Mar 29, 2021 -- Notes

IT 372 -- Apr 3, 2024

Review Exercises

What are some Android API version names?
Answer: Here is a list of these version names, named after deserts or sweets:
apilevels.com
For what are XML files used in Android Studio projects?
Ans: Resource files and layout files. They are also used for the Manifest, which we will look at later.
For what is Java used in Android Studio projects?
Ans: Event handlers, which are methods that are executed by the operating system when the event to which the method is connected occurs. Some common events are Create, Click, LongClick, FocusChange, Destroy.
Java Question: what is the difference between a static method and an instance method?
Ans: A static method is called from a class; an instance method is called from an object. For example, the main method in a traditional Java class is always static because it is called initially before any objects have been created. The String format method is static:
```
String s = String.format("Name:%s; Age:%d", name, age); 
```
The String toUpperCase method is an instance method:
```
String t = s.toUpperCase( );
```
How does the toString method work for a class? What does @Override do?
Ans: The toString method for a class returns a String representation of an object. @Override means that the method is overriding a method in the base class of the current class.

Create an array that contains these strings:

ibm  oracle  microsoft  google

Print the array using a modern for loop.

 
// Answer:
String[ ] names = {"ibm", "oracle", "microsoft", "google"};
for(String names : name) {
    System.out.println(name);
}

Create an ArrayList collection that contains the same strings as the array in Problem 6. Then print the arraylist. Ans:

ArrayList<String> names2 = new ArrayList<String>( );
for(String name : names) {
    names2.add(t);
}
System.out.println(names2);

Recreate the TextDisplay app from April 1; call your app Prob9. Change the layout to LinearLayout and set the text in the TextView widget to "Hello, my name is <your name>" :
```
android:text="Hello, my name is Stan."
```
Then make these changes to the app:
1. Change the title of the TextDisplay Example from "Prob9" to "TextDisplay Example":
  In the file app/src/main/res/values/strings.xml, change the app_name like this:
```
<string name="app_name">TextDisplay Example</string>
```
2. What does the XML attribute android:gravity mean in the layout file? Answer:
```
android:gravity="center"
```
  means that items in the layout will be centered vertically and horizontally.
```
android:gravity="center_horizontal"
```
  means center horizontally but not vertically.
```
android:gravity="center_vertical"
```
  means center vertically but not horizontally.
3. What do the XML attribute values match_parent and wrap_content do? Answer:
```
android:layout_width:match_parent
android:layout_height:match_parent
```
  means that the width and height of the item will expand as much as possible inside the parent layout, which for our example is the linear layout.
```
android:layout_width:wrap_content
android:layout_height:wrap_content
```
  means that the width and height of the item will shink to the height and width of its contents.
4. Change the background color of the layout (android:background) to yellow. Answer:
```
android:background="#FFFF00"
```
  sets the background color to yellow.
5. Change the foreground color (android:textColor) TextView widget to teal (dark bluegreen). Answer:
```
android:textColor="#008080
```

Android Studio Folder Structure

Find these folders in an Android Studio project:
```
app  src  main  java  res
```
Find these files in an Android Studio project:
```
MainActivity.java  activity_main.xml
```

XML Primer

Device Independent Pixels

Because Android devices have a variety of screen sizes and resolutions, using absolute pixels as measure units doesn't work well. A device independent pixel (dp) is defined as 1/160 of an inch or 0.15875 mm. Use dp units to specify units of length in an XML layout file. The abbreviation for device independent pixel used to be dpi, but this abbreviation is now deprecated.
The font size is measured in spale-independent pixels, abbreviated as sp. One sp is the same as one dp = 1/160 of an inch. Use the sp unit to specify the android:textSize attribute. For example:
```
android:textSize="30sp"
```

Some LinearLayout and TextView Attributes

Recall the CSS Box Model. Android widgets also have properties that specify margin and padding.
Create a project named Test1 and change the layout file to:
activity_main.xml
Set the background color and text color of the textview by adding these attributes:
```
android:background="#FFFFEE"
android:textColor="#800000"
```
When we talked about using hex color codes in a layout file, we looked at using a color wheel to help us write the color codes. Here is a color wheel with hex color codes: Hex Color Codes.
Extended HTML Colors
To place the color constant in the colors.xml resource file, click on a hex color code and press Alt-Enter (Android Studio quick fix) and Extract Color Resource. Specify the names of the color constants and press okay. Do this for all the hex color codes in the layout file. Also, create a string constant for the two greetings. (Placing color and string constants in the resource files colors.xml and strings.xml is a best practice for Android projects. Here are the resulting XML files:
activity_main.xml colors.xml strings.xml.

Try out these attributes to modify the position of the LinearLayout widget:

layout_margin  layout_marginLeft  layout_marginRight
padding  paddingLeft  paddingRight

Add another textview to the layout. Add margin and padding attributes to position it.

Setting up a Click Event Handler

To set up a click event handler for the linear layout, add this attribute to the LinearLayout element:
```
android:onClick="changeBackground"
```

Then add this method to the activity file:

public void changeBackground(View view) {
    LinearLayout layout = findViewById(R.id.layout);
    layout.setBackgroundColor(Color.parseColor("black"));
}

Add a click event handler for the textview that changes the text to "Fine, thankyou. How are you?".

Projects 1a and 1b

Look again at Project 1a and Project 1b.