片段的引用未分配给myfragment

Question

import android.app.Fragment;
import android.app.FragmentManager;
import android.app.FragmentTransaction;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

public class MainActivity extends AppCompatActivity {

    Button but1,but2;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        but1 = findViewById(R.id.button1);
        but2 = findViewById(R.id.button2);
    }
    public void setfrag(View but)
    {
        Fragment fragment;
        if (but == findViewById(R.id.button1)) {
            fragment = new Fragment1();
            FragmentManager fm = getFragmentManager();
            FragmentTransaction ft = fm.beginTransaction();
            ft.add(R.id.fragment1, fragment);
            ft.commit();
        }
        if (but == findViewById(R.id.button2)) {
            fragment = new Fragment2();
            FragmentManager fm = getFragmentManager();
            FragmentTransaction ft = fm.beginTransaction();

            ft.commit();
        }
    }
}

我正在尝试在一个活动中的两个片段之间交换，但是当我尝试执行此操作时，我收到一个错误，我无法将Fragment类的引用分配给我所拥有的片段的对象板条箱