Veamos un poco de código

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />


public class MainActivity extends AppCompatActivity {

    private Button button;
    private ImageView iv;
    final int PERMISSION_REQUEST_EXTERNAL_STORAGE =100;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        button = findViewById(R.id.boton);
        iv = findViewById(R.id.imagen);


        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                if(ContextCompat.checkSelfPermission(getApplicationContext(), Manifest.permission.WRITE_EXTERNAL_STORAGE) == PackageManager.PERMISSION_DENIED) {

                        ActivityCompat.requestPermissions(MainActivity.this, new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE}, PERMISSION_REQUEST_EXTERNAL_STORAGE);

                }else{
                    share();
                }


            }
        });


    }

    @Override
    public void onRequestPermissionsResult(int requestCode, String[] permissions,  int[] grantResults) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
        if(requestCode == PERMISSION_REQUEST_EXTERNAL_STORAGE){
            if (grantResults[0] == PackageManager.PERMISSION_GRANTED){
                share();
            } else {
                Toast.makeText(this, "Until you grant the permission, we cannot save the photo", Toast.LENGTH_SHORT).show();
            }
        }
    }
    public void share(){
       ............
    }
}