Some Java knowledge points

1 Importjava.util.ArrayList;2 3  Public classMain {4     5      Public Static voidMain (string[] args) {6StringBuffer buffer =NewStringBuffer ("Hello World."));7         8StringBuffer Buffer_other =buffer;9         TenBuffer.append ("Me too."); One          A         if(Buffer = =buffer_other) { -System.out.println ("OK.")); -         } the          -          -         //buffer.append (null); Error -String str =NULL; + System.out.println (Buffer.append (str)); -         //output: Hello world.me Too.null When the string is null, StringBuffer's append () converts null to a string null plus at the end +          Aarraylist<string> list =NewArraylist<>(); at List.add (str); -System.out.println (list.get (0));//output is null -List.add (NULL); -System.out.println (List.get (1));//output is null -          -         /********************************/ inString string =NULL; -String string2 =NULL; toArraylist<string> ArrayList =NULL; +         if(String = =string2) { -System.out.println ("Y");//Output y the         } *         if(String = =NULL) { $System.out.println ("Y");//Output yPanax Notoginseng         } -         if(NULL==NULL) { theSystem.out.println ("Y");//Output y +         } A         //String = = ArrayList is a syntax error the          +         //if (string.equals (string2)) {//NullPointerException -         //System.out.println ("Y"); $         //} $          -     } -}

Some Java knowledge points