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

 1 package com.example.newrecycleviewdate;
 2
 3 import androidx.appcompat.app.AppCompatActivity;
 4
 5 import android.content.Intent;
 6 import android.content.SharedPreferences;
 7 import android.os.Bundle;
 8 import android.view.View;
 9 import android.widget.Button;
10 import android.widget.CheckBox;
11
12 import java.util.ArrayList;
13 import java.util.List;
14
15 public class Main2Activity extends AppCompatActivity {
16     private CheckBox chk;
17     private Button gotobtn;
18     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~
19     private List<String> list=new ArrayList<>();
20     SharedPreferences.Editor editor;                  //Editer 需先声明，否则取出数据为Null;
21
22     @Override
23     protected void onCreate(Bundle savedInstanceState) {
24         super.onCreate(savedInstanceState);
25         setContentView(R.layout.activity_main2);
26
27         gotobtn=findViewById(R.id.gotobtn);
28
29         gotobtn.setOnClickListener(new View.OnClickListener() {
30             @Override
31             public void onClick(View v) {
32                 for (int i :id){                                   //循环检查复选框状态，选中则加入数组中
33                     chk = findViewById(i);
34                     if (chk.isChecked())
35                         list.add(chk.getText().toString());
36                 }
37                 editor = getSharedPreferences("nameList",MODE_PRIVATE).edit();
38
39                 editor.putInt("listsize",list.size());
40
41
42                 for (int i=0 ;i< list.size();i++){
43                     editor.putString("item"+i,list.get(i));
44                }
45
46
47                 editor.commit();
48                 startActivity(new Intent(Main2Activity.this,MainActivity.class));
49             }
50         });
51
52
53
54
55     }
56 }

2、Main3Activity

 1 package com.example.newrecycleviewdate;
 2
 3 import androidx.appcompat.app.AppCompatActivity;
 4 import androidx.recyclerview.widget.LinearLayoutManager;
 5 import androidx.recyclerview.widget.RecyclerView;
 6 import androidx.recyclerview.widget.StaggeredGridLayoutManager;
 7
 8 import android.content.SharedPreferences;
 9 import android.os.Bundle;
10 import android.widget.TextView;
11
12 import java.util.ArrayList;
13 import java.util.Arrays;
14 import java.util.List;
15
16 public class Main3Activity extends AppCompatActivity {
17     private RecyclerView resultRy;
18     private List<String> data = new ArrayList<>();
19     private int count;
20     //private String[] st={"strind","33fi","33022","dvikeke","88vj3k"};
21     SharedPreferences myNum;                //SharedPreferences 先声明，否则出现取出数据为空的情况
22     private MyRecycleAdapter myRecycleAdapter;
23
24     @Override
25     protected void onCreate(Bundle savedInstanceState) {
26         super.onCreate(savedInstanceState);
27         setContentView(R.layout.activity_main3);
28
29         resultRy = findViewById(R.id.resultRy);
30         count=0;
31
32          myNum = getSharedPreferences("nameList",MODE_PRIVATE);
33
34         count= myNum.getInt("listsize",0);
35         for (int i =0;i<count;i++){
36            data.add(myNum.getString("item"+i,"fale"));
37
38         }
39
40
41
42
43         myRecycleAdapter = new MyRecycleAdapter(Main3Activity.this, data);
44         LinearLayoutManager manager=new LinearLayoutManager(this);
45         StaggeredGridLayoutManager stagger = new StaggeredGridLayoutManager(1, StaggeredGridLayoutManager.VERTICAL);
46         resultRy.setLayoutManager(stagger);
47         resultRy.setAdapter(myRecycleAdapter);
48
49
50
51     }
52 }