feat(class39): 添加 Lambda表达式示例代码

- 新增 LambdaDemo1.java 文件，演示函数式接口和 Lambda 表达式的使用
- 新增 LambdaDemo2.java 文件，展示 Lambda表达式在数组排序和事件监听中的应用
- 通过具体实例说明 Lambda 表达式的简洁性和高效性

src/class39/LambdaDemo1.java

@@ -0,0 +1,31 @@
+package class39;
+
+public class LambdaDemo1 {
+    public static void main(String[] args) {
+        Animal a = new Animal() {
+            @Override
+            public void cry() {
+                System.out.println("cry...");
+            }
+        };
+        a.cry();
+//        Swim s1 = new Swim() {
+//            @Override
+//            public void swimming() {
+//                System.out.println("student is swimming...");
+//            }
+//        };
+
+        Swim s1 = () -> System.out.println("student is swimming...");//lambda表达式，函数式接口的实现
+
+    }
+}
+
+abstract class Animal{
+    public abstract void cry();
+}
+//函数式接口：有且只有一个抽象方法
+@FunctionalInterface //声明函数式接口的注解
+interface Swim{
+    void swimming();
+}

src/class39/LambdaDemo2.java

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+package class39;
+
+import class38.Student;
+
+import javax.swing.*;
+import java.util.Arrays;
+
+public class LambdaDemo2 {
+    public static void main(String[] args) {
+        test2();
+    }
+    public static void test1(){
+        Student[] students = new Student[6];
+        students[0] = new Student("般素素", 35, 171.5, '女');
+        students[1] = new Student("杨幂", 28, 168.5, '女');
+        students[2] = new Student("张无忌", 25, 181.5, '男');
+        students[3] = new Student("小昭", 19, 165.5, '女');
+        students[4] = new Student("赵敏", 27, 167.5, '女');
+        students[5] = new Student("刘亦菲", 36, 168, '女');
+        //IDEA提示简化
+//        Arrays.sort(students, Comparator.comparingInt(Student::getAge));
+        Arrays.sort(students, (o1, o2) -> o1.getAge() - o2.getAge());
+
+        for (Student student : students) {
+            System.out.println(student);
+        }
+    }
+    public static void test2(){
+        JFrame win = new JFrame("登陆窗口");
+        win.setSize(300,200);
+        win.setLocationRelativeTo(null);
+        win.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
+        win.setVisible(true);
+        JPanel panel = new JPanel();
+        win.add(panel);
+        JButton btn = new JButton("登陆");
+        btn.addActionListener(e -> System.out.println("点击了登陆按钮"));
+        panel.add(btn);
+
+
+    }
+}