我怎么知道一个类的实例是否已经存在于内存中？

Question

我怎么知道一个类的实例是否已经存在于内存中？

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我的问题是,如果存在Class的实例,这是我的代码,不想读取方法

private void jButton (java.awt.event.ActionEvent evt) {
    PNLSpcMaster pnlSpc = new PNLSpcMaster();
    jtabbedPanel.addTab("reg",pnlSpc);
}

我想检查实例PNLSpcMaster当然我可以通过静态布尔检查,但我认为这种方式更好.

Answer 1

如果您只想拥有一个"PNLSpcMaster"实例,那么您需要一个单例:

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,Singleton Pattern非常有效.然而,对于更复杂的用法,它可能是危险的.

你可以阅读更多关于它的内容:为什么单身人士会引起争议