記住密碼這個功能大家應該都不陌生，所以在思考邏輯的時候會比較清楚一點，也可以較熟悉SharedPreferences的應用，這個特別的地方是我設了一個boolean狀態給後續執行可以做出判斷，以下是我的做法:

public class Login extends AppCompatActivity {
    private EditText account,password,code;
    private CheckBox remember;
    private String Account,Password,Account2,Password2 ;

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

        account =findViewById(R.id.account2);
        password=findViewById(R.id.password2);
        code=findViewById(R.id.code);
        check=findViewById(R.id.remember);
        output(); //先執行我寫的副程式，可以先參考

        Button login = findViewById(R.id.login);
        login.setOnClickListener(new View.OnClickListener() {  //點擊登入按鈕
            @Override
            public void onClick(View view) {
                Account2=account.getText().toString();  //取得輸入的值並轉成字串
                Password2=password.getText().toString();

                SharedPreferences user =getSharedPreferences("signup",MODE_PRIVATE);
                Account=user.getString("account","");
                Password=user.getString("password",""); //取得之前註冊好的資料


                if (TextUtils.isEmpty(Account2)){
                    Toast.makeText(Login.this, "請輸入帳號", Toast.LENGTH_SHORT).show();
                }else if(TextUtils.isEmpty(Password2)) {
                    Toast.makeText(Login.this, "請輸入密碼", Toast.LENGTH_SHORT).show();
                }else if(Account2.equals(Account) &&
                         Password2.equals(Password)){

                    Intent intent = new Intent();
                    intent.setClass(Login.this, Menu.class); //若帳密皆符合先前註冊資料則跳頁
                    startActivity(intent);
                    input(); ///登入成功的話才執行這個副程式
                }else {
                    Toast.makeText(Login.this, "輸入有誤，請重新確認", Toast.LENGTH_SHORT).show();
                    account.setText(""); //清空
                    passsword.setText("");
                }
                }

        });
    }



    private void input() {

        SharedPreferences user = getSharedPreferences("remember", MODE_PRIVATE);
        SharedPreferences.Editor edit = user.edit();
            if (check.isChecked()) { //如果記住密碼那個選項有打勾就把這次成功登入的資料儲存起來
                edit.putString("account2",account.getText().toString());
                edit.putString("password2",password.getText().toString());
                edit.putBoolean("remember",true);  //把我自己設的remember狀態設為true

            } else {
                edit.remove("account2"); //若上次沒打勾就把它清除記住的資料
                edit.remove("password2");
                edit.putBoolean("remember",false);
            }
                edit.commit();
            }

    private void output(){
        SharedPreferences user =getSharedPreferences("remember",MODE_PRIVATE);
        String account2=user.getString("account2","");  //若沒取得就是沒任何東西 " "
        String password2=user.getString("password2","");
        boolean remember=user.getBoolean("remember",false); 
        //在input()我是寫成若打勾則讓它remember設為true，所以若沒取到則是false

        account.setText(account2); //有取到先前給予的資料的話就會顯示出來
        password.setText(password2);
        remember.setChecked(remember);//並且利用remember的狀態判斷上次是否打勾，有的話就顯示打勾
    }
}