Java字符串的默认值：null与“”

Question

The default value of a Java String is 'null'. However, when I instantiate the string, it seems to be an empty string instead. Can somebody explain this please ?

class Stuff
{
    String a;
    String b = new String();
}

class Demo
{
    public static void main( String[] args )
    {
       Stuff s = new Stuff();
       System.out.println( s.a );
       System.out.println( s.b );

       if( s.b.equals( "" ) )
       {
            System.out.println( "true" );
       }
    }
}

Answer 1

The String() constructor Javaodc says,

Initializes a newly created String object so that it represents an empty character sequence. Note that use of this constructor is unnecessary since Strings are immutable.

Which matches your observed behavior.

Answer 2

In java a default string or a string set as null has no value. By setting a string with "" or with new String() it will create a new instance of the String class. A string which is initiated with "" or new String() is just a String ready to be used. I guess you can think of it as a cup. If something is null you have no cup, but if something, such as this string, is initiated as new String() or "" you now have an empty cup.

Answer 3

Well, the string b is not null anymore because you created a String and assigned it to the variable.

If you use the default constructor of String , you will get an empty String :

public String()

Initializes a newly created String object so that it represents an empty character sequence. Note that use of this constructor is unnecessary since Strings are immutable.

http://docs.oracle.com/javase/6/docs/api/java/lang/String.html#String()