added old projects

CS101/lab_15/.vscode/settings.json

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+{
+    "java.project.sourcePaths": ["src"],
+    "java.project.outputPath": "bin"
+}

CS101/lab_15/README.md

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+## Getting Started
+
+Welcome to the VS Code Java world. Here is a guideline to help you get started to write Java code in Visual Studio Code.
+
+## Folder Structure
+
+The workspace contains two folders by default, where:
+
+- `src`: the folder to maintain sources
+- `lib`: the folder to maintain dependencies
+
+Meanwhile, the compiled output files will be generated in the `bin` folder by default.
+
+> If you want to customize the folder structure, open `.vscode/settings.json` and update the related settings there.
+
+## Dependency Management
+
+The `JAVA PROJECTS` view allows you to manage your dependencies. More details can be found [here](https://github.com/microsoft/vscode-java-dependency#manage-dependencies).

CS101/lab_15/src/App.java

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+public class App {
+    public static void main(String[] args) {
+        QuadraticSolver quadSolve = new QuadraticSolver(1, 2, 3);
+        
+    }
+}

CS101/lab_15/src/NonRealException.java

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+
+
+public class NonRealException extends QuadraticException
+{
+    public NonRealException(String msg)
+    {
+        super(msg);
+    }
+}

CS101/lab_15/src/QuadraticException.java

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+
+
+public class QuadraticException extends ArithmeticException
+{
+    public QuadraticException(String msg)
+    {
+        super(msg);
+    }
+}

CS101/lab_15/src/QuadraticSolver.java

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+public class QuadraticSolver {
+    
+    private int _a;
+    private int _b;
+    private int _c;
+
+    public QuadraticSolver() {
+        _a = 1;
+        _b = 1;
+        _c = 1;
+    }
+
+    public QuadraticSolver(int a, int b, int c) throws QuadraticException
+    {
+        if (a == 0)
+            throw new QuadraticException("Not a real quadratic");
+
+        _a = a;
+        _b = b;
+        _c = c;
+
+    }
+
+    public double discriminant() {
+        return _b*_b - 4 * _a * _c;
+    }
+
+    public boolean realSolution() {
+        return discriminant() >= 0 ;
+    }
+
+    public double firstRoot() throws NonRealException {
+        if (!realSolution())
+            throw new NonRealException("no real solution");
+        return (-1 * _b + discriminant()) / (2 * _a);
+    }
+
+    public double secondRoot() throws NonRealException{
+        if (!realSolution())
+            throw new NonRealException("no real solution");
+        else
+            return (-1 * _b - discriminant()) / (2 * _a);
+    }
+
+    public String toString() {
+        return "f(x) = " + _a + "x^2 + " + _b + "x + " + _c;
+    }
+}