(Android)FrameLayOut 버튼클릭 화면바꾸기

package com.example.my04_framelayout;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

    Button btnChange;
    ImageView imageView1,imageView2,imageView3;
    int selIdx=2;

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

        btnChange = findViewById(R.id.btnChange);

        imageView1=findViewById(R.id.imageView1);
        imageView2=findViewById(R.id.imageView2);
        imageView3=findViewById(R.id.imageView3);

        imageView1.setVisibility(View.VISIBLE);
        imageView2.setVisibility(View.GONE);
        imageView3.setVisibility(View.GONE);

        btnChange.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if(selIdx==1){
                    imageView1.setVisibility(View.VISIBLE);
                    imageView2.setVisibility(View.GONE);
                    imageView3.setVisibility(View.GONE);
                    selIdx=2;
                    Toast.makeText(MainActivity.this, "1번 선택됨", Toast.LENGTH_SHORT).show();
                }else if(selIdx==2){
                    imageView1.setVisibility(View.GONE);
                    imageView2.setVisibility(View.VISIBLE);
                    imageView3.setVisibility(View.GONE);
                    selIdx=3;
                    Toast.makeText(MainActivity.this, "2번 선택됨", Toast.LENGTH_SHORT).show();
                }else if(selIdx==3){
                    imageView1.setVisibility(View.GONE);
                    imageView2.setVisibility(View.GONE);
                    imageView3.setVisibility(View.VISIBLE);
                    selIdx=1;
                    Toast.makeText(MainActivity.this, "3번 선택됨", Toast.LENGTH_SHORT).show();
                }

            }
        });
    }
}