update:demo3

com/study/demo3/Book.java

@@ -0,0 +1,61 @@
+package com.study.demo3;
+
+public class Book {
+    protected String isbn;
+    protected String title;
+    protected boolean available;
+
+   
+    public Book() {
+        this.available = true;
+    }
+
+   
+    public Book(String isbn, String title) {
+        this.isbn = isbn;
+        this.title = title;
+        this.available = true;
+    }
+
+   
+    public Book(String isbn, String title, boolean available) {
+        this.isbn = isbn;
+        this.title = title;
+        this.available = available;
+    }
+
+   
+    public String getIsbn() {
+        return isbn;
+    }
+
+    public void setIsbn(String isbn) {
+        this.isbn = isbn;
+    }
+
+    public String getTitle() {
+        return title;
+    }
+
+    public void setTitle(String title) {
+        this.title = title;
+    }
+
+    public boolean isAvailable() {
+        return available;
+    }
+
+    public void setAvailable(boolean available) {
+        this.available = available;
+    }
+
+   
+    public void returnBook() {
+        if (!this.available) {
+            this.available = true;
+            System.out.println(this.title + " (ISBN: " + this.isbn + ") has been returned and is now available.");
+        } else {
+            System.out.println(this.title + " (ISBN: " + this.isbn + ") is already available.");
+        }
+    }
+}