安卓的Bundle传递fragment数据报错，提示空指针异常

Question

我在尝试利用bundle，传递string数据给fragment，打印log的时候出错，提示我on a null object reference；

相关代码如下，MainAcitivity中：

package com.example.fragmentmanger;
import android.annotation.SuppressLint;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;

import androidx.appcompat.app.AppCompatActivity;
import androidx.fragment.app.Fragment;
import androidx.fragment.app.FragmentManager;
import androidx.fragment.app.FragmentTransaction;

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button btn1 = findViewById(R.id.btn1);
        Button btn2 = findViewById(R.id.btn2);
        Button btn3 = findViewById(R.id.btn3);
        btn1.setOnClickListener(this);
        btn2.setOnClickListener(this);
        btn3.setOnClickListener(this);
    }

    @SuppressLint("NonConstantResourceId")
    @Override
    public void onClick(View v) {
        switch (v.getId()) {
            case R.id.btn1:
                Log.e("Fragment1", "fragment1 click");
                Bundle bundle = new Bundle();
                bundle.putString("message", "i love android");
                BlankFragment1 bf1 = new BlankFragment1();
                bf1.setArguments(bundle);
                replaceFrag(new BlankFragment1());
                break;

            case R.id.btn2:
                Log.e("Fragment2", "fragment2 click");
                replaceFrag(new BlankFragment2());
                break;

            case R.id.btn3:
                Log.e("ItemFragment", "item fragment click");
                replaceFrag(new ItemFragment());
                break;

            default:
                break;
        }
    }

    /**
     * 动态切换fragment
     *
     * @param fragment
     */
    private void replaceFrag(Fragment fragment) {
        FragmentManager fragmentManager = getSupportFragmentManager();
        FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
        fragmentTransaction.replace(R.id.frameLayout1, fragment);
        fragmentTransaction.addToBackStack(null);
        fragmentTransaction.commit();
    }
}

---

在BlankFragment1中代码如下：

package com.example.fragmentmanger;

import android.os.Bundle;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

import androidx.fragment.app.Fragment;

/**
 * A simple {@link Fragment} subclass.
 * Use the {@link BlankFragment1#newInstance} factory method to
 * create an instance of this fragment.
 *
 * @author MAC
 */
public class BlankFragment1 extends Fragment {

    // TODO: Rename parameter arguments, choose names that match
    // the fragment initialization parameters, e.g. ARG_ITEM_NUMBER
    private static final String ARG_PARAM1 = "param1";
    private static final String ARG_PARAM2 = "param2";
    private static final String TAG = "BlankFragment1";

    // TODO: Rename and change types of parameters
    private String mParam1;
    private String mParam2;

    public BlankFragment1() {
        // Required empty public constructor
    }

    /**
     * Use this factory method to create a new instance of
     * this fragment using the provided parameters.
     *
     * @param param1 Parameter 1.
     * @param param2 Parameter 2.
     * @return A new instance of fragment BlankFragment1.
     */
    // TODO: Rename and change types and number of parameters
    public static BlankFragment1 newInstance(String param1, String param2) {
        BlankFragment1 fragment = new BlankFragment1();
        Bundle args = new Bundle();
        args.putString(ARG_PARAM1, param1);
        args.putString(ARG_PARAM2, param2);
        fragment.setArguments(args);
        return fragment;
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Bundle bundle = getArguments();
        String res = bundle.getString("message");
        Log.e(TAG, "onCreate:" + res);
        if (getArguments() != null) {
            mParam1 = getArguments().getString(ARG_PARAM1);
            mParam2 = getArguments().getString(ARG_PARAM2);
        }
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_blank1, container, false);
    }
}

Answer 1

你提交的是一个新的实例，当然空指针了