Value "null" by default in String

I have开发者_C百科 this class

public class Model {    
    private String a;
    private String b;

    public synchronized String getA() {
        return a;
    }
    public synchronized void setA(String a) {
        this.a = a;
    }
    public synchronized String getB() {
        return b;
    }
    public synchronized void setB(String b) {
        this.b = b;
    }   
}

I try to get the value of a, and I know that by default this variable is initialize to null. But, is it possible that if I call the getA() method, afterwards this variable has the String "null" on it (not null but the String)? So a.equals("null") == true.

public static void main(String[] args) throws ParseException {      
Model m = new Model();
String test = m.getA();
m.getA().equals("null");//No Exception occurs

}

And in fact the code where I eval the String is part of an Android Application:

    mAirline = (Airline) extras.getSerializable("airline");     
    @SuppressWarnings("unused")
    String test = mAirline.getPhone(); //(1)
    String test2 = mAirline.getHref(); //(2)

If I check mAirline in (1) mAirline has it fields in null, but in (2) has some of them to "null" And my method for get is

    public synchronized String getPhone() {
    return phone;
}

No, with the code you showed us, it's not possible that you automatically get the String "null".

Note, however, that some methods will convert null to "null". The most notable examples are PrintWriter.println() (as in System.out.println()) and String.valueOf().

System.out.println(null);
System.out.println("null".equals(String.valueOf(null)));

These line will print null (i.e. the 4 characters) and true respectively

Maybe just:

private String a = "null";

Why don't you just check if the string is not null?

Model m = new Model();
String test = m.getA();
if(test==null) {
  //is null
} else {
  //is not null
}

You can also alter the getter method, so it returns your default value if the field is not initialized:

public class Model {    
    private String a;

    public synchronized String getA() {
        //if a is null, return "null", else return a
        return a==null?"null":a;
    }

}

The method String.valueOf() returns "null" if the argument is null or the argument itself if it is a String that is different from null.

    Model m = new Model();
    String test = m.getA();
    if (String.valueOf(a).equals("null")) //No Exception occurs

but this is kind of cryptic, pretty hard to understand what you want to do.
Check for null directly, much easier to read:

    Model m = new Model();
    String test = m.getA();
    if (a == null || a.equals("null")) //No Exception occurs

So you mean, the value of a is the string null, rather than a pointer with value null? That well never happen, unless you set it like that using setA("null");.

If you want a to be initialized as null, write a constructor:

public Model() {
    a = "null";
}

This will fix the problem for you. If the variable is null you will return "null" and if not you will return the value.

public class Model {    
    private String a;
    private String b;

    public synchronized String getA() {
        return (if (a ==null) ? "null" : a);
    }
    public synchronized void setA(String a) {
        this.a = a;
    }
    public synchronized String getB() {
        return (if (b ==null) ? "null" : b);
    }
    public synchronized void setB(String b) {
        this.b = b;
    }   
}