3

Person.java

public class Person {
    public String firstName, lastName;

    public Person(String firstName,
            String lastName) {
        this.firstName = firstName;
        this.lastName = lastName;
    }

    public String getFullName() {
        return(firstName + " " + lastName);
    }
}

PersonTest.java

public class PersonTest {
    public static void main(String[] args) {
        Person[] people = new Person[20];              //this line .
        for(int i=0; i<people.length; i++) {
            people[i] = 
                new Person(NameUtils.randomFirstName(),
                        NameUtils.randomLastName());  //this line
        }
        for(Person person: people) {
            System.out.println("Person's full name: " +
                    person.getFullName());
        }
    }
}

In above code, we used twice "new". Is this code is correct or wrong? First one is for allocation of array. But why the second one? It's from lecture notes.

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3
  • The second "new Person" is correct, assuming you want to insert a new Person with a random first and last name into every entry of the array "people." As far as posting code, it's usually easy to copy-paste the code and for each line that isn't previewed as code you can insert four spaces. I hope this addressed your questions, I wasn't entirely sure what you meant. Commented Apr 6, 2012 at 13:52
  • (There's a help button in the editor (?, top right) that has examples of how to format code. Simply put, select all your code and click on the {} button, or hit Ctrl+K) Commented Apr 6, 2012 at 13:54
  • ok i got how it does. -> after selecting the text ,do ctrl k . thanks.. Commented Apr 6, 2012 at 14:09

3 Answers 3

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10

Yes, it is correct.

The line:

Person[] people = new Person[20]

allocates the array, full of references to null while the line:

new Person(NameUtils.randomFirstName(),
                      NameUtils.randomLastName());  //this line

fills it [the array] by instantiating objects of type Person, and assigning the reference in the array.

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2 Comments

Ok.Then "new" keyword is not only used for allocation.It can be used for calling constructors without allocation.(because allocation may be done previously)
No, new handles both allocation and construction. Arrays don't have constructors, but new Person[20] both allocates and initializes the array.
10

new Person[20] creates an array that can hold 20 references to Person objects. It does not create any actual Person objects.

new Person(...) creates a Person object.

The critical distinction to make here is that unlike in C or C++, new Person[20] does not allocate memory for 20 Person objects. The array does not contain the actual objects; it only contains references to them.

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1 Comment

In Java, allocation is not handled separately from construction. new Person[20] allocates the memory for an array of 20 references, and then initializes it as a Person[]. new Person(...) allocates the memory for a Person object and then calls the Person constructor.
2 
Person[] people = new Person[20];

only allocates memory for objects Person (filled with nulls). Then you need to fill it with particular Persons (with random name and surname int this example).

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