单例模式

public class test_test {
    private test_test(){}
    private static final test_test instance=new test_test();
    public static test_test getInstance(){
        return instance;
    }

}

public class test_test{
    private test_test(){}
    private volatile static  test_test instance=null;
    public  static  test_test getInstance(){
        if(instance==null){
            synchronized(test_test.class){
                if(instance==null){
                    return new test_test();                
                }
            }            
        }
    }    
}

public class test_test{
    private test_test(){}
    //Java的静态内部类
    private static class test_test setInstance(){
        private static final test_test instance=new test_test();        
    }
    public static  test_test getInstance(){
        return setInstance.instance;
    }
}