.Nullable;
+
 import com.termux.R;
 import com.termux.shared.termux.shell.command.runner.terminal.TermuxSession;
 import com.termux.shared.termux.interact.TextInputDialogUtils;
@@ -108,14 +111,14 @@ public class TermuxTerminalSessionClient extends TermuxTerminalSessionClientBase
 
 
     @Override
-    public void onTextChanged(TerminalSession changedSession) {
+    public void onTextChanged(@NonNull TerminalSession changedSession) {
         if (!mActivity.isVisible()) return;
 
         if (mActivity.getCurrentSession() == changedSession) mActivity.getTerminalView().onScreenUpdated();
     }
 
     @Override
-    public void onTitleChanged(TerminalSession updatedSession) {
+    public void onTitleChanged(@NonNull TerminalSession updatedSession) {
         if (!mActivity.isVisible()) return;
 
         if (updatedSession != mActivity.getCurrentSession()) {
@@ -129,7 +132,7 @@ public class TermuxTerminalSessionClient extends TermuxTerminalSessionClientBase
     }
 
     @Override
-    public void onSessionFinished(final TerminalSession finishedSession) {
+    public void onSessionFinished(@NonNull TerminalSession finishedSession) {
         TermuxService service = mActivity.getTermuxService();
 
         if (service == null || service.wantsToStop()) {
@@ -174,7 +177,7 @@ public class TermuxTerminalSessionClient extends TermuxTerminalSessionClientBase
     }
 
     @Override
-    public void onCopyTextToClipboard(TerminalSession session, String text) {
+    public void onCopyTextToClipboard(@NonNull TerminalSession session, String text) {
         if (!mActivity.isVisible()) return;
 
         ClipboardManager clipboard = (ClipboardManager) mActivity.getSystemService(Context.CLIPBOARD_SERVICE);
@@ -182,7 +185,7 @@ public class TermuxTerminalSessionClient extends TermuxTerminalSessionClientBase
     }
 
     @Override
-    public void onPasteTextFromClipboard(TerminalSession session) {
+    public void onPasteTextFromClipboard(@Nullable TerminalSession session) {
         if (!mActivity.isVisible()) return;
 
         ClipboardManager clipboard = (ClipboardManager) mActivity.getSystemService(Context.CLIPBOARD_SERVICE);
@@ -194,7 +197,7 @@ public class TermuxTerminalSessionClient extends TermuxTerminalSessionClientBase
     }
 
     @Override
-    public void onBell(TerminalSession session) {
+    public void onBell(@NonNull TerminalSession session) {
         if (!mActivity.isVisible()) return;
 
         switch (mActivity.getProperties().getBellBehaviour()) {
@@ -213,7 +216,7 @@ public class TermuxTerminalSessionClient extends TermuxTerminalSessionClientBase
     }
 
     @Override
-    public void onColorsChanged(TerminalSession changedSession) {
+    public void onColorsChanged(@NonNull TerminalSession changedSession) {
         if (mActivity.getCurrentSession() == changedSession)
             updateBackgroundColor();
     }
diff --git a/terminal-emulator/build.gradle b/terminal-emulator/build.gradle
index 71a93348..f3d567b3 100644
--- a/terminal-emulator/build.gradle
+++ b/terminal-emulator/build.gradle
@@ -50,6 +50,7 @@ tasks.withType(Test) {
 }
 
 dependencies {
+    implementation 'androidx.annotation:annotation:1.3.0'
     testImplementation 'junit:junit:4.13.2'
 }
 
diff --git a/terminal-emulator/src/main/java/com/termux/terminal/TerminalSessionClient.java b/terminal-emulator/src/main/java/com/termux/terminal/TerminalSessionClient.java
index 0ffe8e43..796c5b73 100644
--- a/terminal-emulator/src/main/java/com/termux/terminal/TerminalSessionClient.java
+++ b/terminal-emulator/src/main/java/com/termux/terminal/TerminalSessionClient.java
@@ -1,5 +1,8 @@
 package com.termux.terminal;
 
+import androidx.annotation.NonNull;
+import androidx.annotation.Nullable;
+
 /**
  * The interface for communication between {@link TerminalSession} and its client. It is used to
  * send callbacks to the client when {@link TerminalSession} changes or for sending other
@@ -7,19 +10,19 @@ package com.termux.terminal;
  */
 public interface TerminalSessionClient {
 
-    void onTextChanged(TerminalSession changedSession);
+    void onTextChanged(@NonNull TerminalSession changedSession);
 
-    void onTitleChanged(TerminalSession changedSession);
+    void onTitleChanged(@NonNull TerminalSession changedSession);
 
-    void onSessionFinished(TerminalSession finishedSession);
+    void onSessionFinished(@NonNull TerminalSession finishedSession);
 
-    void onCopyTextToClipboard(TerminalSession session, String text);
+    void onCopyTextToClipboard(@NonNull TerminalSession session, String text);
 
-    void onPasteTextFromClipboard(TerminalSession session);
+    void onPasteTextFromClipboard(@Nullable TerminalSession session);
 
-    void onBell(TerminalSession session);
+    void onBell(@NonNull TerminalSession session);
 
-    void onColorsChanged(TerminalSession session);
+    void onColorsChanged(@NonNull TerminalSession session);
 
     void onTerminalCursorStateChange(boolean state);
 
diff --git a/termux-shared/src/main/java/com/termux/shared/errors/Errno.java b/termux-shared/src/main/java/com/termux/shared/errors/Errno.java
index ae860915..1ac9fd08 100644
--- a/termux-shared/src/main/java/com/termux/shared/errors/Errno.java
+++ b/termux-shared/src/main/java/com/termux/shared/errors/Errno.java
@@ -25,7 +25,7 @@ public class Errno {
     public static final Errno ERRNO_FAILED = new Errno(TYPE, Activity.RESULT_FIRST_USER + 1, "Failed");
 
     /** The errno type. */
-    protected String type;
+    protected final String type;
     /** The errno code. */
     protected final int code;
     /** The errno message. */
@@ -34,7 +34,7 @@ public class Errno {
     private static final String LOG_TAG = "Errno";
 
 
-    public Errno(final String type, final int code, final String message) {
+    public Errno(@NonNull final String type, final int code, @NonNull final String message) {
         this.type = type;
         this.code = code;
         this.message = message;
@@ -47,19 +47,22 @@ public class Errno {
         return "type=" + type + ", code=" + code + ", message=\"" + message + "\"";
     }
 
-
+    @NonNull
     public String getType() {
         return type;
     }
 
-    public String getMessage() {
-        return message;
-    }
-
     public int getCode() {
         return code;
     }
 
+    @NonNull
+    public String getMessage() {
+        return message;
+    }
+
+
+
     /**
      * Get the {@link Errno} of a specific type and code.
      *
diff --git a/termux-shared/src/main/java/com/termux/shared/file/FileUtils.java b/termux-shared/src/main/java/com/termux/shared/file/FileUtils.java
index 52ffcc6c..0ca949ae 100644
--- a/termux-shared/src/main/java/com/termux/shared/file/FileUtils.java
+++ b/termux-shared/src/main/java/com/termux/shared/file/FileUtils.java
@@ -4,6 +4,7 @@ import android.os.Build;
 import android.system.Os;
 
 import androidx.annotation.NonNull;
+import androidx.annotation.Nullable;
 
 import com.google.common.io.RecursiveDeleteOption;
 import com.termux.shared.file.filesystem.FileType;
@@ -97,6 +98,7 @@ public class FileUtils {
      * @param path The {@code path} to convert.
      * @return Returns the {@code normalized path}.
      */
+    @Nullable
     public static String normalizePath(String path) {
         if (path == null) return null;
 
@@ -247,7 +249,7 @@ public class FileUtils {
     }
 
     /**
-     * Checks the type of file that exists at {@code filePath}.
+     * Get the type of file that exists at {@code filePath}.
      *
      * This function is a wrapper for
      * {@link FileTypes#getFileType(String, boolean)}
@@ -260,6 +262,7 @@ public class FileUtils {
      *                       returned.
      * @return Returns the {@link FileType} of file.
      */
+    @NonNull
     public static FileType getFileType(final String filePath, final boolean followLinks) {
         return FileTypes.getFileType(filePath, followLinks);
     }
@@ -1446,8 +1449,8 @@ public class FileUtils {
     }
 
     public static class ReadSerializableObjectResult {
-        public Error error;
-        public Serializable serializableObject;
+        public final Error error;
+        public final Serializable serializableObject;
 
         ReadSerializableObjectResult(Error error, Serializable serializableObject) {
             this.error = error;
diff --git a/termux-shared/src/main/java/com/termux/shared/file/FileUtilsErrno.java b/termux-shared/src/main/java/com/termux/shared/file/FileUtilsErrno.java
index b39870b8..b7da6637 100644
--- a/termux-shared/src/main/java/com/termux/shared/file/FileUtilsErrno.java
+++ b/termux-shared/src/main/java/com/termux/shared/file/FileUtilsErrno.java
@@ -94,7 +94,7 @@ public class FileUtilsErrno extends Errno {
 
 
     /** Defines the {@link Errno} mapping to get a shorter version of {@link FileUtilsErrno}. */
-    public static Map<Errno, Errno> ERRNO_SHORT_MAPPING = new HashMap<Errno, Errno>() {{
+    public static final Map<Errno, Errno> ERRNO_SHORT_MAPPING = new HashMap<Errno, Errno>() {{
         put(ERRNO_FILE_NOT_FOUND_AT_PATH, ERRNO_FILE_NOT_FOUND_AT_PATH_SHORT);
 
         put(ERRNO_NON_REGULAR_FILE_FOUND, ERRNO_NON_REGULAR_FILE_FOUND_SHORT);
diff --git a/termux-shared/src/main/java/com/termux/shared/file/filesystem/FileTypes.java b/termux-shared/src/main/java/com/termux/shared/file/filesystem/FileTypes.java
index cf599cd2..c739392d 100644
--- a/termux-shared/src/main/java/com/termux/shared/file/filesystem/FileTypes.java
+++ b/termux-shared/src/main/java/com/termux/shared/file/filesystem/FileTypes.java
@@ -82,6 +82,7 @@ public class FileTypes {
      *                       returned.
      * @return Returns the {@link FileType} of file.
      */
+    @NonNull
     public static FileType getFileType(final String filePath, final boolean followLinks) {
         if (filePath == null || filePath.isEmpty()) return FileType.NO_EXIST;
 
diff --git a/termux-shared/src/main/java/com/termux/shared/termux/terminal/TermuxTerminalSessionClientBase.java b/termux-shared/src/main/java/com/termux/shared/termux/terminal/TermuxTerminalSessionClientBase.java
index 6764d5a9..f79cf0fc 100644
--- a/termux-shared/src/main/java/com/termux/shared/termux/terminal/TermuxTerminalSessionClientBase.java
+++ b/termux-shared/src/main/java/com/termux/shared/termux/terminal/TermuxTerminalSessionClientBase.java
@@ -1,5 +1,8 @@
 package com.termux.shared.termux.terminal;
 
+import androidx.annotation.NonNull;
+import androidx.annotation.Nullable;
+
 import com.termux.shared.logger.Logger;
 import com.termux.terminal.TerminalSession;
 import com.termux.terminal.TerminalSessionClient;
@@ -10,31 +13,31 @@ public class TermuxTerminalSessionClientBase implements TerminalSessionClient {
     }
 
     @Override
-    public void onTextChanged(TerminalSession changedSession) {
+    public void onTextChanged(@NonNull TerminalSession changedSession) {
     }
 
     @Override
-    public void onTitleChanged(TerminalSession updatedSession) {
+    public void onTitleChanged(@NonNull TerminalSession updatedSession) {
     }
 
     @Override
-    public void onSessionFinished(final TerminalSession finishedSession) {
+    public void onSessionFinished(@NonNull TerminalSession finishedSession) {
     }
 
     @Override
-    public void onCopyTextToClipboard(TerminalSession session, String text) {
+    public void onCopyTextToClipboard(@NonNull TerminalSession session, String text) {
     }
 
     @Override
-    public void onPasteTextFromClipboard(TerminalSession session) {
+    public void onPasteTextFromClipboard(@Nullable TerminalSession session) {
     }
 
     @Override
-    public void onBell(TerminalSession session) {
+    public void onBell(@NonNull TerminalSession session) {
     }
 
     @Override
-    public void onColorsChanged(TerminalSession changedSession) {
+    public void onColorsChanged(@NonNull TerminalSession changedSession) {
     }
 
     @Override