问题描述
我如何知道某个类的实例是否已经存在于内存中?
How can I know whether an instance of a class already exists in memory?
我的问题是'不要'如果存在类
的实例,则需要读取方法这是我的代码
My problem is that don't want read method if exist instance of Classthis is my code
private void jButton (java.awt.event.ActionEvent evt) {
PNLSpcMaster pnlSpc = new PNLSpcMaster();
jtabbedPanel.addTab("reg",pnlSpc);
}
我想检查 PNLSpcMaster 当然我可以通过静态布尔检查,但我认为这种方式更好。
I want check instance of
PNLSpcMaster
of course I can check by static boolean but I think this way is better.
推荐答案
如果你想只有一个PNLSpcMaster实例然后你:
If you want to have only one instance of "PNLSpcMaster" then you do need a singleton:
这是常见的单身成语:
public class PNLSpcMaster {
/**
* This class attribute will be the only "instance" of this class
* It is private so none can reach it directly.
* And is "static" so it does not need "instances"
*/
private static PNLSpcMaster instance;
/**
* Constructor make private, to enforce the non-instantiation of the
* class. So an invocation to: new PNLSpcMaster() outside of this class
* won't be allowed.
*/
private PNLSpcMaster(){} // avoid instantiation.
/**
* This class method returns the "only" instance available for this class
* If the instance is still null, it gets instantiated.
* Being a class method you can call it from anywhere and it will
* always return the same instance.
*/
public static PNLSpcMaster getInstance() {
if( instance == null ) {
instance = new PNLSpcMaster();
}
return instance;
}
....
}
用法:
private void jButton (java.awt.event.ActionEvent evt) {
// You'll get the "only" instance.
PNLSpcMaster pnlSpc = PNLSpcMaster.getInstace(); //<-- getInstance()
jtabbedPanel.addTab("reg",pnlSpc);
}
或直接:
private void jButton (java.awt.event.ActionEvent evt) {
jtabbedPanel.addTab("reg",PNLSpcMaster.getInstace());
}
对于基本用法,非常有效。但是对于更复杂的用法,它可能很危险。
For basic usages the Singleton Pattern works very well. However for more sophisticated usages it may be dangerous.
您可以阅读更多相关信息:
You could read more about it: Why singletons are controversial
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