Assign a value to a String variable in a String array

Question

I am trying to assign values to String variables within an array:

String details = "passFirst0,passLast0,Molly,Quinn";
Passenger passenger = new Passenger(details);

public class Passenger {

    private String firstNameField;
    private String lastNameField;
    private String firstName;
    private String lastName;

    public Passenger(String details) {
        String[] temp = details.split(",");
        String[] fields = {firstNameField, lastNameField, firstName, lastName};

        for (int ctr = 0; ctr < fields.length; ctr++) {
            fields[ctr] = temp[ctr];
        }

        // Print instance variables - all null
        System.out.println(this.firstNameField);
        System.out.println(this.lastNameField);
        System.out.println(this.firstName);
        System.out.println(this.lastName);

        // Print array - has values
        System.out.println(Arrays.toString(fields));
    }

    // Methods
}

However, the instance variables themselves remain null, while the fields[] has values when you iterate through the array.

Why is this so and how to accomplish this?

It's an array of strings, not an array of string variables. You can't have an array of variables. — user2357112
– user2357112, Commented Apr 30, 2017 at 6:00

Kamil Piwowarski · Accepted Answer · 2017-04-30 05:58:34Z

3

String objects are immutable - you cannot change their value - while doing assignments you are changing object to which the variable (or variable at given array index) is referring to.

When you do:

String[] fields = {firstNameField, lastNameField, firstName, lastName};

you set reference of fields array value with index 0 to same object that firstNameField is referring to (in this case null), index 1 to refer to same object as lastNameField, etc.

Then, if you do:

fields[ctr] = temp[ctr];

you are not changing value of the object fields[ctr] was referring earlier (one of your instance variables) to value of temp[ctr], but rather you are setting fields[ctr] to refer to the same object temp[ctr] is referring to right now.

If you want to refer to your variables as an array, why won't you just declare them as an array from the beginning?

answered Apr 30, 2017 at 5:58

Kamil Piwowarski

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2 Comments

k_rollo Over a year ago

Upvoted. Let's just say the details is being read from an external file. :)

k_rollo Over a year ago

While Andreas' solution is a close contender, I'm choosing this as accepted answer for detailed explanation of "why". Thanks everyone!

Andreas · Accepted Answer · 2017-04-30 05:52:15Z

I would advise against this, but you could use Java 8 method references:

public class Passenger {

    private String firstNameField;
    private String lastNameField;
    private String firstName;
    private String lastName;

    public Passenger(String details) {
        List<Consumer<String>> setters = Arrays.asList(this::setFirstNameField,
                                                       this::setLastNameField,
                                                       this::setFirstName,
                                                       this::setLastName);
        String[] temp = details.split(",");
        for (int i = 0; i < setters.size(); i++) {
            setters.get(i).accept(temp[i]);
        }
    }

    public String getFirstNameField() {
        return this.firstNameField;
    }

    public void setFirstNameField(String firstNameField) {
        this.firstNameField = firstNameField;
    }

    public String getLastNameField() {
        return this.lastNameField;
    }

    public void setLastNameField(String lastNameField) {
        this.lastNameField = lastNameField;
    }

    public String getFirstName() {
        return this.firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return this.lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    @Override
    public String toString() {
        return "Passenger [firstNameField=" + this.firstNameField +
                        ", lastNameField=" + this.lastNameField +
                        ", firstName=" + this.firstName +
                        ", lastName=" + this.lastName + "]";
    }

}

Test

String details = "passFirst0,passLast0,Molly,Quinn";
Passenger passenger = new Passenger(details);
System.out.println(passenger);

Output

Passenger [firstNameField=passFirst0, lastNameField=passLast0, firstName=Molly, lastName=Quinn]

user7391763 · Accepted Answer · 2017-04-30 05:36:10Z

They are null because they haven't been set, you can set them like this;

public class Passenger {

private String firstNameField = "a";
private String lastNameField = "b";
private String firstName = "c";
private String lastName "d";

Or with arguments to the constructor like this;

public Passenger(String details, String f, String l, String fn, String ln) {

   firstNameField = a;
   lastNameField = l;
   firstName = fn;
   lastName = ln;

   .....
}

As stated in the comments, you do overwrite the array values within the for loop, but you try to print out class members;

System.out.println(this.firstNameField);
System.out.println(this.lastNameField);
System.out.println(this.firstName);
System.out.println(this.lastName);

which are uninitialised.

Why not set the array like below;

String[] temp = details.split(",");
    String[] fields = {temp[0], temp[1], temp[2], temp[3]};

And why bother with the other variables, just create getters for each needed variable, eg;

public String getFirstName() {
   return fields[2];
}

Raz Zelinger · Accepted Answer · 2017-04-30 05:37:29Z

0

in "fields" you have the values of the split of "details" that you set in the for loop and in the instance variables you have nothing (They weren't initialized with data)

edited Apr 30, 2017 at 5:37

answered Apr 30, 2017 at 5:33

Raz Zelinger

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7 Comments

Turing85 Over a year ago

While this is true, this does not explain the full problem since OP overwrites the array fields within the for loop.

Raz Zelinger Over a year ago

exactly, Strings are not an objects, it's a reference to an object

Raz Zelinger Over a year ago

you may try to use reflection

Turing85 Over a year ago

@silver ther is no "easy solution" for this. There are ways (or more like hacks) to do so, e.g. reflection mentioned by Raz) but they are not worth it.

Andreas Over a year ago

@silver No, and why do you need to? What's so difficult about writing firstNameField = temp[0]; lastNameField = temp[1]; firstName = temp[2]; lastName = temp[3];? It's really not that much more code than you have.

|

k_rollo · Accepted Answer · 2017-04-30 06:37:39Z

0

As for the "how", I ultimately went with Andreas' first comment:

public Passenger(String details) {
    String[] temp = details.split(",");

    this.firstNameField = temp[0];
    this.lastNameField = temp[1];
    this.firstName = temp[2];
    this.lastName = temp[3];
}

Realizing the simplest approach ain't so bad.

answered Apr 30, 2017 at 6:37

k_rollo

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