The stuffs that I didn’t know before are

• Double brace initialization

	Map map=new HashMap<String,String>() {
		{
			put("key1","value1");
			put("key2","value2");
		}
	};

• Instance initializer

public class Initializer {
	static {
		System.out.println("static");
	}
	{
		System.out.println("instance");
	}
	public Initializer() {
		System.out.println("constructor");
	}
}

• Covariant return type

	abstract class Base {
		abstract List getList();
	}

	class Sub extends Base {
		@Override
		ArrayList getList() {
			return null;
		}
	}

Double brace initialization uses instance initializer because first brace is for declairing annomymous inner class and second brace is for instance initializer block.

http://www.infoq.com/presentations/pampuch-vm-optimizations-language-designers

I wondered under what conditions do profilers detect deadlock and then report it, so I wrote code that reaches deadlock.

package com.grayger.deadlock;

public class SimpleDeadlocker {
	private final static Object OBJ1=new Object();
	private final static Object OBJ2=new Object();

	private final static int LOOP=100;

	public void obj1First() {
		synchronized(OBJ1) {
			printInfo("Hold OBJ1");
			synchronized(OBJ2) {
				printInfo("Hole OBJ2");
			}
			printInfo("Release OBJ2");
		}
		printInfo("Release OBJ1");
	}

	public void obj2First() {
		synchronized(OBJ2) {
			printInfo("Hold OBJ2");
			synchronized(OBJ1) {
				printInfo("Hole OBJ1");
			}
			printInfo("Release OBJ1");
		}
		printInfo("Release OBJ2");
	}

	private void printInfo(String info) {
		System.out.println(Thread.currentThread().getName()+" "+info);
	}

	public static void main(String args[]) {
		final SimpleDeadlocker d=new SimpleDeadlocker();
		Thread t1=new Thread(new Runnable() {
			public void run() {
				for(int i=0;i<loop;i++)>
					d.obj1First();
				}
				d.printInfo("Terminated");
			}
		}, "OBJ1_First");

		Thread t2=new Thread(new Runnable() {
			public void run() {
				for(int i=0;i<loop;i++)>
					d.obj2First();
				}
				d.printInfo("Terminated");
			}
		}, "OBJ2_First");

		t1.start();
		t2.start();
	}
}

Above example can be refactored as follows:

package com.grayger.deadlock;

public class Deadlocker {
	private final static int LOOP=100;

	static class Deadlock1 {
		private final static Object OBJ1=new Object();
		private final static Object OBJ2=new Object();		

		public void obj1First() {
			synchronized(OBJ1) {
				printInfo("Hold OBJ1");
				synchronized(OBJ2) {
					printInfo("Hole OBJ2");
				}
				printInfo("Release OBJ2");
			}
			printInfo("Release OBJ1");
		}
	}

	static class Deadlock2 {
		private final static Object OBJ1=new Object();
		private final static Object OBJ2=new Object();

		public void obj2First() {
			synchronized(OBJ2) {
				printInfo("Hold OBJ2");
				synchronized(OBJ1) {
					printInfo("Hole OBJ1");
				}
				printInfo("Release OBJ1");
			}
			printInfo("Release OBJ2");
		}
	}

	public static void printInfo(String info) {
		System.out.println(Thread.currentThread().getName()+" "+info);
	}

	public static void main(String args[]) {
		final Deadlock1 d1=new Deadlock1();
		final Deadlock2 d2=new Deadlock2();
		Thread t1=new Thread(new Runnable() {
			public void run() {
				for(int i=0;i<loop;i++)>
					d1.obj1First();
				}
				Deadlocker.printInfo("Terminated");
			}
		}, "OBJ1_First");

		Thread t2=new Thread(new Runnable() {
			public void run() {
				for(int i=0;i<loop;i++)>
					d2.obj2First();
				}
				Deadlocker.printInfo("Terminated");
			}
		}, "OBJ2_First");

		t1.start();
		t2.start();
	}
}

Unfortunately I couldn’t find any document on the change from java.sun.com site. Missing this issue from the release note of Java6 seems to be a mistake as http://blogs.sun.com/ahe/entry/override_snafu says.

However it is not a pioneeering book like DP of GoF; it doesn’t address something really new. That makes me a little disppointed because I have expected it would tell me lots of stuffs that I didn’t realize before. It is like a well-documented wikipedia in that sense.

The title of book need to be changed to “Implementation patterns in Java”. It is not just because example code of the book is written in Java but because some topics are very specific to Java so that these topics are not concerns of Non-Java developers.

It is becoming a trend.

* OverrideBothEqualsAndHashcode
* FinalizeDoesNotCallSuperFinalize
* ConstructorCallsOverridableMethod
* ProperCloneImplementation
* CloneThrowsCloneNotSupportedException
* CloneMethodMustImplementCloneable
* ReplaceVectorWithList
* ReplaceHashtableWithMap
* UseArrayListInsteadOfVector
* ReturnFromFinallyBlock
* EmptyCatchBlock
* ExceptionAsFlowControl