[JAVA] About the basics of Android development

Introduction

Hello. I'm Wataku, a server-side programmer who is studying programming at a certain school. : relaxed: Let's develop Android this time as well. I received a request this time, so I will answer the request and write about the basics of Android development in an easy-to-understand manner.

Target person

--A person who can write Java somehow. --People who want to develop Android. --Beginner in Android development.

Do not write this time

--About the installation of Android Studio.

What to make

When you enter a name and press the button, the one entered in the display area below and the character string "-san, nice" are displayed.

screen

I will make a screen with an xml file

activity_button_click_sample.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/tv_name" />

    <EditText
        android:id="@+id/etName"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:inputType="text"/>

    <Button
        android:id="@+id/btClick"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/bt_click"/>

    <TextView
        android:id="@+id/tvOutput"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="25dp"
        android:text=""
        android:textSize="25dp"/>

</LinearLayout>

Commentary

tag

--LinearLayout: Layout used when arranging parts vertically or horizontally. --TextView: Create a display area --EditText: Create an input area --Button: Make a button

attribute

Common attributes

①android:layout_width="〜〜〜"
②android:layout_height="〜〜〜"

① Width attribute ② Height attribute

A value that goes into ~ ~ ~

--match_parent: Expand to fill the screen. --wrap_content: It will be displayed in an appropriate size.

LinearLayout attributes

android:orientation="〜〜〜"

A value that goes into ~ ~ ~

--vertical: Arrange vertically --horizontal: side by side

Attributes used other than LinearLayout

android:text="value value"
* Usually "string".Specify the content to be output in "xml" and read the image from there

Example

android:text="@string/bt_click"

android:id="@+id/ID (name) for acquiring parts in the activity"
→ R value: The file in the res folder and the "" of that file@ + id/Since the value of "" is a management target, it is an int type integer that identifies the file or value.
It is automatically generated.

Example

android:id="@+id/tvOutput"

Attributes used in EditText

android:inputType="〜〜〜"

~ ~ ~: Type of inputType

** ** none Cannot be entered. text Ordinary text. textCapCharacters When typing in all uppercase letters. textCapWords When entering the beginning of a word in uppercase. textCapSentences When entering the beginning of a sentence in uppercase. textAutoCorrect To automatically correct the character input. textAutoComplete When completing characters. textMultiLine When entering multiple lines of characters. textImeMultiLine When multiple input is not allowed during normal character input and multiple line input is set by IME. When entering the textUri URL. textEmailAddress When entering an email address. textEmailSubject When entering the subject of an email. textShortMessage When entering a short message. textLongMessage When entering a long message. textPersonName When entering a person's name. textPostalAddress When entering an address. textPassword When entering a password. textVisiblePassword When entering the password characters by showing them. textWebEditText When entering HTML. textFilter Enter characters filtered by other data. textPhonetic When entering phonetic symbols. number When entering a numerical value. numberSigned When entering a signed number. numberDecimal When entering decimal numbers. phone When entering a phone number. datetime When entering a date time. date When entering a date. time When entering the time.

Activity (processing)

I will write it in java.

ButtonClickSampleActivity.java

public class ButtonClickSampleActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_button_click_sample);

        Button button = findViewById(R.id.btClick);
        ButtonClickListener listener = new ButtonClickListener();
        button.setOnClickListener(listener);
    }

    /**
     *A member class that describes what happens when a button is pressed
     */
    private class ButtonClickListener implements View.OnClickListener {

        @Override
        public void onClick(View view) {
            EditText input = findViewById(R.id.etName);
            String inputStr = input.getText().toString();

            TextView output = findViewById(R.id.tvOutput);
            output.setText(inputStr + "San, nice! !!" );
        }
    }
}

Commentary

Inherit AppCompatActivity.

onCreate () method

• A method that launches an Android app. * → Write what is necessary for initial display etc. in this method. However, the following two lines are required

• super.onCreate(savedInstanceState); --setContentView (screen xml file);

• These two lines are written, but I will write them because they are uniform.

Acquisition of screen parts

Use ** findViewById () ** and specify the R value (ID attached to the part) of the part as an argument.

EditText input = findViewById(R.id.etName)

Get a string

Use ** getText (). ToString () **.

input.getString().toString()

Embed a string

Use ** setText ("embed string") **.

TextView output = findViewById(R.id.tvOutput);//Parts acquisition
output.setText(inputStr + "San, nice! !!" );

Event listener

--Event: The user performs something on the screen. --Event handler: Processing performed in response to an event. --Listener: Validate this event.

• On Android, event processing does not work unless this listener is set.

Listener setting procedure

(1) Create a listener class corresponding to each event as a member class. (2) Write the process in the method defined in the interface. ③ Set the listener by "new" the listener class.

Example) (button click listener)

Button button = findViewById(R.id.btClick);
ButtonClickListener listener = new ButtonClickListener();
button.setOnClickListener(listener);

private class ButtonClickListener implements View.OnClickListener {

        @Override
        public void onClick(View view) {
            EditText input = findViewById(R.id.etName);
            String inputStr = input.getText().toString();

            TextView output = findViewById(R.id.tvOutput);
            output.setText(inputStr + "San, nice! !!" );
        }
}

You can also write using anonymous functions (reference books are often written in this way). What they are doing is the same.

findViewById(R.id.btClick).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                EditText input = findViewById(R.id.etName);
                String inputStr = input.getText().toString();

                TextView output = findViewById(R.id.tvOutput);
                output.setText(inputStr + "San, nice! !!" );
            }
});

that's all. It was the basis of Android development. If you have any suggestions such as something wrong, please contact us. Thank you for reading to the end.