findViewById

• 보통 View의 속성을 정의할때 ID도 작성한 후 onCreate 메소드에서 해당 View를 찾기위해 사용.!
• [예제] id 정의 후 MainActivity.java 또는 kotlin에서 찾아와서 Toast로 띄우기

xml

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">

<Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Button"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.498"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.805" />
     
</androidx.constraintlayout.widget.ConstraintLayout>

java

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        
        findViewById(R.id.button).setOnClickListener(v -> 
                Toast.makeText(getApplicationContext(), "toast", Toast.LENGTH_LONG).show()
        );
    }
}

kotlin

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.widget.Button
import android.widget.Toast

class MainActivity : AppCompatActivity(){

    override fun onCreate(savedInstanceState: Bundle?) { 
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        
        /* id를 찾아온 뒤 클릭리스너를 통해 토스트 창 띄우기 */
        findViewById<Button>(R.id.button).setOnClickListener {
            Toast.makeText(applicationContext, "toast", Toast.LENGTH_LONG).show()
        }
    }

}

결과