public static void main(String[] args) {
        Object msgObject=null; 
        String msgString="HelloWorld";
        Display(msgString); //永远返回false，不会改变msgString
        Display(msgObject); //永远返回false，不会改变msgObject
        System.out.println(msgString);// HelloWorld
        System.out.println(msgObject);//null
        System.out.println(msgString.toString());//HelloWorld
        System.out.println(msgObject.toString());//这个应该报空指针异常才对
}

------解决方案--------------------
Java code

public static void main(String[] args) {
Object msgObject=null; 
String msgString="HelloWorld";
msgString = Display(msgString);
Display(msgObject);
System.out.println(msgString);
System.out.println(msgObject);
System.out.println(msgString.toString());
System.out.println(msgObject.toString());
}

public static String Display(Object msg) {

return "error";
}

public static boolean Display(String msg) {
String mString=new String("error");
msg="error";
return false;
}

------解决方案--------------------
你return的是一个布尔值false。
你要想看到error这个字符串，可以上楼上一样。打印出来。或者函数的返回类型是String，return你的msg
Java code

public static String Display(Object msg) {

String mString=new String("error");
msg= mString;
return msg;
}

public static String Display(String msg) {
String mString=new String("error");
msg="error";
return msg;
}

------解决方案--------------------
public static void main(String[] args) {
Object msgObject=null;  
String msgString="HelloWorld";
Display(msgString);//想让msgString变化成error
Display(msgObject);//想让msgObject变化成error
System.out.println(msgString);
System.out.println(msgObject);
System.out.println(msgString.toString());
System.out.println(msgObject.toString());
}