2011年12月31日
the super keyword in java generic programming is used to declare a template of a specific type. e.g.
List<?
super MyClass> list =
new LinkedList<AnyTypeDerivedByMyClass>();
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2012-12-31 15:35 Sam Zheng 閱讀(245) |
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編輯 收藏int (*test(char *(*)(void)) (int); declares a function named "test" which accepts a pointer to a function accepting no parameter and returning a char pointer, the function "test" returns a pointer to a function which accepts a integer parameter and returns a integer.
Note: a declaration declares either a variable(pointer) or function(pointer), if its name is followed by left parentheses, it is a function, otherwise it is a variable(pointer).
posted @
2012-09-20 14:38 Sam Zheng 閱讀(129) |
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編輯 收藏if you don't feel comfortable with your code, it's time to refactor/re-think about it.
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2012-08-31 11:36 Sam Zheng 閱讀(161) |
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don't get trapped when reading source code of a complex module, first figure out what the module does/its main functionality and its interfaces, once you get into details, you cannot see the module as a whole and forget its main functionality, if you recognize you are trapped in details, try to come out and focus on interfaces, top->down->top->down
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2012-08-15 16:07 Sam Zheng 閱讀(156) |
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The crucial part in design/programming, is to break down complex things, anything should be easy when broken down to manageable pieces, and, only when broken down, can it be easy. when doing so, the most important thing is to think about the granularity and the relationship between every pieces/modules, again, reasonability.
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2012-04-18 12:10 Sam Zheng 閱讀(147) |
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when analyzing/programming, think about reasonability against requirements
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2012-04-17 12:55 Sam Zheng 閱讀(315) |
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1. what does the program/module/class/method do?
2. what is its input, if any?
3. what is its output, if any?
for a class, understanding its lifecycle is crucial:
1. how is it created? by who? from a factory? what are the parameters required to create it?
2. does it have any enforced lifecycle methods? e.g. initialize, dispose..., who is responsible to call these methods, in which order?
3. what it can do at each stage of its lifecycle?
4. who is going to use it/what is its client?
posted @
2012-03-09 10:52 Sam Zheng 閱讀(238) |
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An interface or a class should only do things that it knows, never do things it is not sure about. to define the responsibility of an interface/class clearly and precisely is critical.
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2012-02-27 14:12 Sam Zheng 閱讀(171) |
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The main difference between SoftReference and WeakReference is that GC uses different algorithm to determine when to reclaim the objects they refer to, they can be used interchangablely in most situation.
// soft reference
o = new Object();
ReferenceQueue<Object> sq = new ReferenceQueue<Object>();
SoftReference<Object> sf = new SoftReference<Object>(o, sq);
System.out.println(sf.get());
o = null;
System.gc();
r = (Reference) sq.poll();
if (r != null) {
System.out.println(r.get());
}
// weak reference
Object o = new Object();
ReferenceQueue<Object> q = new ReferenceQueue<Object>();
WeakReference<Object> w = new WeakReference<Object>(o, q);
System.out.println(w.get());
o = null;
System.gc();
Reference r = (Reference) q.poll();
if (r != null) {
System.out.println(r.get());
}
// phantom reference
o = new Object();
ReferenceQueue<Object> pq = new ReferenceQueue<Object>();
PhantomReference<Object> pf = new PhantomReference<Object>(o, sq);
System.out.println(pf.get());
o = null;
System.gc();
r = (Reference) pq.poll();
if (r != null) {
System.out.println(r.get());
}
from http://www.ibm.com/developerworks/library/j-refs/
The SoftReference class
A typical use of the SoftReference class is for a memory-sensitive cache. The idea of a SoftReference is that you hold a reference to an object with the guarantee that all of your soft references will be cleared before the JVM reports an out-of-memory condition. The key point is that when the garbage collector runs, it may or may not free an object that is softly reachable. Whether the object is freed depends on the algorithm of the garbage collector as well as the amount of memory available while the collector is running.
The WeakReference class
A typical use of the WeakReference class is for canonicalized mappings. In addition, weak references are useful for objects that would otherwise live for a long time and are also inexpensive to re-create. The key point is that when the garbage collector runs, if it encounters a weakly reachable object, it will free the object the WeakReference refers to. Note, however, that it may take multiple runs of the garbage collector before it finds and frees a weakly reachable object.
The PhantomReference class
The PhantomReference class is useful only to track the impending collection of the referring object. As such, it can be used to perform pre-mortem cleanup operations. A PhantomReference must be used with the ReferenceQueue class. The ReferenceQueue is required because it serves as the mechanism of notification. When the garbage collector determines an object is phantomly reachable, the PhantomReference object is placed on its ReferenceQueue. The placing of the PhantomReference object on the ReferenceQueue is your notification that the object the PhantomReference object referred to has been finalized and is ready to be collected. This allows you to take action just prior to the object memory being reclaimed.
posted @
2012-01-08 13:06 Sam Zheng 閱讀(129) |
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編輯 收藏Because some native platforms only provide limited buffer size for standard input and output streams, failure to promptly write the input stream or read the output stream of the subprocess may cause the subprocess to block, and even deadlock.
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2012-01-07 22:06 Sam Zheng 閱讀(377) |
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JAAS generally has the following two steps:
1, Authentication: define principals, implement LoginModule and callback handler, associate designated principals with the subject object in the commit method of LoginModule implementation, configure the module implementation in a login configuration file, i.e
Sample {
sample.module.SampleLoginModule required debug=true;
};
lc = new LoginContext("Sample", new MyCallbackHandler());
Subject s = lc.getSubject();
Subject.doAs(s, new PrivilegedAction(){...}); // the authenticated subject s will be associated with the current access control context.
2. Authorization: write policy file to grant principals access right to any critical resources. e.g.
grant
Principal sample.principal.SamplePrincipal "testUser" {
permission java.util.PropertyPermission "java.home", "read";
permission java.util.PropertyPermission "user.home", "read";
permission java.io.FilePermission "foo.txt", "read";
};
posted @
2011-12-31 14:32 Sam Zheng 閱讀(137) |
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