[Android / Java] Understand the description type in listener settings (button installation)

Introduction

Various Listeners that appear in Android application development. There are two major patterns of writing in a java file, and it was confusing, so I will write it as a memorandum.

This sample is an application that displays a message when you press a button.

• Even if the button is a button, the method of installing the button (option menu) in the ActionBar is completely different, so please refer to this. https://qiita.com/kilalabu/items/62a39e63ee2c6d4293bd

Confirmation of terms

Event: An operation to be performed on the screen (example: click) Event handler ・ ・ ・ Processing to be performed for the event (Example: Display text when clicked) Listener: Something that detects an event (example: button)

code

The layout is suitable.

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">
    
    <TextView
        android:id="@+id/text"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/btn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="button"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toBottomOf="@id/text" />

</androidx.constraintlayout.widget.ConstraintLayout>

Pattern 1: Describe event & event handler and listener separately

MainActivity.java

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        
        //Listener setting (event detection button installed)
        Button button1 = findViewById(R.id.btn);
        button1.setOnClickListener(new TestListener());
    }

    //Creating a listener class(Click the button+Text display)
    private class TestListener implements View.OnClickListener {
        @Override
        public void onClick(View view) {
            TextView textView = findViewById(R.id.text);
            textView.setText("I clicked button1");
        }
    }
}

Pattern 2: Describe events & event handlers & listeners together

MainActivity.java

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        //Describe all together
        Button button2 = findViewById(R.id.btn);
        button2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                TextView textView = findViewById(R.id.text);
                textView.setText("I clicked button2");
            }
        });
    }
}

Finally

In Button ・ SetOnClickListener ・ View.OnClickListener ・ OnClick Since the flow is decided, recognize it as one set.

Postscript (2020/11/1)

I introduced two, but in View.OnClickListener, it seems to be set in the basic anonymous class. Therefore, pattern 1 should be used as a reference for understanding the structure!