public class MainActivity extends AppCompatActivity {
    TextView tv1,tv2;
    Button bOpen;
    EditText et;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        tv1=(TextView)this.findViewById(R.id.tv1);
        tv2=(TextView)this.findViewById(R.id.textView2);
        bOpen=(Button)this.findViewById(R.id.button);
        et=(EditText)this.findViewById(R.id.editText);
        bOpen.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String str=loadFromSDFile(et.getText().toString().trim());
                tv2.setText(str);
            }


        });
    }

    private String loadFromSDFile(String fname) {
        fname="/"+fname;
        String result=null;
        try {
            File f=new File(Environment.getExternalStorageDirectory().getPath()+fname);
            int length=(int)f.length();
            byte[] buff=new byte[length];
            FileInputStream fin=new FileInputStream(f);
            fin.read(buff);
            fin.close();
            result=new String(buff,"UTF-8");
        }catch (Exception e){
            e.printStackTrace();
            Toast.makeText(MainActivity.this,"没有找到指定文件",Toast.LENGTH_SHORT).show();
        }
        return result;
    }
}