Java Concurrency in Practice ：基础知识（正确的同步 -- 客户端加锁）

正确的同步 -- 客户端加锁

1、错误的示范

public class ListHepler<E>{
public List<E> list = Collections.synchronizedList(new ArrayList<E>);
public synchronized boolean putIfAbsent(E x){
boolean absent = !list.contains(x);
if(absent) list.add(x);
return absent;
}
}
}

2、正确的例子

public class ListHepler<E>{
public List<E> list = Collections.synchronizedList(new ArrayList<E>);
public boolean putIfAbsent(E x){
synchronized(list){
boolean absent = !list.contains(x);
if(absent) list.add(x);
return absent;
}
}
}

3、有啥区别？