与凌空Java / Android的HTTP请求

我正在尝试从页面发出http请求，但是我的应用程序崩溃了。这是我的第一个Java应用程序，所以我是一个初学者。我研究了一段时间，但找不到解决方案：

    package com.lookingunique.splashstockcontrol;


import android.app.Activity;
import android.os.Bundle;
import android.text.Editable;
import android.text.TextWatcher;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;
import com.android.volley.Request;
import com.android.volley.RequestQueue;
import com.android.volley.Response;
import com.android.volley.VolleyError;
import com.android.volley.toolbox.StringRequest;
import com.android.volley.toolbox.Volley;


public class MainActivity extends Activity {

EditText barcode;

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

    barcode = (EditText) findViewById(R.id.etbarcode);
    final Button addBtn = (Button) findViewById(R.id.button);

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

            String barcodeval = barcode.getText().toString();
            // Instantiate the RequestQueue.
            RequestQueue queue = Volley.newRequestQueue(getApplicationContext());
            String url ="http://xxzx.com/zzz/xzxx/yyy.php?barcode="+ barcodeval +"&action=check";

            // Request a string response from the provided URL.
            StringRequest stringRequest = new StringRequest( Request.Method.GET, url,
                    new Response.Listener<String>() {
                        @Override
                        public void onResponse(String response) {
                            // Display the first 500 characters of the response string.
                            String test = response.substring(0,100);
                            Toast.makeText(getApplicationContext(), test, Toast.LENGTH_SHORT).show();
                        }
                    }, new Response.ErrorListener() {
                @Override
                public void onErrorResponse(VolleyError error) {
                    String test = "That didn't work!";
                    Toast.makeText(getApplicationContext(), test, Toast.LENGTH_SHORT).show();
                }
            });
            // Add the request to the RequestQueue.
            queue.add(stringRequest);




        }
    });

    barcode.addTextChangedListener(new TextWatcher() {
        @Override
        public void beforeTextChanged(CharSequence s, int start, int count, int after) {

        }

        @Override
        public void onTextChanged(CharSequence s, int start, int before, int count) {
            addBtn.setEnabled(!barcode.getText().toString().trim().isEmpty());
        }

        @Override
        public void afterTextChanged(Editable s) {

        }
    });
}


}

08-25 10：15：15.336 2029-2029 / com.lookingunique.splashstockcontrol E / AndroidRuntime致命异常：主进程：com.lookingunique.splashstockcontrol，PID：2029 java.lang.StringIndexOutOfBoundsException：length = 0; regionStart = 0; regionLength = 100 at java.lang.String.startEndAndLength（String.java:298）at java.lang.String.substring（String.java:1087）at com.lookingunique.splashstockcontrol.MainActivity $ 1 $ 1.onResponse（MainActivity.java： 47）在com.lookingunique.splashstockcontrol.MainActivity $ 1 $ 1.onResponse（MainActivity.java:43）在com.android.volley.toolbox.StringRequest.deliverResponse（StringRequest.java:60）在com.android.volley.toolbox.StringRequest com.android.volley.ExecutorDelivery $ ResponseDeliveryRunnable.run（ExecutorDelivery.java:99）的android.os.Handler.handleCallback（Handler.java:739）上的.deliverResponse（StringRequest.java:30）。