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Android Build a Simple Android App Improving Our Code Dynamically Changing the Background Color

Posted by Malcolm Mutambanengwe
Malcolm Mutambanengwe
4,205 Points

java.lang.NullPointerException (Look around MealActivity.java line 16)

What does java.lang.NullPointerException mean and how can I make the background color green?

MealActivity.java 
import android.os.Bundle;
import android.widget.TextView;

public class MealActivity extends Activity {

  private RelativeLayout mRelativeLayout;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_meal);

        TextView foodLabel = (TextView) findViewById(R.id.foodTextView);
        TextView drinkLabel = (TextView) findViewById(R.id.drinkTextView);
        RelativeLayout mealLayout = (RelativeLayout) findViewById(R.id.mealLayout);
        mRelativeLayout.setBackgroundColor(Color.GREEN);
    }
}

2 Answers

Steve Hunter
Steve Hunter
57,712 Points

Hi Malcolm,

Technically, what Gunjeet has posted is the reason for your NullPointerException. However, you need to reverse the variable names to pass the challenge as the challenge provides you with a RelativeLayout called mealLayout. The test is expecting the background colour of a variable called mealLayout to equal Color.GREEN.

You need to add one line only:

    public void onCreate(Bundle savedInstanceState) {
        // provided code
        .
        .
        RelativeLayout mealLayout = (RelativeLayout) findViewById(R.id.mealLayout); // provided
        // your line below
        mealLayout.setBackgroundColor(Color.GREEN);
     }

I hope that helps,

Steve.

Malcolm Mutambanengwe
Malcolm Mutambanengwe
4,205 Points

Hi Steve. finally understood what was needed. Thanks for your help. The coding force is strong in you, jedi Steve.

Steve Hunter
Steve Hunter
57,712 Points

A challenging language, Android is. :+1: :smile:

Gunjeet Hattar
Gunjeet Hattar
14,483 Points
Gunjeet Hattar
Gunjeet Hattar
14,483 Points

Hi Malcom,

Let me try and help you out on this.
So if you look closely the relativeLayout has already been defined at the top with the name mRelativeLayout 

private RelativeLayout mRelativeLayout;

Then at the bottom you're trying to define it agin using a different name called mealLayout

RelativeLayout mealLayout = (RelativeLayout) findViewById(R.id.mealLayout);

Therefore when you call this line

mRelativeLayout.setBackgroundColor(Color.GREEN);

the compiler gets confused because you've initialised mealLayout and no mRelativeLayout. Since mRelativeLayout has not been initialised but only declared it gives you a null pointer exception

Therefore as a solution 

RelativeLayout mealLayout = (RelativeLayout) findViewById(R.id.mealLayout);
//instead of this this line type
mRelativeLayout = (RelativeLayout) findViewById(R.id.mealLayout);

// now when you call 
mRelativeLayout.setBackgroundColor(Color.GREEN);

// it will refer to the correct relative layout

Hoep that helps. Feel free to post again if you need any help

Malcolm Mutambanengwe
Malcolm Mutambanengwe
4,205 Points

Thanks Gunjeet. It's making sense, I finally got it. thanks. Learning programming is like learning a new language, literary.