update faq

doc/faq/faq_java.md

@@ -6,6 +6,19 @@
 - https://cloud.tencent.com/developer/article/2183300
 
 
+### Java memoery model
+
+### a++ VS ++a ?
+
+- a++ : assign a first, then do a += 1
+- ++a : do a += 1 first, then assign value
+
+```java
+// java
+b = a++;
+b = ++a;
+```
+
 ### Java basic data type (基本數據類型) VS reference data type (引用數據類型) **** ?
 
 <img src ="https://github.com/yennanliu/CS_basics/blob/master/doc/pic/basic_ref_data_type.jpeg" width="500" height="300">
@@ -25,6 +38,8 @@
 			- override : compare attr (value)
 		- all class are child class of Object class
 
+	- Note : some reference class' equals method already overriden (e.g. String), so its equals method already compare attr
+
 	```java
 	// java
 	public boolean equals(Object obj) {
@@ -50,7 +65,89 @@
 - https://blog.csdn.net/weixin_45658089/article/details/120248191
 - https://javaguide.cn/java/basis/java-basic-questions-02.html#object-%E7%B1%BB%E7%9A%84%E5%B8%B8%E8%A7%81%E6%96%B9%E6%B3%95%E6%9C%89%E5%93%AA%E4%BA%9B
 
-### String、StringBuffer、StringBuilder ?
+
+### Java boxing (裝箱) VS unboxing (拆箱) ?
+
+- boxing : basic data type -> reference type
+- unboxing : reference type -> basic data type
+
+```java
+// java
+Integer i = 1; // boxing
+int n = i; // unboxing
+```
+
+### String constant pool ?
+
+- Avoid repeatedly create same String, for better performance
+
+```java
+// java
+// 在堆中创建字符串对象”ab“
+// 将字符串对象”ab“的引用保存在字符串常量池中
+String aa = "ab";
+// 直接返回字符串常量池中字符串对象”ab“的引用
+String bb = "ab";
+System.out.println(aa==bb);// true
+```
+
+### 成員變量 (member variable) VS 局部變量 (local variable) ?
+
+- member variable
+	- belong to class
+
+- local variable
+
+```java
+// java
+public class VariableExample {
+
+    // 成员变量
+    private String name;
+    private int age;
+
+    // 方法中的局部变量
+    public void method() {
+        int num1 = 10; // 栈中分配的局部变量
+        String str = "Hello, world!"; // 栈中分配的局部变量
+        System.out.println(num1);
+        System.out.println(str);
+    }
+
+    // 带参数的方法中的局部变量
+    public void method2(int num2) {
+        int sum = num2 + 10; // 栈中分配的局部变量
+        System.out.println(sum);
+    }
+
+    // 构造方法中的局部变量
+    public VariableExample(String name, int age) {
+        this.name = name; // 对成员变量进行赋值
+        this.age = age; // 对成员变量进行赋值
+        int num3 = 20; // 栈中分配的局部变量
+        String str2 = "Hello, " + this.name + "!"; // 栈中分配的局部变量
+        System.out.println(num3);
+        System.out.println(str2);
+    }
+}
+
+```
+
+### String VS StringBuffer VS StringBuilder ?
+
+- String : 
+	- unchangeable
+	- thread safety, constant
+
+- StringBuilder :
+	- parent class : AbstractStringBuilder
+	- thread unsafety
+	- use case : single thread
+
+- StringBuffer :
+	- parent class : AbstractStringBuilder
+	- thread safety (lock)
+	- use case : multi thread
 
 ### Exception VS Error ?