creating an instance of a class type variable

Question

I'm new to OOP and I was wondering how to set something that is not like int, string, double etc.

I have two classes, Foo and Bar, and some instance variables How can I set the Bar type instance variables?

public class Foo
{
    //instance variables
    private String name;        
    Bar test1, test2;

    //default constructor
    public Foo()
    {
        name = "";
        //how would I set test1 and test 2?
    }
}

public class Bar
{
    private String nameTest; 

    //constructors
    public Bar()
    {
        nameTest = "";
    }
}

Bar test1 = new Bar(); Bar test2 = new Bar(); thats how you create an instance of Bar class. — Branislav Lazic
– Branislav Lazic, Commented Sep 6, 2012 at 1:13

Dave Newton · Accepted Answer · 2012-09-06 01:33:45Z

3

Same way as anything else, create an instance of a Bar and set it on the instance property.

You can create those instances in a constructor, pass them to a constructor, or set with a setter:

public class Foo {

    private Bar test1;

    public Foo() {
        test1 = new Bar();
    }

    public Foo(Bar bar1) {
        test1 = bar1;
    }

    public void setTest1(Bar bar) {
        test1 = bar;
    }

    public static void main(String[] args) {
        Foo f1 = new Foo();
        Foo f2 = new Foo(new Bar());
        f2.setTest1(new Bar());
    }

}

edited Sep 6, 2012 at 1:33

answered Sep 6, 2012 at 1:13

Dave Newton

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2 Comments

Dog Over a year ago

Just found a feature in Eclipse IDE that generates getters and setters for you. But it is good to know otherwise, thanks Dave and everybody else

Dave Newton Over a year ago

@user1615805 Correct, most IDEs will have various code templates, although that's not really related to the question. IIRC Eclipse also has a ctor generation that will take selected properties.

João Silva · Accepted Answer · 2012-09-06 01:13:07Z

3

You need to create a new instance of Bar, using the new operator, and assign them to your member variables:

public Foo() {
  name = "";
  test1 = new Bar();
  test2 = new Bar();
}

References:

answered Sep 6, 2012 at 1:13

João Silva

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Comments

jahroy · Accepted Answer · 2012-09-06 01:46:16Z

If you want to set a Bar in your default constructor, you'll have to instantiate it.

This is done using the new operator.

Bar someBar = new Bar();

You can also create constructors with parameters.

Here's how you would create a Bar constructor that takes a String as a parameter:

class Bar {

    private String name;

    public Bar(String n) {
        name = n;
    }

}

Here's how to use your new Bar constructor in Foo's default constructor:

class Foo {

    private String name;
    private Bar theBar;

    public Foo() {
        name = "Sam";
        theBar = new Bar("Cheers");
    }

}

To be even more clever, you could create a new Foo constructor that takes two parameters:

class Foo {

    private String name;
    private Bar theBar;

    public Foo(String fooName, String barName) {
        name = fooName;
        theBar = new Bar(barName);
    }  

}

mssb · Accepted Answer · 2012-09-06 04:13:04Z

1

Try this example

public class Person {

    private String name;
    private int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }        
    //getters and setters        
}


public class Student {

    private String school;
    private Person person;

    public Student(Person person, String school) {
        this.person = person;
        this.school = school;
    }        
    //code here
}

class Main {

   public static void main(String args[]) {
      Person p = new Person("name", 10);
      Student s = new Student(p, "uwu");
   }

}

String , Integer, Double etc. also classes like person

answered Sep 6, 2012 at 4:13

mssb

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