diff --git a/src/configurationProvider.ts b/src/configurationProvider.ts
index 69b9f0f1..9505bfb4 100644
--- a/src/configurationProvider.ts
+++ b/src/configurationProvider.ts
@@ -73,7 +73,7 @@ export class JavaDebugConfigurationProvider implements vscode.DebugConfiguration
             try {
                 // See https://github.com/microsoft/vscode-java-debug/issues/778
                 // Merge the platform specific properties to the global config to simplify the subsequent resolving logic.
-                this.mergePlatformProperties(folder, config);
+                this.mergePlatformProperties(config, folder);
                 return this.resolveAndValidateDebugConfiguration(folder, config);
             } catch (ex) {
                 utility.showErrorMessage({
@@ -110,7 +110,7 @@ export class JavaDebugConfigurationProvider implements vscode.DebugConfiguration
                     const launchConfigs = mainClasses.map((item) => {
                         return {
                             ...defaultLaunchConfig,
-                            name: this.constructLaunchConfigName(item.mainClass, item.projectName, cache),
+                            name: this.constructLaunchConfigName(item.mainClass, cache, item.projectName),
                             mainClass: item.mainClass,
                             projectName: item.projectName,
                         };
@@ -127,7 +127,7 @@ export class JavaDebugConfigurationProvider implements vscode.DebugConfiguration
         });
     }
 
-    private mergePlatformProperties(_folder: vscode.WorkspaceFolder, config: vscode.DebugConfiguration) {
+    private mergePlatformProperties(config: vscode.DebugConfiguration, _folder?: vscode.WorkspaceFolder) {
         if (config && platformName && config[platformName]) {
             try {
                 for (const key of Object.keys(config[platformName])) {
@@ -140,7 +140,7 @@ export class JavaDebugConfigurationProvider implements vscode.DebugConfiguration
         }
     }
 
-    private constructLaunchConfigName(mainClass: string, projectName: string, cache: {[key: string]: any}) {
+    private constructLaunchConfigName(mainClass: string, cache: {[key: string]: any}, projectName?: string) {
         const prefix = "Debug (Launch)-";
         let name = prefix + mainClass.substr(mainClass.lastIndexOf(".") + 1);
         if (projectName !== undefined) {
@@ -333,7 +333,8 @@ export class JavaDebugConfigurationProvider implements vscode.DebugConfiguration
         return Object.keys(config).filter((key: string) => key !== "noDebug").length === 0;
     }
 
-    private async resolveLaunchConfig(folder: vscode.Uri | undefined, config: vscode.DebugConfiguration): Promise<lsPlugin.IMainClassOption> {
+    private async resolveLaunchConfig(folder: vscode.Uri | undefined,
+                                      config: vscode.DebugConfiguration): Promise<lsPlugin.IMainClassOption | undefined> {
         if (!config.mainClass || this.isFile(config.mainClass)) {
             const currentFile = config.mainClass ||  _.get(vscode.window.activeTextEditor, "document.uri.fsPath");
             if (currentFile) {
@@ -350,7 +351,7 @@ export class JavaDebugConfigurationProvider implements vscode.DebugConfiguration
         }
 
         const containsExternalClasspaths = !_.isEmpty(config.classPaths) || !_.isEmpty(config.modulePaths);
-        const validationResponse = await lsPlugin.validateLaunchConfig(folder, config.mainClass, config.projectName, containsExternalClasspaths);
+        const validationResponse = await lsPlugin.validateLaunchConfig(config.mainClass, config.projectName, containsExternalClasspaths, folder);
         if (!validationResponse.mainClass.isValid || !validationResponse.projectName.isValid) {
             return this.fixMainClass(folder, config, validationResponse);
         }
@@ -391,7 +392,7 @@ export class JavaDebugConfigurationProvider implements vscode.DebugConfiguration
                 const selectedFix = await mainClassPicker.showQuickPick(validationResponse.proposals,
                     "Please select main class<project name>.", false);
                 if (selectedFix) {
-                    sendInfo(null, {
+                    sendInfo("", {
                         fix: "yes",
                         fixMessage: errors.join(os.EOL),
                     });
diff --git a/src/debugCodeLensProvider.ts b/src/debugCodeLensProvider.ts
index 66f1e9bc..6821fe09 100644
--- a/src/debugCodeLensProvider.ts
+++ b/src/debugCodeLensProvider.ts
@@ -134,11 +134,11 @@ function debugJavaProgram(mainClass: string, projectName: string, uri: vscode.Ur
     return startDebugging(mainClass, projectName, uri, false);
 }
 
-async function constructDebugConfig(mainClass: string, projectName: string, workspace: vscode.Uri): Promise<vscode.DebugConfiguration> {
+async function constructDebugConfig(mainClass: string, projectName: string, workspace?: vscode.Uri): Promise<vscode.DebugConfiguration> {
     const launchConfigurations: vscode.WorkspaceConfiguration = vscode.workspace.getConfiguration("launch", workspace);
     const rawConfigs: vscode.DebugConfiguration[] = launchConfigurations.configurations;
 
-    let debugConfig: vscode.DebugConfiguration = _.find(rawConfigs, (config) => {
+    let debugConfig: vscode.DebugConfiguration | undefined = _.find(rawConfigs, (config) => {
         return config.mainClass === mainClass && _.toString(config.projectName) === _.toString(projectName);
     });
 
@@ -174,7 +174,7 @@ async function constructDebugConfig(mainClass: string, projectName: string, work
     return _.cloneDeep(debugConfig);
 }
 
-async function launchJsonExists(workspace: vscode.Uri): Promise<boolean> {
+async function launchJsonExists(workspace?: vscode.Uri): Promise<boolean> {
     if (!workspace) {
         return false;
     }
@@ -185,8 +185,8 @@ async function launchJsonExists(workspace: vscode.Uri): Promise<boolean> {
 }
 
 export async function startDebugging(mainClass: string, projectName: string, uri: vscode.Uri, noDebug: boolean): Promise<boolean> {
-    const workspaceFolder: vscode.WorkspaceFolder = vscode.workspace.getWorkspaceFolder(uri);
-    const workspaceUri: vscode.Uri = workspaceFolder ? workspaceFolder.uri : undefined;
+    const workspaceFolder: vscode.WorkspaceFolder | undefined = vscode.workspace.getWorkspaceFolder(uri);
+    const workspaceUri: vscode.Uri | undefined = workspaceFolder ? workspaceFolder.uri : undefined;
     const onClasspath = await isOnClasspath(uri.toString());
     if (workspaceUri && onClasspath === false && !(await addToClasspath(uri))) {
         return false;
diff --git a/src/extension.ts b/src/extension.ts
index de522f2f..eab2847e 100644
--- a/src/extension.ts
+++ b/src/extension.ts
@@ -165,7 +165,7 @@ function specifyProgramArguments(context: vscode.ExtensionContext): Thenable<str
 }
 
 async function applyHCR(hcrStatusBar: NotificationBar) {
-    const debugSession: vscode.DebugSession = vscode.debug.activeDebugSession;
+    const debugSession: vscode.DebugSession | undefined = vscode.debug.activeDebugSession;
     if (!debugSession) {
         return;
     }
@@ -246,7 +246,7 @@ async function runJavaFile(uri: vscode.Uri, noDebug: boolean) {
         throw ex;
     }
 
-    const activeEditor: vscode.TextEditor = vscode.window.activeTextEditor;
+    const activeEditor: vscode.TextEditor | undefined = vscode.window.activeTextEditor;
     if (!uri && activeEditor && _.endsWith(path.basename(activeEditor.document.fileName), ".java")) {
         uri = activeEditor.document.uri;
     }
@@ -279,7 +279,7 @@ async function runJavaFile(uri: vscode.Uri, noDebug: boolean) {
     } else {
         const launchMainChoice: string = "main() method";
         const launchTestChoice: string = "unit tests";
-        const choice: string = await vscode.window.showQuickPick(
+        const choice: string | undefined = await vscode.window.showQuickPick(
             [launchMainChoice, launchTestChoice],
             { placeHolder: "Please select which kind of task you would like to launch" },
         );
@@ -317,7 +317,7 @@ async function launchMain(mainMethods: IMainClassOption[], uri: vscode.Uri, noDe
         return;
     }
 
-    await startDebugging(pick.mainClass, pick.projectName, uri, noDebug);
+    await startDebugging(pick.mainClass, pick.projectName || "", uri, noDebug);
 }
 
 async function runJavaProject(node: any, noDebug: boolean) {
@@ -342,13 +342,13 @@ async function runJavaProject(node: any, noDebug: boolean) {
         return;
     }
 
-    const projectName: string = pick.projectName;
+    const projectName: string | undefined = pick.projectName;
     const mainClass: string = pick.mainClass;
-    const filePath: string = pick.filePath;
-    const workspaceFolder: vscode.WorkspaceFolder = filePath ? vscode.workspace.getWorkspaceFolder(vscode.Uri.file(filePath)) : undefined;
+    const filePath: string | undefined = pick.filePath;
+    const workspaceFolder: vscode.WorkspaceFolder | undefined = filePath ? vscode.workspace.getWorkspaceFolder(vscode.Uri.file(filePath)) : undefined;
     const launchConfigurations: vscode.WorkspaceConfiguration = vscode.workspace.getConfiguration("launch", workspaceFolder);
     const existingConfigs: vscode.DebugConfiguration[] = launchConfigurations.configurations;
-    const existConfig: vscode.DebugConfiguration = _.find(existingConfigs, (config) => {
+    const existConfig: vscode.DebugConfiguration | undefined = _.find(existingConfigs, (config) => {
         return config.mainClass === mainClass && _.toString(config.projectName) === _.toString(projectName);
     });
     const debugConfig = existConfig || {
diff --git a/src/javaDebugAdapterDescriptorFactory.ts b/src/javaDebugAdapterDescriptorFactory.ts
index b82e77b3..0fd731f8 100644
--- a/src/javaDebugAdapterDescriptorFactory.ts
+++ b/src/javaDebugAdapterDescriptorFactory.ts
@@ -8,8 +8,9 @@ import { Type } from "./logger";
 import { convertErrorToMessage, showErrorMessageWithTroubleshooting } from "./utility";
 
 export class JavaDebugAdapterDescriptorFactory implements DebugAdapterDescriptorFactory {
-    public async createDebugAdapterDescriptor(_session: DebugSession, _executable: DebugAdapterExecutable): Promise<DebugAdapterDescriptor> {
-        let error: Error;
+    public async createDebugAdapterDescriptor(_session: DebugSession,
+                                              _executable: DebugAdapterExecutable): Promise<DebugAdapterDescriptor | undefined> {
+        let error: Error| undefined;
         try {
             const debugServerPort = <number> (await startDebugSession());
             if (debugServerPort) {
diff --git a/src/languageServerPlugin.ts b/src/languageServerPlugin.ts
index 8bedfe5b..1c7cd59b 100644
--- a/src/languageServerPlugin.ts
+++ b/src/languageServerPlugin.ts
@@ -49,14 +49,14 @@ export function resolveClasspath(mainClass: string, projectName: string) {
     return commands.executeJavaLanguageServerCommand(commands.JAVA_RESOLVE_CLASSPATH, mainClass, projectName);
 }
 
-export function resolveMainClass(workspaceUri: vscode.Uri): Promise<IMainClassOption[]> {
+export function resolveMainClass(workspaceUri?: vscode.Uri): Promise<IMainClassOption[]> {
     if (workspaceUri) {
         return <Promise<IMainClassOption[]>>commands.executeJavaLanguageServerCommand(commands.JAVA_RESOLVE_MAINCLASS, workspaceUri.toString());
     }
     return <Promise<IMainClassOption[]>>commands.executeJavaLanguageServerCommand(commands.JAVA_RESOLVE_MAINCLASS);
 }
 
-export function validateLaunchConfig(workspaceUri: vscode.Uri, mainClass: string, projectName: string, containsExternalClasspaths: boolean):
+export function validateLaunchConfig(mainClass: string, projectName: string, containsExternalClasspaths: boolean, workspaceUri?: vscode.Uri):
     Promise<ILaunchValidationResponse> {
     return <Promise<ILaunchValidationResponse>>commands.executeJavaLanguageServerCommand(commands.JAVA_VALIDATE_LAUNCHCONFIG,
         workspaceUri ? workspaceUri.toString() : undefined, mainClass, projectName, containsExternalClasspaths);
diff --git a/src/logger.ts b/src/logger.ts
index c145da04..470a96a1 100644
--- a/src/logger.ts
+++ b/src/logger.ts
@@ -16,7 +16,7 @@ const SENSITIVE_PROPS = ["message", "stacktrace", "detailmessage"];
 
 // Deprecate
 class Logger implements vscode.Disposable {
-    private reporter: TelemetryReporter = null;
+    private reporter: TelemetryReporter | null = null;
 
     public initialize(context: vscode.ExtensionContext, firstParty?: boolean): void {
         if (this.reporter) {
diff --git a/src/mainClassPicker.ts b/src/mainClassPicker.ts
index 4d66c5de..caee665d 100644
--- a/src/mainClassPicker.ts
+++ b/src/mainClassPicker.ts
@@ -88,16 +88,19 @@ class MainClassPicker {
             return this.getMRUTimestamp(b) - this.getMRUTimestamp(a);
         });
 
-        const mostRecentlyUsedOption: IMainClassOption = (options.length && this.contains(options[0])) ? options[0] : undefined;
+        const mostRecentlyUsedOption: IMainClassOption | undefined = (options.length && this.contains(options[0])) ? options[0] : undefined;
         const isMostRecentlyUsed = (option: IMainClassOption): boolean => {
-            return mostRecentlyUsedOption
+            return !!mostRecentlyUsedOption
                 && mostRecentlyUsedOption.mainClass === option.mainClass
                 && mostRecentlyUsedOption.projectName === option.projectName;
         };
         const isFromActiveEditor = (option: IMainClassOption): boolean => {
-            const activeEditor: TextEditor = window.activeTextEditor;
+            const activeEditor: TextEditor | undefined = window.activeTextEditor;
             const currentActiveFile: string = _.get(activeEditor, "document.uri.fsPath");
-            return option.filePath && currentActiveFile && path.relative(option.filePath, currentActiveFile) === "";
+            if (option.filePath && currentActiveFile) {
+                return path.relative(option.filePath, currentActiveFile) === "";
+            }
+            return false;
         };
         const isPrivileged = (option: IMainClassOption): boolean => {
             return isMostRecentlyUsed(option) || isFromActiveEditor(option);
@@ -124,7 +127,7 @@ class MainClassPicker {
                 adjustedDetail.push("$(clock) recently used");
             }
 
-            if (isFromActiveEditor(option)) {
+            if (isFromActiveEditor(option) && option.filePath) {
                 adjustedDetail.push(`$(file-text) active editor (${path.basename(option.filePath)})`);
             }
 
diff --git a/src/processPicker.ts b/src/processPicker.ts
index 8dc5a2bd..a9f11ff0 100644
--- a/src/processPicker.ts
+++ b/src/processPicker.ts
@@ -46,7 +46,7 @@ function convertToJavaProcess(pid: number, command: string, args: string): IJava
     return undefined;
 }
 
-export async function pickJavaProcess(): Promise<IJavaProcess> {
+export async function pickJavaProcess(): Promise<IJavaProcess | undefined> {
     const javaProcesses: IJavaProcess[] = [];
     try {
         await getProcesses((pid: number, _ppid: number, command: string, args: string, _date: number) => {
diff --git a/src/terminalLinkProvider.ts b/src/terminalLinkProvider.ts
index 6d5db9d1..d50927bb 100644
--- a/src/terminalLinkProvider.ts
+++ b/src/terminalLinkProvider.ts
@@ -20,7 +20,7 @@ export class JavaTerminalLinkProvder implements TerminalLinkProvider<IJavaTermin
         }
 
         const regex = new RegExp("(\\sat\\s+)([\\w$\\.]+\\/)?(([\\w$]+\\.)+[<\\w$>]+)\\(([\\w-$]+\\.java:\\d+)\\)");
-        const result: RegExpExecArray = regex.exec(context.line);
+        const result: RegExpExecArray | null = regex.exec(context.line);
         if (result && result.length) {
             const stackTrace = `${result[2] || ""}${result[3]}(${result[5]})`;
             const sourceLineNumber = Number(result[5].split(":")[1]);
diff --git a/src/threadOperations.ts b/src/threadOperations.ts
index 4c3f3ecc..23ca45c6 100644
--- a/src/threadOperations.ts
+++ b/src/threadOperations.ts
@@ -23,7 +23,7 @@ export function initializeThreadOperations(context: vscode.ExtensionContext) {
 }
 
 async function operateThread(request: string, threadId: any): Promise<void> {
-    const debugSession: vscode.DebugSession = vscode.debug.activeDebugSession;
+    const debugSession: vscode.DebugSession | undefined = vscode.debug.activeDebugSession;
     if (!debugSession) {
         return;
     }
diff --git a/src/utility.ts b/src/utility.ts
index c2e12d53..6746c57a 100644
--- a/src/utility.ts
+++ b/src/utility.ts
@@ -55,11 +55,11 @@ function logMessage(message: ILoggingMessage): void {
         }
         sendError(error);
     } else {
-        sendInfo(null, { message: message.message });
+        sendInfo("", { message: message.message });
     }
 
     // Deprecated
-    logger.log(message.type, { message: message.message, stack: message.stack });
+    logger.log(message.type, { message: message.message, stack: message.stack || "" });
 }
 
 export async function showInformationMessage(message: ILoggingMessage, ...items: string[]): Promise<string | undefined> {
@@ -79,20 +79,20 @@ export async function showErrorMessage(message: ILoggingMessage, ...items: strin
 
 export async function showInformationMessageWithTroubleshooting(message: ITroubleshootingMessage, ...items: string[]): Promise<string | undefined> {
     const choice = await showInformationMessage(message, ...items, LEARN_MORE);
-    return handleTroubleshooting(choice, message.message, message.anchor);
+    return handleTroubleshooting(message.message, choice, message.anchor);
 }
 
 export async function showWarningMessageWithTroubleshooting(message: ITroubleshootingMessage, ...items: string[]): Promise<string | undefined> {
     const choice = await showWarningMessage(message, ...items, LEARN_MORE);
-    return handleTroubleshooting(choice, message.message, message.anchor);
+    return handleTroubleshooting(message.message, choice, message.anchor);
 }
 
 export async function showErrorMessageWithTroubleshooting(message: ITroubleshootingMessage, ...items: string[]): Promise<string | undefined> {
     const choice = await showErrorMessage(message, ...items, LEARN_MORE);
-    return handleTroubleshooting(choice, message.message, message.anchor);
+    return handleTroubleshooting(message.message, choice, message.anchor);
 }
 
-function handleTroubleshooting(choice: string, message: string, anchor: string): string | undefined {
+function handleTroubleshooting(message: string, choice?: string, anchor?: string): string | undefined {
     if (choice === LEARN_MORE) {
         openTroubleshootingPage(message, anchor);
         return undefined;
@@ -101,9 +101,9 @@ function handleTroubleshooting(choice: string, message: string, anchor: string):
     return choice;
 }
 
-export function openTroubleshootingPage(message: string, anchor: string) {
+export function openTroubleshootingPage(message: string, anchor?: string) {
     vscode.commands.executeCommand("vscode.open", vscode.Uri.parse(anchor ? `${TROUBLESHOOTING_LINK}#${anchor}` : TROUBLESHOOTING_LINK));
-    sendInfo(null, {
+    sendInfo("", {
         troubleshooting: "yes",
         troubleshootingMessage: message,
     });
@@ -183,7 +183,7 @@ export function getJavaExtensionAPI(): Thenable<any> {
     return extension.activate();
 }
 
-export function getJavaExtension(): vscode.Extension<any> {
+export function getJavaExtension(): vscode.Extension<any> | undefined {
     return vscode.extensions.getExtension(JAVA_EXTENSION_ID);
 }
 
@@ -194,11 +194,11 @@ export function isJavaExtEnabled(): boolean {
 
 export function isJavaExtActivated(): boolean {
     const javaExt = vscode.extensions.getExtension(JAVA_EXTENSION_ID);
-    return javaExt && javaExt.isActive;
+    return !!javaExt && javaExt.isActive;
 }
 
 export function getLauncherScriptPath() {
-    const ext = vscode.extensions.getExtension(DEBUGGER_EXTENSION_ID);
+    const ext = vscode.extensions.getExtension(DEBUGGER_EXTENSION_ID)!;
     return path.join(ext.extensionPath, "scripts", "launcher.bat");
 }
 
diff --git a/test/extension.test.ts b/test/extension.test.ts
index e342097b..a2127763 100644
--- a/test/extension.test.ts
+++ b/test/extension.test.ts
@@ -12,7 +12,7 @@ suite("Extension Tests", () => {
 
     test("should activate", function() {
         this.timeout(1 * 60 * 1000);
-        return vscode.extensions.getExtension("vscjava.vscode-java-debug").activate().then((_api) => {
+        return vscode.extensions.getExtension("vscjava.vscode-java-debug")!.activate().then((_api) => {
             assert.ok(true);
         });
     });
diff --git a/tsconfig.json b/tsconfig.json
index 1f32c464..5e963013 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -13,6 +13,7 @@
         "noImplicitAny": true,
         "noImplicitReturns": true,
         "noUnusedParameters": true,
+        "strictNullChecks": true,
         "alwaysStrict": true
     },
     "exclude": [