(Android) 按下后退时，尝试在空对象引用上调用虚拟方法“int java.lang.String.hashCode()”

Posted 2023-04-18

【中文标题】(Android) 按下后退时，尝试在空对象引用上调用虚拟方法“int java.lang.String.hashCode()”

【英文标题】：(Android) When Back pressed, Attempt to invoke virtual method 'int java.lang.String.hashCode()' on a null object reference

【发布时间】：2022-01-04 16:48:48

【问题描述】：

我一直在做一个 android 项目（一个基本的大学信息应用程序），我有三个主要活动，用户进入就像，Goal/Course Select activity-> CollegeList activity-> College Details Activity。现在，当我按回 College Details Activity 时，它会因以下错误而崩溃：

尝试在空对象引用上调用虚拟方法“int java.lang.String.hashCode()”

以下是我认为可能导致此问题的文件/代码.....

CollegeListActivity.java 文件

package com.anurag.college_information.activities;

import static com.anurag.college_information.activities.CareerGoalActivity.GOAL;
import static com.anurag.college_information.activities.CareerGoalActivity.SHARED_PREFS;

import android.app.AlertDialog;
import android.content.DialogInterface;
import android.content.Intent;
import android.content.SharedPreferences;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

import androidx.appcompat.app.AppCompatActivity;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import com.anurag.college_information.R;
import com.anurag.college_information.adapters.RecyclerAdapter;
import com.anurag.college_information.models.ModelClass;

import java.util.ArrayList;
import java.util.List;

public class CollegeListActivity extends AppCompatActivity 

    private RecyclerAdapter.RecyclerViewClickListener listener;

    //ListView collegeList;
    TextView collegeListTitle;
    Button courseChange;
    RecyclerView recyclerView;
    LinearLayoutManager LayoutManager;
    RecyclerAdapter recyclerAdapter;
    List<ModelClass> cList;

    String courseName;

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

        collegeListTitle = findViewById(R.id.college_list_title);
        courseChange = findViewById(R.id.btn_change_course);

        //collegeListTitle.setText(goal + "Colleges");

        collegeListTitle.setText(getIntent().getStringExtra("Title") + " Colleges");

        initData();
        initRecyclerView();

        //collegeList = findViewById(R.id.lv_college_list);


        courseChange.setTransformationMethod(null);
        courseChange.setOnClickListener(new View.OnClickListener() 
            @Override
            public void onClick(View v) 

                SharedPreferences sharedPreferences = getSharedPreferences(SHARED_PREFS, MODE_PRIVATE);
                SharedPreferences.Editor editor = sharedPreferences.edit();
                editor.clear();
                editor.apply();

                Intent i = new Intent(CollegeListActivity.this, CareerGoalActivity.class);
                startActivity(i);
                //finish();

            
        );

    

    private void initData() 
        cList = new ArrayList<>();
        courseName = getIntent().getStringExtra("Title");

        switch (courseName) 

            case "BE/BTech":
                cList.add(new ModelClass("https://images.static-collegedunia.com/public/college_data/images/campusimage/1479294300b-5.jpg", "A.P. Shah College of Engineering", "Thane", "8.0"));
                break;

            case "Pharmacy":
                cList.add(new ModelClass("https://images.static-collegedunia.com/public/college_data/images/campusimage/14382400753.jpg", "Bombay College Of Pharmacy", "Mumbai", "9.0"));
                break;
        
    

    private void initRecyclerView() 
        setOnClickListener();

        recyclerView = findViewById(R.id.recycler_view);
        LayoutManager = new LinearLayoutManager(this);
        LayoutManager.setOrientation(RecyclerView.VERTICAL);
        recyclerView.setLayoutManager(LayoutManager);
        recyclerAdapter = new RecyclerAdapter(cList, listener);
        recyclerView.setAdapter(recyclerAdapter);
    

    private void setOnClickListener() 
        listener = new RecyclerAdapter.RecyclerViewClickListener() 
            @Override
            public void onClick(View v, int position) 
                Intent i = new Intent(CollegeListActivity.this, CollegeDetailsActivity.class);
                startActivity(i);
            
        ;
    

    @Override
    public void onBackPressed() 
        new AlertDialog.Builder(this)
                .setMessage("Are you sure you want to exit?")
                .setPositiveButton("Yes", new DialogInterface.OnClickListener() 
                    public void onClick(DialogInterface dialog, int id) 
                        finish();
                    
                )
                .setNegativeButton("No", null)
                .show();
    

RecyclerAdapter.java

package com.anurag.college_information.adapters;

import android.content.Context;
import android.content.Intent;
import android.telecom.Call;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageView;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.recyclerview.widget.RecyclerView;

import com.anurag.college_information.activities.CollegeDetailsActivity;
import com.anurag.college_information.models.ModelClass;
import com.anurag.college_information.R;
import com.squareup.picasso.Picasso;

import java.util.List;

public class RecyclerAdapter extends RecyclerView.Adapter<RecyclerAdapter.ViewHolder> 


    private List<ModelClass> collegeList ;
    private RecyclerViewClickListener listener;

    List<String> imageUrl, collegeName, collegeLocation, collegeRating;


    public RecyclerAdapter(List<ModelClass> collegeList, RecyclerViewClickListener listener)
        this.collegeList=collegeList;
        this.listener = listener;

    

    @NonNull
    @Override
    public RecyclerAdapter.ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) 
        View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.college_list_single_row, parent, false);
        return new ViewHolder(view);
    

    @Override
    public void onBindViewHolder(@NonNull RecyclerAdapter.ViewHolder holder, int position) 

        String imageLink = collegeList.get(position).getImageLink();
        //int img = collegeList.get(position).getCollegeImage();
        String cName = collegeList.get(position).getCollegeName();
        String cRating = collegeList.get(position).getCollegeRating();
        String cLocation = collegeList.get(position).getLocation();
        Picasso.get().load(imageLink).into(holder.imageView);

        //holder.setData(img, cName, cRating);
        holder.setData(imageLink, cName, cLocation ,cRating);

    

    @Override
    public int getItemCount() 
        return collegeList.size();
    

    public class ViewHolder extends RecyclerView.ViewHolder implements View.OnClickListener 

        private ImageView imageView;
        private TextView collegeName, collegeRating, collegeLocation;


        public ViewHolder(@NonNull View itemView) 
            super(itemView);

            imageView = itemView.findViewById(R.id.college_image);
            collegeName = itemView.findViewById(R.id.college_name);
            collegeRating = itemView.findViewById(R.id.college_rating);
            collegeLocation = itemView.findViewById(R.id.college_location);

            itemView.setOnClickListener(this);

        

        public void setData(String imageLink, String cName, String cLocation, String cRating) 

            //imageView.setImageResource(img);
            Picasso.get().load(imageLink).error(R.drawable.error).into(imageView);
            collegeName.setText(cName);
            collegeRating.setText(cRating);
            collegeLocation.setText(cLocation);


        

        @Override
        public void onClick(View v) 
            listener.onClick(v, getAdapterPosition());

            Intent i = new Intent(v.getContext(), CollegeDetailsActivity.class);

            i.putExtra("collegeImage", collegeList.get(getAdapterPosition()).getImageLink());
            i.putExtra("collegeName", collegeList.get(getAdapterPosition()).getCollegeName());
            i.putExtra("collegeRating", collegeList.get(getAdapterPosition()).getCollegeRating());
            i.putExtra("collegeLocation", collegeList.get(getAdapterPosition()).getLocation());

            v.getContext().startActivity(i);

        

    
    public interface RecyclerViewClickListener
        void onClick(View v, int position);
    

CollegeDetailsActivity.java

package com.anurag.college_information.activities;

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

import androidx.appcompat.app.AppCompatActivity;

import com.anurag.college_information.R;
import com.squareup.picasso.Picasso;

public class CollegeDetailsActivity extends AppCompatActivity 

    Button btnApply;
    ImageView dCollegeImage;
    TextView dCollegeName, dCollegeRating, dCollegeLocation;

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

        dCollegeImage = findViewById(R.id.details_college_image);
        dCollegeName = findViewById(R.id.details_college_name);
        dCollegeRating = findViewById(R.id.details_college_rating);
        dCollegeLocation = findViewById(R.id.details_college_location);

        btnApply = findViewById(R.id.btn_apply);

        Intent i = getIntent();
        String cn = i.getStringExtra("collegeName");
        String cr = i.getStringExtra("collegeRating");
        String ci = i.getStringExtra("collegeImage");
        String cl = i.getStringExtra("collegeLocation");

        Picasso.get().load(ci).error(R.drawable.error).into(dCollegeImage);
        dCollegeName.setText(cn);
        dCollegeRating.setText(cr);
        dCollegeLocation.setText(cl);


        btnApply.setOnClickListener(new View.OnClickListener() 
            @Override
            public void onClick(View v) 
                Toast.makeText(getApplicationContext(), "The institute will be notified, of your application", Toast.LENGTH_LONG).show();
                Toast.makeText(getApplicationContext(), "The college will contact you, Thank you", Toast.LENGTH_LONG).show();
            
        );
    

    @Override
    public void onBackPressed() 

        Intent i = new Intent(CollegeDetailsActivity.this, CollegeListActivity.class);
        startActivity(i);
    

这是错误截图：

我对 android 还很陌生，我只参与了 4 到 5 个项目， 任何帮助将不胜感激，谢谢

注释掉的代码，只是我实现的一个普通列表视图，以防万一，我必须删除回收器视图。

【参考方案1】：

如果您不在CollegeDetailsActivity 中覆盖onBackPressed，这可能会消失。您发布的代码不会返回具有有效“标题”字符串的活动，而是转到未定义“标题”的新活动，然后得到 NullPointerException，因为courseName 在@987654325 中将为空@（错误消息告诉您在该方法的第 81 行导致错误）。在开关中使用空字符串会导致that type of error

只需在CollegeDetailsActivity 中完全删除您的onBackPressed。

【讨论】：

所以，现在应用程序没有崩溃，但是，按 back 一次显示相同的CollegeDetailsActivty，没有任何数据，只是一个空白布局，有所有视图，但是没有数据。有解决办法吗？编辑：当按两次返回时，它会进入上一个屏幕

您有两个地方可以启动详细信息活动 - 一个在 RecyclerAdapter 中，另一个在点击侦听器中。您将启动详细信息活动两次，因此按“返回”将返回其中的第一个。你会想要解决这个问题，所以它只启动一次。删除 CollegeListActivity 中的一组 - 它不会将任何参数传递给新活动，或者修复那个并且不在 RecyclerAdapter 中启动意图（无论哪种方式 - 只是不要做两次）