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Build a Simple Android App with Java

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Dynamically Changing the Background Color

3:51 with Ben Deitch

The last thing we need to do is pick a new background color each time we display a new fact. Try it yourself, then watch this video for my explanation.

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Android recently updated the findViewById method to be generic. This means, in most cases, that a cast (e.g. (RelativeLayout)) is no longer necessary. If you don't see an error when using findViewById, that just means you're on a newer version of Android than I am :)

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    We're almost done. 0:00
    The last thing we need to do is pick a new background color 0:01
    each time we display a new fun fact. 0:04
    Sounds simple enough, right? 0:07
    If you're feeling adventurous, why don't you pause me and try it yourself? 0:09
    Then come back and see how I do it. 0:13
    Here are the steps you need to do. 0:15
    Step 1, declare a field or member variable with a data type of relative layout. 0:17
    The format is just like our other View fields, but with a different name and 0:23
    data type. 0:27
    Step 2, Set this field in the onCreate method just like the other Views. 0:29
    You may need to add an ID to the RelativeLayout in the layout file. 0:33
    Step 3, where we set a new fun fact, 0:38
    call the 'setBackgroundColor()' method on this new variable. 0:40
    Use Color.RED as the parameter. 0:44
    All right, did you get it working? 0:48
    Let's switch to fun facts activity and start from the top. 0:50
    Declare a field with the data type of RelativeLayout. 0:54
    Let's add a new line below our showFactButton declaration and 0:57
    type private RelativeLayout and let's name it RelativeLayout. 1:01
    Okay, on to step two. 1:07
    Set this field and the onCreate method. 1:09
    Let's add a new line below where we initialized showFactButton. 1:12
    And then type relativeLayout =, 1:16
    then a cast to a relativeLayout, followed by findViewById. 1:20
    And then in parenthesis, type R.ID. 1:28
    And that's right, we haven't given our relative layout an ID yet. 1:32
    If we had, we'd be able to see that ID in this list. 1:37
    Let's open our layout file, so open the project pane, Res Directory, 1:41
    layout Activity fun facts and then I'll close this directory again. 1:45
    And inside this first relative layout tag, let's add an ID property. 1:52
    Let's add a line after padding and 1:57
    type android:id= and then in quotation marks, 2:02
    "@+id/relativeLayout with a lowercase r. 2:08
    Remember, we could have also set the id from the design tab 2:15
    by selecting the relativeLayout from the component tree and 2:18
    then setting the id in the property pane. 2:21
    Looks good. 2:23
    Let's go back to our activity and reference our new id. 2:24
    Let's hit Ctrl+Space to bring autocomplete back up. 2:27
    And then select Relative Layout. 2:31
    And add a semicolon. 2:35
    So much for step two. 2:38
    Next up, step three, where we set a new fun fact called the setback 2:39
    onColor method on our new variable using color.red as the parameter. 2:44
    This is referring to the onClick method. 2:49
    At the bottom, let's add a reference to our new variable, RelativeLayout, 2:52
    and then using auto complete as our guide let's add a dot and type sbc. 3:01
    That's right, just typing the first letter of each word in a method 3:08
    is another way to let auto complete know what we're looking for. 3:12
    So let's select the setBackgroundColor method, and 3:16
    it takes an int color as a parameter. 3:19
    For this example, we're just using a system color. 3:22
    Inside the parentheses, let's type the class name, 3:25
    color with a capital C, and then add .capital RED. 3:29
    All right, let's give it a shot. 3:33
    Now when we first tap the button, it should change the background color to red. 3:38
    And it does, cool. 3:44
    Let's pause for a moment and then we will make some changes to use a new color for 3:46
    each fact. 3:50

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