SharedPreferences、内外部存储

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<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".MainActivity">
    <EditText
        android:id="@+id/username"
        android:hint="账号"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="100dp"
        />
    <EditText
        android:id="@+id/password"
        android:hint="密码"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="20dp"
        android:inputType="textPassword"
        />
    <Button
        android:id="@+id/login"
        android:text="登录"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="40dp"/>
</LinearLayout>

package com.lindingdu.app04;
import android.content.Intent;
import android.content.SharedPreferences;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

        private String Lindingdu="Lindingdu",heat3534331="heat3534331";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button login=findViewById(R.id.login);
        final EditText passwordET=findViewById(R.id.password);
        final EditText usernameET=findViewById(R.id.username);
        //本应用权限,指定保存文件名Lindingdu
        final SharedPreferences sp=getSharedPreferences("Lindingdu",MODE_PRIVATE);
        //自动登录功能
        String username=sp.getString("username",null);
        String password=sp.getString("password",null);
        if(password!=null&&username!=null) {
            if (username.equals(Lindingdu) && password.equals(heat3534331)) {
                Intent intent = new Intent(MainActivity.this, messageActivity.class);
                startActivity(intent);
            }
        }else { //实现手动登录并储存密码
            login.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    String in_username=usernameET.getText().toString();//获取账号密码
                    String in_password=passwordET.getText().toString();
                    SharedPreferences.Editor editor=sp.edit();//获取Editor对象保存账号密码
                    if (in_password.equals(heat3534331)&&in_username.equals(Lindingdu)){
                            editor.putString("username",in_username);
                            editor.putString("password",in_password);
                            editor.commit();
                            startActivity(new
                                    Intent(MainActivity.this,messageActivity.class));
                        Toast.makeText(MainActivity.this, "账号密码已保存"
                                , Toast.LENGTH_SHORT).show();
                    }else {
                        Toast.makeText(MainActivity.this, "密码错误", Toast.LENGTH_SHORT).show();
                    }
                }
            });
        }
    }
}

java中规定，内部类只能访问外部类中的成员变量，不能访问方法中定义的变量，如果要访问方法中的变量，就要把方法中的变量声明为final（常量）的，因为这样可以使变量全局化，就相当于是在外部定义的而不是在方法里定义的
Variable is accessed from within inner class, needs to be declared final

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<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">
    <EditText
        android:id="@+id/editText"
        android:layout_marginRight="20dp"
        android:layout_marginLeft="20dp"
        android:layout_marginTop="190dp"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />
    <Button
        android:layout_marginLeft="60dp"
        android:layout_alignParentBottom="true"
        android:text="保存"
        android:id="@+id/btn_save"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="60dp"
        />
    <Button
        android:layout_marginRight="60dp"
        android:layout_alignParentBottom="true"
        android:text="取消"
        android:id="@+id/btn_cancel"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentRight="true"
        android:layout_marginBottom="60dp"
        />
</RelativeLayout>

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;

public class MainActivity extends AppCompatActivity {

   byte[]buffer=null;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button btn_save=findViewById(R.id.btn_save);
        Button btn_cancel=findViewById(R.id.btn_cancel);
        final EditText editText=findViewById(R.id.editText);
        btn_save.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String str=editText.getText().toString();//获取EditText信息
                FileOutputStream fos=null;

                try {
                    fos=openFileOutput("a",MODE_PRIVATE);//获得文件输出流对象
                    fos.write(str.getBytes());//保存信息
                    fos.flush();//清除缓存

                } catch (FileNotFoundException e) {
                    e.printStackTrace();
                } catch (IOException e) {
                    e.printStackTrace();
                }finally {
                    if (fos != null) {
                        try {
                            fos.close();//关闭输出流
                            Toast.makeText(MainActivity.this, "保存成功！", Toast.LENGTH_SHORT).show();
                        } catch (IOException e) {
                            e.printStackTrace();
                        }
                    }
                }
            }
        });
       // 读取信息
        FileInputStream fis=null;
        try {
            fis=openFileInput("a");//获取文件输入流对象
            buffer=new byte[fis.available()];//实例化字节数组
            fis.read(buffer);//从输入流中读取数据到字节数组中
            buffer.toString();//不能用toSting要用new String（）

        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }finally {
            if (fis!=null) {
                try {
                    fis.close();
                    String string = new String(buffer);//字节数组转化为字符串
                    editText.setText(string);//将转化的字符串显示出来
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
        btn_cancel.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                finish();//退出当前应用
            }
        });
    }
}

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<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">
    <EditText
        android:id="@+id/editText"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginLeft="20dp"
        android:layout_marginRight="20dp"
        android:layout_marginTop="200dp"
        />
    <Button
        android:layout_marginTop="100dp"
        android:layout_below="@+id/editText"
        android:text="保存"
        android:layout_marginLeft="60dp"
        android:id="@+id/btn_save"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
    <Button
        android:layout_marginTop="100dp"
        android:layout_below="@+id/editText"
        android:text="取消"
        android:layout_alignParentRight="true"
        android:layout_marginRight="60dp"
        android:id="@+id/btn_cancel"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="TextView"
        android:layout_marginLeft="180dp"
        android:layout_marginTop="90dp"
         />
</RelativeLayout>

  <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

import android.os.Environment;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;

public class MainActivity extends AppCompatActivity {

    byte[]buffer=null;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button btn_save=findViewById(R.id.btn_save);
        Button btn_cancel=findViewById(R.id.btn_cancel);
        TextView textView=findViewById(R.id.textView);
        final EditText editText=findViewById(R.id.editText);
        final File file=new File(Environment.getExternalStorageDirectory(),"a.txt");//实例化文件对象

        btn_save.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                FileOutputStream fos=null;
                String etext=editText.getText().toString();

                try {
                    fos=new FileOutputStream(file);
                    fos.write(etext.getBytes());
                    fos.flush();
                } catch (FileNotFoundException e) {
                    e.printStackTrace();
                } catch (IOException e) {
                    e.printStackTrace();
                }finally {
                    if(fos!=null){
                    try {
                        fos.close();
                        Toast.makeText(MainActivity.this, "保存成功", Toast.LENGTH_SHORT).show();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                    }
                }
            }
        });

        //读取保存的信息并显示
        FileInputStream fis=null;

        try {
            fis=new FileInputStream(file);
            buffer=new byte[fis.available()];
            fis.read(buffer);
            String string=new String(buffer);
            textView.setText(string);
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }finally {
            if (fis!=null) {
                try {
                    fis.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }      
        btn_cancel.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                finish();
            }
        });
    }
}