SharedPreferences 数据传输中遇到的一些问题总结

原构想：MainActivity 设置两个按钮，btn1——跳转Main2Activity通过复选框组选择并提交，btn2——跳转Main3Activity通过RecycleView显示选择结果。

主要经验：SharedPreferences 使用前要先声明，否则读取为Null；

1、Main2Activity.java

package com.example.newrecycleviewdate;

import androidx.appcompat.app.AppCompatActivity;

import android.content.Intent;
import android.content.SharedPreferences;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.CheckBox;

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

public class Main2Activity extends AppCompatActivity {
    private CheckBox chk;
    private Button gotobtn;
    private int[] id={R.id.ck1,R.id.ck2,R.id.ck3,R.id.ck4,R.id.ck5,R.id.ck6,R.id.ck7,R.id.ck8,R.id.ck9,R.id.ck10};   数组储存重复的复选框ID，减少键盘磨损~
    private List<String> list=new ArrayList<>();
    SharedPreferences.Editor editor;                  //Editer 需先声明，否则取出数据为Null;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main2);

        gotobtn=findViewById(R.id.gotobtn);

        gotobtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                for (int i :id){                                   //循环检查复选框状态，选中则加入数组中
                    chk = findViewById(i);
                    if (chk.isChecked())
                        list.add(chk.getText().toString());
                }
                editor = getSharedPreferences("nameList",MODE_PRIVATE).edit();

                editor.putInt("listsize",list.size());


                for (int i=0 ;i< list.size();i++){
                    editor.putString("item"+i,list.get(i));
               }


                editor.commit();
                startActivity(new Intent(Main2Activity.this,MainActivity.class));
            }
        });




    }
}

2、Main3Activity

package com.example.newrecycleviewdate;

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

import android.content.SharedPreferences;
import android.os.Bundle;
import android.widget.TextView;

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

public class Main3Activity extends AppCompatActivity {
    private RecyclerView resultRy;
    private List<String> data = new ArrayList<>();
    private int count;
    //private String[] st={"strind","33fi","33022","dvikeke","88vj3k"};
    SharedPreferences myNum;                //SharedPreferences 先声明，否则出现取出数据为空的情况
    private MyRecycleAdapter myRecycleAdapter;          

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main3);

        resultRy = findViewById(R.id.resultRy);
        count=0;

         myNum = getSharedPreferences("nameList",MODE_PRIVATE);

        count= myNum.getInt("listsize",0);
        for (int i =0;i<count;i++){
           data.add(myNum.getString("item"+i,"fale"));

        }




        myRecycleAdapter = new MyRecycleAdapter(Main3Activity.this, data);
        LinearLayoutManager manager=new LinearLayoutManager(this);
        StaggeredGridLayoutManager stagger = new StaggeredGridLayoutManager(1, StaggeredGridLayoutManager.VERTICAL);
        resultRy.setLayoutManager(stagger);
        resultRy.setAdapter(myRecycleAdapter);



    }
}