add demo for constant pool

add demo for fix deadlock

src/main/java/jvm/ConstantPoolDemo.java

@@ -0,0 +1,14 @@
+package jvm;
+
+public class ConstantPoolDemo {
+
+    public static void main(String[] args) {
+        //this will be equal since it is first time to create
+        String greeting = new StringBuilder("Hello ").append("Jason").toString();
+        System.out.println(greeting == greeting.intern());
+
+        //This will not be equal since it is already created before and cached in constant pool
+        String java = new StringBuilder("ja").append("va").toString();
+        System.out.println(java == java.intern());
+    }
+}

src/main/java/threaddemo/DeadLockFixDemo.java

@@ -0,0 +1,41 @@
+package threaddemo;
+
+/**
+ * fix deadlock by keeping the lock in the same order
+ */
+public class DeadLockFixDemo {
+    private static final Object resource1 = new Object();
+    private static final Object resource2 = new Object();
+    public static void main(String[] args) {
+        new Thread(() -> {
+            synchronized (resource1) {
+                System.out.println(STR."\{Thread.currentThread()}get resource1");
+                try {
+                    Thread.sleep(1000);
+                } catch (InterruptedException e) {
+                    e.printStackTrace();
+                }
+                System.out.println(STR."\{Thread.currentThread()} waiting get resource2");
+                synchronized (resource2) {
+                    System.out.println(STR."\{Thread.currentThread()} get resource2");
+                }
+            }
+        },"t1").start();
+
+        new Thread(() -> {
+            synchronized (resource1) {
+                System.out.println(Thread.currentThread() + "get resource1");
+                try {
+                    Thread.sleep(1000);
+                } catch (InterruptedException e) {
+                    e.printStackTrace();
+                }
+
+                System.out.println(Thread.currentThread() + "waiting get resource2");
+                synchronized (resource2) {
+                    System.out.println(Thread.currentThread() + "get resource2");
+                }
+            }
+        },"t2").start();
+    }
+}