String testStr = null;

            System.out.println(testStr);

            if (testStr == null) {

                System.out.println("testStr == null");  // 能正常打印

            }

            if (testStr.equals("null")) {

                System.out.println("testStr.equals null ");   //Exception in thread "main" java.lang.NullPointerException

            }   

           String testStr="";

            System.out.println("aaaa"+testStr);  //能打印

            if (testStr == null) {

                System.out.println("testStr == null");  //不能打印

            }

            if (testStr.equals("null")) {

                System.out.println("testStr.equals null ");  ////不能打印

            }    

　　　　　　　　System.out.println("length:"+testStr.length());   // 打印length:0