diff --git a/src/main/java/com/examplehub/basics/Final.java b/src/main/java/com/examplehub/basics/Final.java
deleted file mode 100644
index 926e7aed..00000000
--- a/src/main/java/com/examplehub/basics/Final.java
+++ /dev/null
@@ -1,16 +0,0 @@
-package com.examplehub.basics;
-
-public class Final {
-  public static void main(String[] args) {
-    final double PI = 3.1415926;
-
-    /*
-    PI = 3.14; // can't be assigned again
-     */
-
-    final int number;
-    /*
-    System.out.println(number); //Variable 'number' might not have been initialized
-     */
-  }
-}
diff --git a/src/main/java/com/examplehub/basics/constant/FinalExample.java b/src/main/java/com/examplehub/basics/constant/FinalExample.java
new file mode 100644
index 00000000..4af26c7b
--- /dev/null
+++ b/src/main/java/com/examplehub/basics/constant/FinalExample.java
@@ -0,0 +1,4 @@
+package com.examplehub.basics.constant;
+
+public class FinalExample {
+}
diff --git a/src/main/java/com/examplehub/basics/TypeCast.java b/src/main/java/com/examplehub/basics/conversion/TypeCast.java
similarity index 91%
rename from src/main/java/com/examplehub/basics/TypeCast.java
rename to src/main/java/com/examplehub/basics/conversion/TypeCast.java
index e4737b13..56324215 100644
--- a/src/main/java/com/examplehub/basics/TypeCast.java
+++ b/src/main/java/com/examplehub/basics/conversion/TypeCast.java
@@ -1,4 +1,4 @@
-package com.examplehub.basics;
+package com.examplehub.basics.conversion;
 
 public class TypeCast {
   public static void main(String[] args) {
diff --git a/src/main/java/com/examplehub/basics/conversion/TypeConversionExample.java b/src/main/java/com/examplehub/basics/conversion/TypeConversionExample.java
new file mode 100644
index 00000000..c21d3deb
--- /dev/null
+++ b/src/main/java/com/examplehub/basics/conversion/TypeConversionExample.java
@@ -0,0 +1,4 @@
+package com.examplehub.basics.conversion;
+
+public class TypeConversionExample {
+}
diff --git a/src/main/java/com/examplehub/basics/function/ModifyFunctionArgs.java b/src/main/java/com/examplehub/basics/function/ModifyFunctionArgs.java
new file mode 100644
index 00000000..32d419ae
--- /dev/null
+++ b/src/main/java/com/examplehub/basics/function/ModifyFunctionArgs.java
@@ -0,0 +1,15 @@
+package com.examplehub.basics.function;
+
+public class ModifyFunctionArgs {
+  static class MyObj{
+    public int age = -1;
+  }
+
+  public static void modify(int num, String str, Integer integer, int[] array, MyObj obj) {
+    num = num + 1;
+    str = "<" + str + ">";
+    integer = integer + 1;
+    array[0] = -array[0];
+    obj.age = -obj.age;
+  }
+}
diff --git a/src/main/java/com/examplehub/designpatterns/singleton/SingletonExample1.java b/src/main/java/com/examplehub/designpatterns/singleton/SingletonExample1.java
new file mode 100644
index 00000000..2860143a
--- /dev/null
+++ b/src/main/java/com/examplehub/designpatterns/singleton/SingletonExample1.java
@@ -0,0 +1,11 @@
+package com.examplehub.designpatterns.singleton;
+
+public class SingletonExample1 {
+  private static final SingletonExample1 INSTANCE = new SingletonExample1();
+  private SingletonExample1(){
+
+  }
+  public static SingletonExample1 getInstance() {
+    return INSTANCE;
+  }
+}
diff --git a/src/main/java/com/examplehub/designpatterns/singleton/SingletonExample2.java b/src/main/java/com/examplehub/designpatterns/singleton/SingletonExample2.java
new file mode 100644
index 00000000..dcece1bd
--- /dev/null
+++ b/src/main/java/com/examplehub/designpatterns/singleton/SingletonExample2.java
@@ -0,0 +1,17 @@
+package com.examplehub.designpatterns.singleton;
+
+public class SingletonExample2 {
+  private static final SingletonExample2 INSTANCE;
+
+  static {
+    /*
+     * do more work  here
+     */
+    INSTANCE = new SingletonExample2();
+  }
+  private SingletonExample2(){
+  }
+  public static SingletonExample2 getInstance() {
+    return INSTANCE;
+  }
+}
diff --git a/src/main/java/com/examplehub/designpatterns/singleton/SingletonExample3.java b/src/main/java/com/examplehub/designpatterns/singleton/SingletonExample3.java
new file mode 100644
index 00000000..9ac01d17
--- /dev/null
+++ b/src/main/java/com/examplehub/designpatterns/singleton/SingletonExample3.java
@@ -0,0 +1,5 @@
+package com.examplehub.designpatterns.singleton;
+
+public enum SingletonExample3 {
+  INSTANCE
+}
diff --git a/src/main/java/com/examplehub/designpatterns/singleton/SingletonExample4.java b/src/main/java/com/examplehub/designpatterns/singleton/SingletonExample4.java
new file mode 100644
index 00000000..dd033262
--- /dev/null
+++ b/src/main/java/com/examplehub/designpatterns/singleton/SingletonExample4.java
@@ -0,0 +1,15 @@
+package com.examplehub.designpatterns.singleton;
+
+public class SingletonExample4 {
+  private static SingletonExample4 INSTANCE;
+
+  private SingletonExample4(){
+  }
+
+  public static SingletonExample4 getInstance() {
+    if (INSTANCE == null) {
+      INSTANCE = new SingletonExample4();
+    }
+    return INSTANCE;
+  }
+}
diff --git a/src/main/java/com/examplehub/designpatterns/singleton/SingletonExample5.java b/src/main/java/com/examplehub/designpatterns/singleton/SingletonExample5.java
new file mode 100644
index 00000000..fe763f74
--- /dev/null
+++ b/src/main/java/com/examplehub/designpatterns/singleton/SingletonExample5.java
@@ -0,0 +1,19 @@
+package com.examplehub.designpatterns.singleton;
+
+public class SingletonExample5 {
+  private static SingletonExample5 INSTANCE;
+
+  private SingletonExample5(){
+  }
+
+  public static SingletonExample5 getInstance() {
+    if (INSTANCE == null) {
+      synchronized (SingletonExample5.class) {
+        if (INSTANCE == null) {
+          INSTANCE = new SingletonExample5();
+        }
+      }
+    }
+    return INSTANCE;
+  }
+}
diff --git a/src/main/java/com/examplehub/designpatterns/singleton/SingletonExample6.java b/src/main/java/com/examplehub/designpatterns/singleton/SingletonExample6.java
new file mode 100644
index 00000000..648725c1
--- /dev/null
+++ b/src/main/java/com/examplehub/designpatterns/singleton/SingletonExample6.java
@@ -0,0 +1,15 @@
+package com.examplehub.designpatterns.singleton;
+
+public class SingletonExample6 {
+
+  private SingletonExample6(){
+  }
+
+  private static final class InstanceHolder {
+    private static final SingletonExample6 INSTANCE = new SingletonExample6();
+  }
+
+  public static SingletonExample6 getInstance() {
+    return InstanceHolder.INSTANCE;
+  }
+}
diff --git a/src/test/java/com/examplehub/basics/chars/CharExampleTest.java b/src/test/java/com/examplehub/basics/chars/CharExampleTest.java
index 2a874099..7dcd997f 100644
--- a/src/test/java/com/examplehub/basics/chars/CharExampleTest.java
+++ b/src/test/java/com/examplehub/basics/chars/CharExampleTest.java
@@ -23,4 +23,13 @@ void testUnicode() {
     assertEquals('A', '\u0041');
     assertEquals('中', '\u4e2d');
   }
+
+  @Test
+  void testAddAndSub() {
+    assertEquals('9', '0' + 9);
+    assertEquals(43, '0' - 5);
+    assertEquals(32, 'a' - 'A');
+    assertEquals(9, '9' - '0');
+    assertEquals(0, 'a' - 97);
+  }
 }
diff --git a/src/test/java/com/examplehub/basics/chars/StringBuilderExampleTest.java b/src/test/java/com/examplehub/basics/chars/StringBuilderExampleTest.java
index 2ac20bba..a63fff58 100644
--- a/src/test/java/com/examplehub/basics/chars/StringBuilderExampleTest.java
+++ b/src/test/java/com/examplehub/basics/chars/StringBuilderExampleTest.java
@@ -11,4 +11,14 @@ void testAppend() {
     builder.append("Hello").append(",").append("World").append("!");
     assertEquals("Hello,World!", builder.toString());
   }
+
+  @Test
+  void testInitCapacity() {
+    String firstStr = "123456789";
+    String secondStr = "987654321";
+    String thirdStr = "abcef";
+
+    StringBuilder builder = new StringBuilder(firstStr.length() + secondStr.length() + thirdStr.length());
+    assertEquals("123456789987654321abcef", builder.append(firstStr).append(secondStr).append(thirdStr).toString());
+  }
 }
diff --git a/src/test/java/com/examplehub/basics/chars/StringExampleTest.java b/src/test/java/com/examplehub/basics/chars/StringExampleTest.java
index a3c1103f..3f701452 100644
--- a/src/test/java/com/examplehub/basics/chars/StringExampleTest.java
+++ b/src/test/java/com/examplehub/basics/chars/StringExampleTest.java
@@ -21,6 +21,14 @@ void testInit() {
     assertEquals("abc", abc);
   }
 
+  @Test
+  void testPlusOperator() {
+    assertEquals("Hi, Java", "Hi, " + "Java");
+    assertEquals("Hi 3", "Hi " + 3);
+    assertEquals("Hi 3.14", "Hi " + 3.14);
+    assertEquals("3Hi4", 3 + "Hi" + 4);
+    assertEquals("Hi10", "Hi" + 0xA);
+  }
   @Test
   void testEqual() {
     String s1 = "hello";
@@ -88,11 +96,9 @@ void testSubstring() {
   void testConcat() {
     String s1 = "hello";
     String s2 = " world!";
-    assertEquals("hello world!", s1 + s2);
     assertEquals("hello world!", s1.concat(s2));
-
     int age = 25;
-    assertEquals("I'm 25.", "I'm ".concat(25 + "."));
+    assertEquals("I'm 25.", "I'm ".concat(age + "."));
   }
 
   //    @Test
diff --git a/src/test/java/com/examplehub/basics/constant/FinalExampleTest.java b/src/test/java/com/examplehub/basics/constant/FinalExampleTest.java
new file mode 100644
index 00000000..f0d453c9
--- /dev/null
+++ b/src/test/java/com/examplehub/basics/constant/FinalExampleTest.java
@@ -0,0 +1,36 @@
+package com.examplehub.basics.constant;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class FinalExampleTest {
+  @Test
+  void test() {
+    final double PI = 3.1415927;
+    assertEquals(3.1415927, PI);
+
+    //PI = 3.14; // cannot assign a value to final variable PI
+  }
+
+  @Test
+  void testWithoutInit() {
+    final double PI;
+    //System.out.println(PI); // variable PI might not have been initialized
+  }
+
+  @Test
+  void testInitLater() {
+    final double PI;
+    PI = 3.1415927;
+    assertEquals(3.1415927, PI);
+  }
+
+  @Test
+  void testStatic() {
+     class MyMath {
+      public static final double PI = 3.1415927;
+    }
+    assertEquals(3.1415927, MyMath.PI);
+  }
+}
\ No newline at end of file
diff --git a/src/test/java/com/examplehub/basics/conversion/TypeConversionExampleTest.java b/src/test/java/com/examplehub/basics/conversion/TypeConversionExampleTest.java
new file mode 100644
index 00000000..a4c8be76
--- /dev/null
+++ b/src/test/java/com/examplehub/basics/conversion/TypeConversionExampleTest.java
@@ -0,0 +1,28 @@
+package com.examplehub.basics.conversion;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.function.Executable;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class TypeConversionExampleTest {
+  @Test
+  void testObjectConversion() {
+    class A {
+
+    }
+    class B {
+
+    }
+    // A a = new B(); // incompatible types: B cannot be converted to A
+
+    class Father {
+    }
+    class Son extends Father {
+    }
+    Father father = new Son(); // ok
+    assertThrows(ClassCastException.class, () -> {
+      Son son = (Son) new Father();
+    });
+  }
+}
\ No newline at end of file
diff --git a/src/test/java/com/examplehub/basics/function/ModifyFunctionArgsTest.java b/src/test/java/com/examplehub/basics/function/ModifyFunctionArgsTest.java
new file mode 100644
index 00000000..016f83a0
--- /dev/null
+++ b/src/test/java/com/examplehub/basics/function/ModifyFunctionArgsTest.java
@@ -0,0 +1,22 @@
+package com.examplehub.basics.function;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class ModifyFunctionArgsTest {
+  @Test
+  void test() {
+    int num = 1;
+    String str = "java";
+    Integer integer = 55;
+    int[] array = {1, 2, 3, 4, 5};
+    ModifyFunctionArgs.MyObj obj = new ModifyFunctionArgs.MyObj();
+    ModifyFunctionArgs.modify(num, str, integer, array, obj);
+    assertEquals(1, num);
+    assertEquals("java", str);
+    assertEquals(55, integer);
+    assertEquals(-1, array[0]);
+    assertEquals(1, obj.age);
+  }
+}
\ No newline at end of file
diff --git a/src/test/java/com/examplehub/designpatterns/singleton/SingletonExample1Test.java b/src/test/java/com/examplehub/designpatterns/singleton/SingletonExample1Test.java
new file mode 100644
index 00000000..3ee01ed2
--- /dev/null
+++ b/src/test/java/com/examplehub/designpatterns/singleton/SingletonExample1Test.java
@@ -0,0 +1,14 @@
+package com.examplehub.designpatterns.singleton;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class SingletonExample1Test {
+  @Test
+  void test() {
+    SingletonExample1 firstInstance = SingletonExample1.getInstance();
+    SingletonExample1 secondInstance = SingletonExample1.getInstance();
+    assertEquals(firstInstance.hashCode(), secondInstance.hashCode());
+  }
+}
\ No newline at end of file
diff --git a/src/test/java/com/examplehub/designpatterns/singleton/SingletonExample2Test.java b/src/test/java/com/examplehub/designpatterns/singleton/SingletonExample2Test.java
new file mode 100644
index 00000000..9bf71702
--- /dev/null
+++ b/src/test/java/com/examplehub/designpatterns/singleton/SingletonExample2Test.java
@@ -0,0 +1,14 @@
+package com.examplehub.designpatterns.singleton;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class SingletonExample2Test {
+  @Test
+  void test() {
+    SingletonExample2 firstInstance = SingletonExample2.getInstance();
+    SingletonExample2 secondInstance = SingletonExample2.getInstance();
+    assertEquals(firstInstance.hashCode(), secondInstance.hashCode());
+  }
+}
\ No newline at end of file
diff --git a/src/test/java/com/examplehub/designpatterns/singleton/SingletonExample3Test.java b/src/test/java/com/examplehub/designpatterns/singleton/SingletonExample3Test.java
new file mode 100644
index 00000000..99de9ba2
--- /dev/null
+++ b/src/test/java/com/examplehub/designpatterns/singleton/SingletonExample3Test.java
@@ -0,0 +1,14 @@
+package com.examplehub.designpatterns.singleton;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class SingletonExample3Test {
+  @Test
+  void test() {
+    SingletonExample3 firstInstance = SingletonExample3.INSTANCE;
+    SingletonExample3 secondInstance = SingletonExample3.INSTANCE;
+    assertEquals(firstInstance.hashCode(), secondInstance.hashCode());
+  }
+}
\ No newline at end of file
diff --git a/src/test/java/com/examplehub/designpatterns/singleton/SingletonExample4Test.java b/src/test/java/com/examplehub/designpatterns/singleton/SingletonExample4Test.java
new file mode 100644
index 00000000..97e2dae1
--- /dev/null
+++ b/src/test/java/com/examplehub/designpatterns/singleton/SingletonExample4Test.java
@@ -0,0 +1,19 @@
+package com.examplehub.designpatterns.singleton;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class SingletonExample4Test {
+  @Test
+  void test() {
+    SingletonExample4 firstInstance = SingletonExample4.getInstance();
+    SingletonExample4 secondInstance = SingletonExample4.getInstance();
+    assertSame(firstInstance, secondInstance);
+  }
+
+  @Test
+  void testMultiThreads() {
+    //TODO
+  }
+}
\ No newline at end of file
diff --git a/src/test/java/com/examplehub/designpatterns/singleton/SingletonExample5Test.java b/src/test/java/com/examplehub/designpatterns/singleton/SingletonExample5Test.java
new file mode 100644
index 00000000..40e7b846
--- /dev/null
+++ b/src/test/java/com/examplehub/designpatterns/singleton/SingletonExample5Test.java
@@ -0,0 +1,19 @@
+package com.examplehub.designpatterns.singleton;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class SingletonExample5Test {
+  @Test
+  void test() {
+    SingletonExample5 firstInstance = SingletonExample5.getInstance();
+    SingletonExample5 secondInstance = SingletonExample5.getInstance();
+    assertSame(firstInstance, secondInstance);
+  }
+
+  @Test
+  void testMultiThreads() {
+    //TODO
+  }
+}
\ No newline at end of file
diff --git a/src/test/java/com/examplehub/designpatterns/singleton/SingletonExample6Test.java b/src/test/java/com/examplehub/designpatterns/singleton/SingletonExample6Test.java
new file mode 100644
index 00000000..394615f3
--- /dev/null
+++ b/src/test/java/com/examplehub/designpatterns/singleton/SingletonExample6Test.java
@@ -0,0 +1,19 @@
+package com.examplehub.designpatterns.singleton;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class SingletonExample6Test {
+  @Test
+  void test() {
+    SingletonExample6 firstInstance = SingletonExample6.getInstance();
+    SingletonExample6 secondInstance = SingletonExample6.getInstance();
+    assertSame(firstInstance, secondInstance);
+  }
+
+  @Test
+  void testMultiThreads() {
+    //TODO
+  }
+}
\ No newline at end of file