Java Programming for Android Developers For Dummies, 2nd Edition
Book image
Explore Book Buy On Amazon
Objects can be a very useful tool in your Android app. Check out the code below which illustrates Java in action. This code contains real-life Java code to create two objects.

package com.allmycode.a09_02;

import android.support.v7.app.AppCompatActivity;

import android.os.Bundle;

import android.widget.TextView;

import com.allmycode.a09_01.BagOfCheese;

public class MainActivity extends AppCompatActivity {

TextView textView;

@Override

protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);

textView = (TextView) findViewById(R.id.textView);

BagOfCheese bag1 = new BagOfCheese();

bag1.kind = "Cheddar";

bag1.weight = 2.43;

bag1.daysAged = 30;

bag1.isDomestic = true;

BagOfCheese bag2 = new BagOfCheese();

bag2.kind = "Blue";

bag2.weight = 5.987;

bag2.daysAged = 90;

bag2.isDomestic = false;

textView.setText("");

textView.append(bag1.kind + ", " + bag1.weight + ", " +

bag1.daysAged + ", " + bag1.isDomestic + "\n");

textView.append(bag2.kind + ", " + bag2.weight + ", " +

bag2.daysAged + ", " + bag2.isDomestic + "\n");

}

}

A run of the code is shown here.

java-programming-for-android-developers-2e-running-code
Running the code.

The code above creates “two BagOfCheese objects” or “two BagOfCheese instances,” or you might say that the new BagOfCheese() statements instantiate the BagOfCheese class. One way or another, the code declares the existence of one class and declares another class — a class that declares the existence of two objects.

Each use of the words new BagOfCheese() is a constructor call.

The code uses ten statements to create two bags of cheese. The first statement (BagOfCheese bag1 = new BagOfCheese()) does three things:

  • With the words

BagOfCheese bag1

the first statement declares that the variable bag1 refers to a bag of cheese.

  • With the words

new BagOfCheese()

the first statement creates a bag with no particular cheese in it. (If it helps, you can think of it as an empty bag reserved for eventually storing cheese.)

  • Finally, with the equal sign, the first statement makes the bag1 variable refer to the newly created bag.

The next four statements assign values to the fields of bag1:

bag1.kind = "Cheddar";

bag1.weight = 2.43;

bag1.daysAged = 30;

bag1.isDomestic = true;

To refer to one of an object's fields, follow a reference to the object with a dot and then the field's name. (For example, follow bag1 with a dot and then the field name kind.)

The next five statements do the same for a second variable, bag2, and a second bag of cheese.

About This Article

This article is from the book:

About the book author:

Barry Burd, PhD, is a professor in the Department of Mathematics and Computer Science at Drew University in Madison, New Jersey. He has lectured at conferences in the United States, Europe, Australia, and Asia. He hosts podcasts and videos about software and other technology topics. He is the author of many articles and books, including Java For Dummies.

This article can be found in the category: