From c9f52b584438c04db84b1d2554a04398c17b3371 Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Sat, 1 Feb 2025 16:33:47 +0800
Subject: [PATCH 01/33] Committed on or around 2025/02/01

---
 Miscellaneous/Cpp/Array_of_objects_3.java | 38 +++++++++++++++++++++++
 1 file changed, 38 insertions(+)
 create mode 100644 Miscellaneous/Cpp/Array_of_objects_3.java

diff --git a/Miscellaneous/Cpp/Array_of_objects_3.java b/Miscellaneous/Cpp/Array_of_objects_3.java
new file mode 100644
index 0000000..f55fc88
--- /dev/null
+++ b/Miscellaneous/Cpp/Array_of_objects_3.java
@@ -0,0 +1,38 @@
+package Cpp;
+
+class MyClasses{
+	private int x;
+	
+	void set_x(int i) {
+		x = i;
+	}
+	
+	int get_x() {
+		return x;
+	}
+}
+
+public class Array_of_objects_3 {
+
+	public static void main(String[] args) {
+		
+		
+		MyClasses[] objects = new MyClasses[4];
+		
+		objects[0] = new MyClasses();
+		objects[0].set_x(11);
+		
+		objects[1].set_x(22);
+		
+		objects[2].set_x(33);
+		
+		
+//		for (int i = 0; i < objects.length; i++) 
+//			objects[i].set_x(i);
+		
+		for (int i = 0; i < objects.length; i++) 
+			System.out.println("Objects[" + i + "].get_x(): " + objects[i].get_x());
+
+	}
+
+}

From 6bd4b3d45ceb50383d8d1ce229d7b29792edac9a Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Sat, 1 Feb 2025 16:34:54 +0800
Subject: [PATCH 02/33] Committed on or around 2025/02/01

---
 Miscellaneous/Cpp/Array_of_objects_3.java | 1 -
 1 file changed, 1 deletion(-)

diff --git a/Miscellaneous/Cpp/Array_of_objects_3.java b/Miscellaneous/Cpp/Array_of_objects_3.java
index f55fc88..7ef0b9d 100644
--- a/Miscellaneous/Cpp/Array_of_objects_3.java
+++ b/Miscellaneous/Cpp/Array_of_objects_3.java
@@ -23,7 +23,6 @@ public static void main(String[] args) {
 		objects[0].set_x(11);
 		
 		objects[1].set_x(22);
-		
 		objects[2].set_x(33);
 		
 		

From 504ec14cc7285d0b2bddbb586c08cf255ad2948a Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Sat, 1 Feb 2025 16:37:24 +0800
Subject: [PATCH 03/33] Committed on or around 2025/02/01

---
 Miscellaneous/Cpp/Array_of_objects_3.java | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/Miscellaneous/Cpp/Array_of_objects_3.java b/Miscellaneous/Cpp/Array_of_objects_3.java
index 7ef0b9d..aa1f547 100644
--- a/Miscellaneous/Cpp/Array_of_objects_3.java
+++ b/Miscellaneous/Cpp/Array_of_objects_3.java
@@ -15,16 +15,17 @@ int get_x() {
 public class Array_of_objects_3 {
 
 	public static void main(String[] args) {
-		
-		
+	
 		MyClasses[] objects = new MyClasses[4];
 		
 		objects[0] = new MyClasses();
 		objects[0].set_x(11);
 		
-		objects[1].set_x(22);
-		objects[2].set_x(33);
-		
+		objects[1] = new MyClasses();
+		objects[1].set_x(99);
+	
+		objects[2].set_x(22);
+		objects[3].set_x(33);
 		
 //		for (int i = 0; i < objects.length; i++) 
 //			objects[i].set_x(i);

From 637d666250259d2cb606604be692a6eada0aa0f8 Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Sat, 1 Feb 2025 18:08:12 +0800
Subject: [PATCH 04/33] Committed on or around 2025/02/01

---
 Chapter7/Abstract_Class.java | 78 ++++++++++++++++++++++++++++++++++++
 1 file changed, 78 insertions(+)
 create mode 100644 Chapter7/Abstract_Class.java

diff --git a/Chapter7/Abstract_Class.java b/Chapter7/Abstract_Class.java
new file mode 100644
index 0000000..e9d4cc6
--- /dev/null
+++ b/Chapter7/Abstract_Class.java
@@ -0,0 +1,78 @@
+
+abstract class Abstract_TwoDShape {
+	
+	private double width;
+	private double height;
+	private String name;
+	
+	// A default constructor
+	public Abstract_TwoDShape() {
+		width = height = 0.0;
+		name = "none";
+	}
+	
+	// Parameterized constructor.
+	Abstract_TwoDShape(double w, double h, String n){
+		width = w;
+		height = h;
+		name = n;
+	}
+	
+	// Construct object with equal width and height. 
+	Abstract_TwoDShape(double x, String n){
+		width = height = x;
+		name = n;
+	}
+	
+	// Construct an object from an object. 
+	Abstract_TwoDShape(Abstract_TwoDShape ob){
+		width = ob.width;
+		height = ob.height;
+		name = ob.name;
+	}
+	
+	// Accessor methods for width and height. 
+	double getWidth() {
+		return width;
+	}
+	
+	double getHeight() {
+		return height;
+	}
+	
+	void setWidth(double w) {
+		width = w;
+	}
+	
+	void setHeight(double h) {
+		height = h;
+	}
+	
+	String getName() {
+		return name;
+	}
+	
+	void showDim() {
+		System.out.println("Width and height are " + width + " and " + height);
+	}
+	
+	// declare an abstract method
+	abstract double area();
+
+	public static void main(String[] args) {
+		// TODO Auto-generated method stub
+
+	}
+
+}
+
+
+
+public class Abstract_Class {
+
+	public static void main(String[] args) {
+		// TODO Auto-generated method stub
+
+	}
+
+}

From d2df6aa4515aaa8ac7c097c6047fb3f73d462520 Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Sat, 1 Feb 2025 18:12:10 +0800
Subject: [PATCH 05/33] Committed on or around 2025/02/01

---
 Chapter7/Abstract_Class.java | 45 ++++++++++++++++++++++++++++++++----
 1 file changed, 41 insertions(+), 4 deletions(-)

diff --git a/Chapter7/Abstract_Class.java b/Chapter7/Abstract_Class.java
index e9d4cc6..f6af48e 100644
--- a/Chapter7/Abstract_Class.java
+++ b/Chapter7/Abstract_Class.java
@@ -59,15 +59,52 @@ void showDim() {
 	// declare an abstract method
 	abstract double area();
 
-	public static void main(String[] args) {
-		// TODO Auto-generated method stub
+}
 
-	}
 
+class Triangle_extends_abstract_superclass2 extends Abstract_TwoDShape {
+	
+	private String styleString;
+	
+	// A default constructor
+	public Triangle_extends_abstract_superclass2() {
+		// TODO Auto-generated constructor stub
+		super();
+		
+		styleString = "none";
+		
+	}
+	
+	// Constructor for Triangle
+	Triangle_extends_abstract_superclass2(String s, double w, double h){
+		super(w, h, "triangle");
+		
+		styleString = s;
+	}
+	
+	// One argument constructor.
+	Triangle_extends_abstract_superclass2(double x){
+		super(x, "triangle");  // call superclass constructor
+		
+		styleString = "filled";
+	}
+	
+	// Construct an object from an object.
+	Triangle_extends_abstract_superclass2(Triangle_extends_abstract_superclass ob){
+		super(ob);  // pass object to TwoDShape constructor
+		styleString = ob.styleString;
+	}
+	
+	double area() {
+		return getWidth() * getHeight() / 2;
+	}
+	
+	void showStyle() {
+		System.out.println("Triangle is " + styleString);
+	}
 }
 
 
-
 public class Abstract_Class {
 
 	public static void main(String[] args) {

From fcb3e4956e433ce812c9cdab04b31538d0af8654 Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Sat, 1 Feb 2025 18:17:13 +0800
Subject: [PATCH 06/33] Committed on or around 2025/02/01

---
 Chapter7/Abstract_Class.java | 54 ++++++++++++++++++++++++++++++++++--
 1 file changed, 52 insertions(+), 2 deletions(-)

diff --git a/Chapter7/Abstract_Class.java b/Chapter7/Abstract_Class.java
index f6af48e..0c329a3 100644
--- a/Chapter7/Abstract_Class.java
+++ b/Chapter7/Abstract_Class.java
@@ -90,7 +90,7 @@ public Triangle_extends_abstract_superclass2() {
 	}
 	
 	// Construct an object from an object.
-	Triangle_extends_abstract_superclass2(Triangle_extends_abstract_superclass ob){
+	Triangle_extends_abstract_superclass2(Triangle_extends_abstract_superclass2 ob){
 		super(ob);  // pass object to TwoDShape constructor
 		styleString = ob.styleString;
 	}
@@ -105,10 +105,60 @@ void showStyle() {
 }
 
 
+class Rectangle_extends_abstract_superclass2 extends Abstract_TwoDShape {
+	
+	// A default constructor
+	public Rectangle_extends_abstract_superclass2() {
+		// TODO Auto-generated constructor stub
+		super();
+	}
+	
+	// Constructor for Rectangle
+	Rectangle_extends_abstract_superclass2(double w, double h){
+		super(w, h, "rectangle");  // Call superclass constructor
+	}
+	
+	// Construct a square
+	Rectangle_extends_abstract_superclass2(double x) {
+		super(x, "rectangle");  // call superclass constructor
+	}
+	
+	// Construct an object from an object. 
+	Rectangle_extends_abstract_superclass2(Rectangle_extends_abstract_superclass ob){
+		super(ob); // pass object to TwoDShape constructor.
+	}
+	
+	boolean isSquare() {
+		
+		if ( getWidth() == getHeight() )
+			return true;
+		
+		return false;
+	}
+	
+	double area() {
+		return getWidth() * getHeight();
+	}
+
+}
+
 public class Abstract_Class {
 
 	public static void main(String[] args) {
-		// TODO Auto-generated method stub
+		
+		Abstract_TwoDShape shapes[] = new Abstract_TwoDShape[4];
+		
+		shapes[0] = new Triangle_extends_abstract_superclass2("outlined", 8.0, 12.0);
+		shapes[1] = new Rectangle_extends_abstract_superclass2(10);
+		shapes[2] = new Rectangle_extends_abstract_superclass2(10, 4);
+		shapes[3] = new Triangle_extends_abstract_superclass2(7.0);
+		
+		for (int i = 0; i < shapes.length; i++) {
+			System.out.println("Object is " + shapes[i].getName());
+			System.out.println("Area is " + shapes[i].area());
+			
+			System.out.println();
+		}
 
 	}
 

From 60a95db1bf78060ee7f770d6ac453a2e6599f44c Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Sat, 1 Feb 2025 18:17:40 +0800
Subject: [PATCH 07/33] Committed on or around 2025/02/01

---
 Chapter7/Abstract_Class.java | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Chapter7/Abstract_Class.java b/Chapter7/Abstract_Class.java
index 0c329a3..11a8783 100644
--- a/Chapter7/Abstract_Class.java
+++ b/Chapter7/Abstract_Class.java
@@ -124,7 +124,7 @@ public Rectangle_extends_abstract_superclass2() {
 	}
 	
 	// Construct an object from an object. 
-	Rectangle_extends_abstract_superclass2(Rectangle_extends_abstract_superclass ob){
+	Rectangle_extends_abstract_superclass2(Rectangle_extends_abstract_superclass2 ob){
 		super(ob); // pass object to TwoDShape constructor.
 	}
 	

From 7ca224008e6c9799de035eb0818a1a782f2edc83 Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Tue, 27 May 2025 14:00:22 +0800
Subject: [PATCH 08/33] Committed on or around 2025/05/27

---
 .../C/Uninitialized_array_element_content.java      | 13 +++++++++++++
 1 file changed, 13 insertions(+)
 create mode 100644 Miscellaneous/C/Uninitialized_array_element_content.java

diff --git a/Miscellaneous/C/Uninitialized_array_element_content.java b/Miscellaneous/C/Uninitialized_array_element_content.java
new file mode 100644
index 0000000..b032f0f
--- /dev/null
+++ b/Miscellaneous/C/Uninitialized_array_element_content.java
@@ -0,0 +1,13 @@
+package C;
+
+public class Uninitialized_array_element_content {
+
+	public static void main(String[] args) {
+		
+		final int ALL_INPUT_LINES_LONGER_THAN_EIGHTY = 500; 
+		
+		char more_than_80_characters[] = new char[ALL_INPUT_LINES_LONGER_THAN_EIGHTY];
+
+	}
+
+}

From b94649a14d2bb6d09459f438a683ea89e43311ce Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Tue, 27 May 2025 14:11:46 +0800
Subject: [PATCH 09/33] Committed on or around 2025/05/27

---
 Miscellaneous/C/Uninitialized_array_element_content.java | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Miscellaneous/C/Uninitialized_array_element_content.java b/Miscellaneous/C/Uninitialized_array_element_content.java
index b032f0f..7f8213a 100644
--- a/Miscellaneous/C/Uninitialized_array_element_content.java
+++ b/Miscellaneous/C/Uninitialized_array_element_content.java
@@ -7,6 +7,13 @@ public static void main(String[] args) {
 		final int ALL_INPUT_LINES_LONGER_THAN_EIGHTY = 500; 
 		
 		char more_than_80_characters[] = new char[ALL_INPUT_LINES_LONGER_THAN_EIGHTY];
+		
+		if(more_than_80_characters[0] == 0)
+			System.out.println("more_than_80_characters[0] is 0\n");
+		else 
+			
+			System.out.println("more_than_80_characters[0] is " + more_than_80_characters[0]);
+		
 
 	}
 

From 7a8c58eff5229f8c0edcba360966ed591911161b Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Tue, 27 May 2025 14:13:00 +0800
Subject: [PATCH 10/33] Committed on or around 2025/05/27

---
 Miscellaneous/C/Uninitialized_array_element_content.java | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Miscellaneous/C/Uninitialized_array_element_content.java b/Miscellaneous/C/Uninitialized_array_element_content.java
index 7f8213a..c3ecf32 100644
--- a/Miscellaneous/C/Uninitialized_array_element_content.java
+++ b/Miscellaneous/C/Uninitialized_array_element_content.java
@@ -9,7 +9,7 @@ public static void main(String[] args) {
 		char more_than_80_characters[] = new char[ALL_INPUT_LINES_LONGER_THAN_EIGHTY];
 		
 		if(more_than_80_characters[0] == 0)
-			System.out.println("more_than_80_characters[0] is 0\n");
+			System.out.println("more_than_80_characters[0] is 0");
 		else 
 			
 			System.out.println("more_than_80_characters[0] is " + more_than_80_characters[0]);

From bf8ed5a80395d49e614195b5b98dc8a34d68bb6f Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Tue, 27 May 2025 14:27:17 +0800
Subject: [PATCH 11/33] Committed on or around 2025/05/27

---
 Miscellaneous/C/Uninitialized_array_element_content.java | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Miscellaneous/C/Uninitialized_array_element_content.java b/Miscellaneous/C/Uninitialized_array_element_content.java
index c3ecf32..2905e86 100644
--- a/Miscellaneous/C/Uninitialized_array_element_content.java
+++ b/Miscellaneous/C/Uninitialized_array_element_content.java
@@ -9,10 +9,10 @@ public static void main(String[] args) {
 		char more_than_80_characters[] = new char[ALL_INPUT_LINES_LONGER_THAN_EIGHTY];
 		
 		if(more_than_80_characters[0] == 0)
-			System.out.println("more_than_80_characters[0] is 0");
+			System.out.println("Look! more_than_80_characters[0] is 0");
 		else 
 			
-			System.out.println("more_than_80_characters[0] is " + more_than_80_characters[0]);
+			System.out.println("See! more_than_80_characters[0] is " + more_than_80_characters[0]);
 		
 
 	}

From ab76f9d45af23a1017013be5504611d9cb64fd00 Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Tue, 27 May 2025 14:55:53 +0800
Subject: [PATCH 12/33] Committed on or around 2025/05/27

---
 Miscellaneous/C/Uninitialized_array_element_content.java | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/Miscellaneous/C/Uninitialized_array_element_content.java b/Miscellaneous/C/Uninitialized_array_element_content.java
index 2905e86..112d943 100644
--- a/Miscellaneous/C/Uninitialized_array_element_content.java
+++ b/Miscellaneous/C/Uninitialized_array_element_content.java
@@ -18,3 +18,5 @@ public static void main(String[] args) {
 	}
 
 }
+
+// Adapted from https://github.com/ResilientSpring/The-C-Programming-Language/blob/main/Blend/Uninitialized%20array%20element's%20content%202/bruise%203.c
\ No newline at end of file

From 51670140d633241d0495a32cb33632c6fe8e075b Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Tue, 27 May 2025 17:36:48 +0800
Subject: [PATCH 13/33] Committed on or around 2025/05/27

---
 Miscellaneous/C/Uninitialized_array_element_content.java | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/Miscellaneous/C/Uninitialized_array_element_content.java b/Miscellaneous/C/Uninitialized_array_element_content.java
index 112d943..77b95ac 100644
--- a/Miscellaneous/C/Uninitialized_array_element_content.java
+++ b/Miscellaneous/C/Uninitialized_array_element_content.java
@@ -19,4 +19,6 @@ public static void main(String[] args) {
 
 }
 
-// Adapted from https://github.com/ResilientSpring/The-C-Programming-Language/blob/main/Blend/Uninitialized%20array%20element's%20content%202/bruise%203.c
\ No newline at end of file
+// Adapted from https://github.com/ResilientSpring/The-C-Programming-Language/blob/main/Blend/Uninitialized%20array%20element's%20content%202/bruise%203.c
+
+// Important Note: https://chatgpt.com/c/68349761-6ad8-8008-8673-728152e81f3a
\ No newline at end of file

From 92a1ccdaf1a4b6801673b4f769156623a2ff5c51 Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Sun, 6 Jul 2025 19:07:50 +0800
Subject: [PATCH 14/33] Committed on or around 2025/07/06

---
 .../Programming_No1.java                      | 27 +++++++++++++++++++
 1 file changed, 27 insertions(+)
 create mode 100644 Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1.java

diff --git a/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1.java b/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1.java
new file mode 100644
index 0000000..3f924be
--- /dev/null
+++ b/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1.java
@@ -0,0 +1,27 @@
+package L4_Civil_Servant_Admission_Exam;
+
+public class Programming_No1 {
+
+	long Catalan_Number(int n) {
+		
+		long sum;
+		
+		if(n  == 0)
+			return 1;
+		
+		for (int i = 0; i < n; i++) {
+			
+			
+			
+		}
+		
+		return sum;
+		
+	}
+	
+	public static void main(String[] args) {
+		
+
+	}
+
+}

From 11452347ff7056e75d8e8690073e4f812bc00b4e Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Sun, 6 Jul 2025 19:32:10 +0800
Subject: [PATCH 15/33] Committed on or around 2025/07/06

---
 .../Programming_No1_2.java                    | 34 +++++++++++++++++++
 1 file changed, 34 insertions(+)
 create mode 100644 Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_2.java

diff --git a/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_2.java b/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_2.java
new file mode 100644
index 0000000..763daa6
--- /dev/null
+++ b/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_2.java
@@ -0,0 +1,34 @@
+package L4_Civil_Servant_Admission_Exam;
+
+public class Programming_No1_2 {
+
+	long Catalan_Number(int n) {
+		
+		long sum;
+		
+		if(n  == 0) {
+			
+			sum = 1;
+			
+			return sum;
+			
+		}
+			
+		
+		for (int i = 0; i < n; i++) {
+			
+			
+			
+		}
+		
+		return sum;
+		
+	}
+	
+	
+	public static void main(String[] args) {
+		// TODO Auto-generated method stub
+
+	}
+
+}

From 0a4d2671462abe44b47522ac7f58834b08f8884d Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Sun, 6 Jul 2025 19:36:08 +0800
Subject: [PATCH 16/33] Committed on or around 2025/07/06

---
 .../L4_Civil_Servant_Admission_Exam/Programming_No1_2.java    | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_2.java b/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_2.java
index 763daa6..033484f 100644
--- a/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_2.java
+++ b/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_2.java
@@ -17,7 +17,7 @@ long Catalan_Number(int n) {
 		
 		for (int i = 0; i < n; i++) {
 			
-			
+			sum = Catalan_Number(i) * Catalan_Number(n-1-i);
 			
 		}
 		
@@ -27,7 +27,7 @@ long Catalan_Number(int n) {
 	
 	
 	public static void main(String[] args) {
-		// TODO Auto-generated method stub
+		
 
 	}
 

From 3577b5be7a39895cdbf476bc124b07c286f69f1b Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Sun, 6 Jul 2025 19:38:16 +0800
Subject: [PATCH 17/33] Committed on or around 2025/07/06

---
 .../Programming_No1_2.java                    | 29 +++++++++----------
 1 file changed, 13 insertions(+), 16 deletions(-)

diff --git a/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_2.java b/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_2.java
index 033484f..28e9b8a 100644
--- a/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_2.java
+++ b/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_2.java
@@ -3,31 +3,28 @@
 public class Programming_No1_2 {
 
 	long Catalan_Number(int n) {
-		
+
 		long sum;
-		
-		if(n  == 0) {
-			
+
+		if (n == 0) {
+
 			sum = 1;
-			
+
 			return sum;
-			
+
 		}
-			
-		
+
 		for (int i = 0; i < n; i++) {
-			
-			sum = Catalan_Number(i) * Catalan_Number(n-1-i);
-			
+
+			sum = Catalan_Number(i) * Catalan_Number(n - 1 - i);
+
 		}
-		
+
 		return sum;
-		
+
 	}
-	
-	
+
 	public static void main(String[] args) {
-		
 
 	}
 

From f7cbc85ca73ae5f56e4739afcc10b16e2d64f4ef Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Sun, 6 Jul 2025 19:49:56 +0800
Subject: [PATCH 18/33] Committed on or around 2025/07/06

---
 .../L4_Civil_Servant_Admission_Exam/Programming_No1_2.java | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_2.java b/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_2.java
index 28e9b8a..2fffd89 100644
--- a/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_2.java
+++ b/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_2.java
@@ -1,8 +1,10 @@
 package L4_Civil_Servant_Admission_Exam;
 
+import Cpp.static_variable;
+
 public class Programming_No1_2 {
 
-	long Catalan_Number(int n) {
+	long static Catalan_Number(int n) {
 
 		long sum;
 
@@ -26,6 +28,9 @@ long Catalan_Number(int n) {
 
 	public static void main(String[] args) {
 
+		System.out.println(Catalan);
+		
+		
 	}
 
 }

From 4efbf3e3cb304ab61e3fc9a7c68647ca5abc8ac2 Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Sun, 6 Jul 2025 19:52:30 +0800
Subject: [PATCH 19/33] Committed on or around 2025/07/06

---
 .../Programming_No1_3.java                    | 32 +++++++++++++++++++
 1 file changed, 32 insertions(+)
 create mode 100644 Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_3.java

diff --git a/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_3.java b/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_3.java
new file mode 100644
index 0000000..5086efb
--- /dev/null
+++ b/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_3.java
@@ -0,0 +1,32 @@
+package L4_Civil_Servant_Admission_Exam;
+
+public class Programming_No1_3 {
+	
+	static long Catalan_Number(int n) {
+
+		long sum;
+
+		if (n == 0) {
+
+			sum = 1;
+
+			return sum;
+
+		}
+
+		for (int i = 0; i < n; i++) {
+
+			sum = Catalan_Number(i) * Catalan_Number(n - 1 - i);
+
+		}
+
+		return sum;
+
+	}
+
+	public static void main(String[] args) {
+		// TODO Auto-generated method stub
+
+	}
+
+}

From 0543d0e78d1ace989b70d72c70dfa3eb9e395d14 Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Sun, 6 Jul 2025 19:53:06 +0800
Subject: [PATCH 20/33] Committed on or around 2025/07/06

---
 .../L4_Civil_Servant_Admission_Exam/Programming_No1_3.java      | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_3.java b/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_3.java
index 5086efb..59c2748 100644
--- a/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_3.java
+++ b/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_3.java
@@ -25,7 +25,7 @@ static long Catalan_Number(int n) {
 	}
 
 	public static void main(String[] args) {
-		// TODO Auto-generated method stub
+		
 
 	}
 

From 5402a4c60b6e21924cab1efb1b34e426cd07ed82 Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Mon, 7 Jul 2025 00:49:45 +0800
Subject: [PATCH 21/33] Committed on or around 2025/07/07

---
 .../Programming_No1_4.java                    | 44 +++++++++++++++++++
 1 file changed, 44 insertions(+)
 create mode 100644 Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_4.java

diff --git a/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_4.java b/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_4.java
new file mode 100644
index 0000000..7903b3e
--- /dev/null
+++ b/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_4.java
@@ -0,0 +1,44 @@
+package L4_Civil_Servant_Admission_Exam;
+
+public class Programming_No1_4 {
+	
+	static long Catalan_Number(int n) {
+
+		// But Catalan_Number will call itself later, which might restart the sum from zero every time 
+		// Catalan_Number is called and cause the intermediate result of the sum to be lost.
+		//
+		// Each call to Catalan_Number(n) has its own independent copy of the sum variable on the call stack.
+		// So even though sum is initialized to 0 every time the function is called, 
+		// this happens in the scope of that one call only.[1]
+		long sum = 0;
+
+		if (n == 0) {
+
+			sum = 1;
+
+			return sum;
+
+		}
+
+		for (int i = 0; i < n; i++) {
+
+			sum = Catalan_Number(i) * Catalan_Number(n - 1 - i);
+
+		}
+
+		return sum;
+
+	}
+
+	public static void main(String[] args) {
+		// TODO Auto-generated method stub
+
+	}
+
+}
+
+/* References:
+ * 
+ * 1. https://chatgpt.com/c/6869ef92-f4c8-8008-b7b6-c9d0ee97820b
+ * 
+ * */

From ec12fa9fb504d4232df35e03c6d037cfa7e80f73 Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Mon, 7 Jul 2025 00:50:26 +0800
Subject: [PATCH 22/33] Committed on or around 2025/07/07

---
 .../L4_Civil_Servant_Admission_Exam/Programming_No1_4.java     | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_4.java b/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_4.java
index 7903b3e..e3e0087 100644
--- a/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_4.java
+++ b/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_4.java
@@ -31,7 +31,8 @@ static long Catalan_Number(int n) {
 	}
 
 	public static void main(String[] args) {
-		// TODO Auto-generated method stub
+		
+		
 
 	}
 

From 6db5c8fa130aab92fb77167112a6351446b3b927 Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Mon, 7 Jul 2025 00:51:30 +0800
Subject: [PATCH 23/33] Committed on or around 2025/07/07

---
 .../L4_Civil_Servant_Admission_Exam/Programming_No1_4.java      | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_4.java b/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_4.java
index e3e0087..4a43b43 100644
--- a/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_4.java
+++ b/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_4.java
@@ -32,7 +32,7 @@ static long Catalan_Number(int n) {
 
 	public static void main(String[] args) {
 		
-		
+		System.out.println(Catalan_Number(4));
 
 	}
 

From 64df1769425a7ae8604219b77e1b425435738c1b Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Mon, 7 Jul 2025 01:00:27 +0800
Subject: [PATCH 24/33] Committed on or around 2025/07/07

---
 .../Programming_No1_5.java                    | 45 +++++++++++++++++++
 1 file changed, 45 insertions(+)
 create mode 100644 Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_5.java

diff --git a/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_5.java b/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_5.java
new file mode 100644
index 0000000..f12f7ae
--- /dev/null
+++ b/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_5.java
@@ -0,0 +1,45 @@
+package L4_Civil_Servant_Admission_Exam;
+
+public class Programming_No1_5 {
+	
+	static long Catalan_Number(int n) {
+
+		// But Catalan_Number will call itself later, which might restart the sum from zero every time 
+		// Catalan_Number is called and cause the intermediate result of the sum to be lost.
+		//
+		// Each call to Catalan_Number(n) has its own independent copy of the sum variable on the call stack.
+		// So even though sum is initialized to 0 every time the function is called, 
+		// this happens in the scope of that one call only.[1]
+		long sum = 0;
+
+		if (n == 0) {
+
+			sum = 1;
+
+			return sum;
+
+		}
+
+		for (int i = 0; i < n; i++) {
+
+			sum = Catalan_Number(i) * Catalan_Number(n - 1 - i);
+
+		}
+
+		return sum;
+
+	}
+
+	public static void main(String[] args) {
+		
+		System.out.println(Catalan_Number(4));
+
+	}
+
+}
+
+/* References:
+ * 
+ * 1. https://chatgpt.com/c/6869ef92-f4c8-8008-b7b6-c9d0ee97820b
+ * 
+ * */

From c66d5b90b0a61d060b290b0c3eced61a1d974194 Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Mon, 7 Jul 2025 01:01:01 +0800
Subject: [PATCH 25/33] Committed on or around 2025/07/07

---
 .../L4_Civil_Servant_Admission_Exam/Programming_No1_5.java      | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_5.java b/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_5.java
index f12f7ae..9f3e472 100644
--- a/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_5.java
+++ b/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_5.java
@@ -22,7 +22,7 @@ static long Catalan_Number(int n) {
 
 		for (int i = 0; i < n; i++) {
 
-			sum = Catalan_Number(i) * Catalan_Number(n - 1 - i);
+			sum = sum + Catalan_Number(i) * Catalan_Number(n - 1 - i);
 
 		}
 

From 40a745395303409eb460d958ceac499dea3a3f00 Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Mon, 7 Jul 2025 01:07:16 +0800
Subject: [PATCH 26/33] Committed on or around 2025/07/07

---
 .../L4_Civil_Servant_Admission_Exam/Programming_No1_5.java      | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_5.java b/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_5.java
index 9f3e472..d6a41b3 100644
--- a/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_5.java
+++ b/Miscellaneous/L4_Civil_Servant_Admission_Exam/Programming_No1_5.java
@@ -22,6 +22,8 @@ static long Catalan_Number(int n) {
 
 		for (int i = 0; i < n; i++) {
 
+			// Catalan Formula involves Sigma, so you must add all operands.[1] 
+			// Sigma means to "add" all operands, rather than to "multiply" all operands.[1] 
 			sum = sum + Catalan_Number(i) * Catalan_Number(n - 1 - i);
 
 		}

From f6750f035ce1275a6ec1511fec9fbae2e0cd935f Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Mon, 10 Nov 2025 23:58:17 +0800
Subject: [PATCH 27/33] Committed on or around 2025/11/10

---
 Miscellaneous/SOE/HappyNumber.java | 10 ++++++++++
 1 file changed, 10 insertions(+)
 create mode 100644 Miscellaneous/SOE/HappyNumber.java

diff --git a/Miscellaneous/SOE/HappyNumber.java b/Miscellaneous/SOE/HappyNumber.java
new file mode 100644
index 0000000..154f990
--- /dev/null
+++ b/Miscellaneous/SOE/HappyNumber.java
@@ -0,0 +1,10 @@
+package SOE;
+
+public class HappyNumber {
+
+	public static void main(String[] args) {
+		// TODO Auto-generated method stub
+
+	}
+
+}

From 0f4d0ad2aef284e0bd09628133e24b7afcf195ce Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Tue, 11 Nov 2025 12:29:44 +0800
Subject: [PATCH 28/33] Committed on or around 2025/11/11

---
 Miscellaneous/SOE/HappyNumber.java | 43 +++++++++++++++++++++++++++---
 1 file changed, 40 insertions(+), 3 deletions(-)

diff --git a/Miscellaneous/SOE/HappyNumber.java b/Miscellaneous/SOE/HappyNumber.java
index 154f990..4507bc9 100644
--- a/Miscellaneous/SOE/HappyNumber.java
+++ b/Miscellaneous/SOE/HappyNumber.java
@@ -1,10 +1,47 @@
 package SOE;
 
+import java.util.HashSet;
+import java.util.Set;
+
 public class HappyNumber {
 
-	public static void main(String[] args) {
-		// TODO Auto-generated method stub
+    // 計算各位數平方和
+    static int next(int n) {
+        int sum = 0;
+        while (n > 0) {
+            int d = n % 10;
+            sum += d * d;
+            n /= 10;
+        }
+        return sum;
+    }
+
+    // 遞迴版 isHappy()
+    static boolean isHappy(int n) {
+        return isHappyHelper(n, new HashSet<>());
+    }
+
+    // 遞迴輔助函式：傳遞 seen 集合
+    private static boolean isHappyHelper(int n, Set<Integer> seen) {
+        if (n == 1)
+            return true; // 終止條件：快樂數
+        if (seen.contains(n))
+            return false; // 終止條件：進入循環
+
+        seen.add(n);
+        return isHappyHelper(next(n), seen);
+    }
 
-	}
+    // 測試
+    public static void main(String[] args) {
+        int[] nums = {1, 7, 19, 20, 25, 4};
 
+        for (int n : nums) {
+            System.out.printf("%d -> %s%n", n, isHappy(n));
+        }
+    }
 }
+
+// Source: https://chatgpt.com/c/69104fa3-4828-8321-ade1-f983f1f8c14c
+
+

From 68a61a0213e05ff3ca1fd3ff7e109c687ead7d80 Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Sun, 19 Apr 2026 12:06:37 +0800
Subject: [PATCH 29/33] Committed on or around 2026/04/19

---
 .../Intro_to_CS_No17_2026.java                | 29 +++++++++++++++++++
 1 file changed, 29 insertions(+)
 create mode 100644 Miscellaneous/L4_Civil_Servant_Admission_Exam/Intro_to_CS_No17_2026.java

diff --git a/Miscellaneous/L4_Civil_Servant_Admission_Exam/Intro_to_CS_No17_2026.java b/Miscellaneous/L4_Civil_Servant_Admission_Exam/Intro_to_CS_No17_2026.java
new file mode 100644
index 0000000..3ea27b6
--- /dev/null
+++ b/Miscellaneous/L4_Civil_Servant_Admission_Exam/Intro_to_CS_No17_2026.java
@@ -0,0 +1,29 @@
+package L4_Civil_Servant_Admission_Exam;
+
+public class Intro_to_CS_No17_2026 {
+
+	public static void main(String[] args) {
+		
+		int month = 1;
+		
+		while(month <= 4) {
+			
+			switch(month) { 
+			
+			case 1: 
+				System.out.println("31");
+			
+			case 2:
+				System.out.println("28");
+				
+			case 4:
+				System.out.println("30");
+			
+			}
+			
+			month++;
+		}
+
+	}
+
+}

From 9c635208ecb38e4eb9387a0cf75d2f70d068513f Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Thu, 11 Jun 2026 16:22:07 +0800
Subject: [PATCH 30/33] Committed on 2025/06/11

---
 Chapter7/SupSubRef2.java | 41 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 41 insertions(+)
 create mode 100644 Chapter7/SupSubRef2.java

diff --git a/Chapter7/SupSubRef2.java b/Chapter7/SupSubRef2.java
new file mode 100644
index 0000000..2e19ecc
--- /dev/null
+++ b/Chapter7/SupSubRef2.java
@@ -0,0 +1,41 @@
+// A superclass reference can refer to a subclass object. 
+
+class X3 {
+	int a;
+	
+	public X3(int i) {
+		// TODO Auto-generated constructor stub
+		
+		a = i;
+	}
+}
+
+class Y3 extends X3 {
+	int b;
+	
+	public Y3(int i, int j) {
+		// TODO Auto-generated constructor stub
+		super(j);
+		b = i;
+	}
+}
+
+public class SupSubRef2 {
+
+	public static void main(String[] args) {
+		
+		X3 x = new X3(10);
+		Y3 y = new Y3(5, 6);
+		
+		System.out.println("X.a: " + x.a);
+		
+		x = y; 
+		System.out.println("X.a: " + x.a);
+		
+		// X references know only about X members
+		x.a = 19;  // OK
+// 		x2.b = 27;  // Error, X does not have a b member. 
+
+	}
+
+}

From 129bb515c3813c1afd514c616516ab2c543bafef Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Thu, 11 Jun 2026 16:26:55 +0800
Subject: [PATCH 31/33] Committed on or around 2026/06/11

---
 Chapter7/SupSubRef2.java | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Chapter7/SupSubRef2.java b/Chapter7/SupSubRef2.java
index 2e19ecc..ce10dac 100644
--- a/Chapter7/SupSubRef2.java
+++ b/Chapter7/SupSubRef2.java
@@ -34,7 +34,7 @@ public static void main(String[] args) {
 		
 		// X references know only about X members
 		x.a = 19;  // OK
-// 		x2.b = 27;  // Error, X does not have a b member. 
+// 		x.b = 27;  // Error, X does not have a b member. 
 
 	}
 

From f0837941c52bd708db12fa700a2545b8078901c7 Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Thu, 11 Jun 2026 16:37:57 +0800
Subject: [PATCH 32/33] Committed on 2026/06/11

---
 Chapter7/Dynamic_Disptach_Demo_2.java | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)
 create mode 100644 Chapter7/Dynamic_Disptach_Demo_2.java

diff --git a/Chapter7/Dynamic_Disptach_Demo_2.java b/Chapter7/Dynamic_Disptach_Demo_2.java
new file mode 100644
index 0000000..318154e
--- /dev/null
+++ b/Chapter7/Dynamic_Disptach_Demo_2.java
@@ -0,0 +1,24 @@
+
+public class Dynamic_Disptach_Demo_2 {
+
+	public static void main(String[] args) {
+		
+		Sup superOb = new Sup();
+		Sub1 subOb1 = new Sub1();
+		Sub2 subOb2 = new Sub2();
+
+		Sup supRef;  // Declaring a variable called supRef of the class type Sup. This variable does not define an object. Instead, it is simply a variable that can refer to an object.
+		
+		supRef = superOb;
+		supRef.who();       // In each case, the version of who() to call is determined at run time by ...
+		
+		supRef = subOb1;
+		supRef.who();       // ... the type of object being referred to.
+		
+		supRef = subOb2;
+		supRef.who();
+		
+
+	}
+
+}

From 98bd68093662581b3e24552686a1ab0b046dec80 Mon Sep 17 00:00:00 2001
From: Ceiling_roof <48134466+ResilientSpring@users.noreply.github.com>
Date: Thu, 11 Jun 2026 16:52:58 +0800
Subject: [PATCH 33/33] Committed on or around 2026/06/11

---
 Chapter7/Dynamic_Disptach_Demo_2.java | 15 +++++----------
 Chapter7/SupSubRef2.java              | 14 +++++++-------
 2 files changed, 12 insertions(+), 17 deletions(-)

diff --git a/Chapter7/Dynamic_Disptach_Demo_2.java b/Chapter7/Dynamic_Disptach_Demo_2.java
index 318154e..9babea9 100644
--- a/Chapter7/Dynamic_Disptach_Demo_2.java
+++ b/Chapter7/Dynamic_Disptach_Demo_2.java
@@ -4,19 +4,14 @@ public class Dynamic_Disptach_Demo_2 {
 	public static void main(String[] args) {
 		
 		Sup superOb = new Sup();
-		Sub1 subOb1 = new Sub1();
-		Sub2 subOb2 = new Sub2();
 
-		Sup supRef;  // Declaring a variable called supRef of the class type Sup. This variable does not define an object. Instead, it is simply a variable that can refer to an object.
+		superOb.who();       // In each case, the version of who() to call is determined at run time by ...
 		
-		supRef = superOb;
-		supRef.who();       // In each case, the version of who() to call is determined at run time by ...
+		superOb = new Sub1();
+		superOb.who();       // ... the type of object being referred to.
 		
-		supRef = subOb1;
-		supRef.who();       // ... the type of object being referred to.
-		
-		supRef = subOb2;
-		supRef.who();
+		superOb = new Sub2();
+		superOb.who();
 		
 
 	}
diff --git a/Chapter7/SupSubRef2.java b/Chapter7/SupSubRef2.java
index ce10dac..3a9a0a3 100644
--- a/Chapter7/SupSubRef2.java
+++ b/Chapter7/SupSubRef2.java
@@ -24,17 +24,17 @@ public class SupSubRef2 {
 
 	public static void main(String[] args) {
 		
-		X3 x = new X3(10);
-		Y3 y = new Y3(5, 6);
+		X3 x3 = new X3(10);
+		Y3 y3 = new Y3(5, 6);
 		
-		System.out.println("X.a: " + x.a);
+		System.out.println("X.a: " + x3.a);
 		
-		x = y; 
-		System.out.println("X.a: " + x.a);
+		x3 = y3; 
+		System.out.println("X.a: " + x3.a);
 		
 		// X references know only about X members
-		x.a = 19;  // OK
-// 		x.b = 27;  // Error, X does not have a b member. 
+		x3.a = 19;  // OK
+// 		x3.b = 27;  // Error, X does not have a b member. 
 
 	}