update/add faq, add pic

doc/faq/faq_java.md

@@ -1,12 +1,56 @@
-# JAVA FAQ
+# Java FAQ
 
 ### 0) Ref
 
 - https://javaguide.cn/java/basis/java-basic-questions-01.html
 - https://cloud.tencent.com/developer/article/2183300
 
 
-### Java basic data type VS reference data type **** ?
+### 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">
+
+- basic data type : byte, short, int, long, float, dobule, char, boolean ...
+- reference data type : Class, interface, Array
+
+- Note :
+	- basic data type :
+		- "==" : compare attr (value)
+		- equals : compare attr (value)
+
+	- reference data type :
+		- "==" : compare storage address
+		- equals : 
+			- NOT override : Still compare storage address (same as "==")
+			- override : compare attr (value)
+		- all class are child class of Object class
+
+	```java
+	// java
+	public boolean equals(Object obj) {
+     return (this == obj);
+	}
+	```
+
+	```java
+	// java
+
+	// new Class : use a new storage in heap (內存)
+    String a = new String("123");
+    String b = new String("123");
+
+    System.out.println(a==b); // false ( "new" will use new address in internal storage)
+
+	// without new : will fetch from constant pool
+    String c = b;
+
+    System.out.println(c==b); // true ( c will fetch from constant pool (常量池))
+	```
+
+- 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 ?
 
 ### Exception VS Error ?
 

doc/faq/java_multi_thread.md

@@ -0,0 +1,7 @@
+# Java Multi-thread FAQ
+
+- https://javaguide.cn/java/concurrent/java-concurrent-questions-01.html
+- https://javaguide.cn/java/concurrent/java-concurrent-questions-02.html
+- https://javaguide.cn/java/concurrent/java-concurrent-questions-03.html
+
+### Java multi thread ?

leetcode_java/src/main/java/dev/workSpace2.java

@@ -12,12 +12,22 @@ public static void main(String[] args) {
 //
 //        }
 
-        String x = "abcd";
-        System.out.println(x.substring(1,3));
-        StringBuilder sb = new StringBuilder(x.substring(1,3));
-        System.out.println(sb.reverse());
+//        String x = "abcd";
+//        System.out.println(x.substring(1,3));
+//        StringBuilder sb = new StringBuilder(x.substring(1,3));
+//        System.out.println(sb.reverse());
+//
+//        System.out.println(x.substring(x.length()-1, x.length()));
+
+
+        String a = new String("123");
+        String b = new String("123");
+
+        System.out.println(a==b); // false ( "new" will use new address in internal storage)
+
+        String c = b;
 
-        System.out.println(x.substring(x.length()-1, x.length()));
+        System.out.println(c==b); // true ( c will fetch from constant pool (常量池))
 
     }