无关紧要

src/chapter4/Example25.java

@@ -1,4 +1,14 @@
 package chapter4;
 
-public class Examaple25 {
+public class Example25 {
+    public static void main(String[] args) {
+        int result = divide(4, 0);
+        System.out.println("结果：" + result);
+
+        System.out.println("程序继续运行...");
+    }
+
+    public static int divide(int x, int y) {
+        return x / y;
+    }
 }

src/chapter4/example14/Example14.java

@@ -1,2 +1,36 @@
-package chapter4.example14;public class Example14 {
+package chapter4.example14;
+
+abstract class Animal {
+    abstract void shout();
+}
+
+class Cat extends Animal {
+    public void shout() {
+        System.out.println("喵喵...");
+    }
+}
+
+class Dog extends Animal {
+    public void shout() {
+        System.out.println("汪汪...");
+    }
+}
+
+class Example14 {
+    public static void main(String[] args) {
+        Animal cat = new Cat();
+        Animal dog = new Dog();
+
+        cat.shout();
+        dog.shout();
+    }
+}
+
+class Example15 {
+    public static void main(String[] args) {
+        Dog dog = new Dog();
+        Animal an = dog;
+
+        an.shout();
+    }
 }

src/chapter4/object/Test.java

@@ -1,5 +1,7 @@
 package chapter4.object;
 
+import java.util.HashSet;
+
 class Student {
     private String name;
     private int age;
@@ -9,11 +11,17 @@ class Student {
         this.age = age;
     }
 
-    public String toString() {
+//    public String toString() {
-        return "Student {name=" + name + ", age=" + age + "}";
+//        return "Student {name=" + name + ", age=" + age + "}";
-    }
+//    }
 }
 
 public class Test {
+    public static void main(String[] args) {
+        Student s1 = new Student("张三", 23);
 
+        System.out.println(s1);
+
+
+    }
 }