Java – How do I change my password programmatically?

I’m trying to create an Android app that can change the device password. I read Device Administration Application
I tried running this sample , here is my main activity code:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    final Activity context = this;
    final String new_pass = ((EditText)findViewById(R.id.editext)).getText().toString();
    ((Button)findViewById(R.id.button)).setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            DevicePolicyManager devicePolicyManager =
                    (DevicePolicyManager)context.getSystemService(context. DEVICE_POLICY_SERVICE);
            ComponentName demoDeviceAdmin = new ComponentName(context, MainActivity.class);

devicePolicyManager.setPasswordQuality(
                    demoDeviceAdmin,DevicePolicyManager.PASSWORD_QUALITY_UNSPECIFIED);
            devicePolicyManager.setPasswordMinimumLength(demoDeviceAdmin, 5);

boolean result = devicePolicyManager.resetPassword("123456",
                    DevicePolicyManager.RESET_PASSWORD_REQUIRE_ENTRY);

Toast.makeText(context,
                    "button_lock_password_device..."+result,
                    Toast.LENGTH_LONG).show();
        }
    });
}

I got this error :

08-26 22:36:51.280  15249-15249/co.rishe.secretpolice.app E/AndroidRuntime﹕ FATAL EXCEPTION: main
java.lang.SecurityException: No active admin ComponentInfo{com.example.secretpolice.app/com.example.secretpolice.app.MainActivity}
        at android.os.Parcel.readException(Parcel.java:1425)
        at android.os.Parcel.readException(Parcel.java:1379)
        at android.app.admin.IDevicePolicyManager$Stub$Proxy.setPasswordQuality(IDevicePolicyManager.java:1359)
        at android.app.admin.DevicePolicyManager.setPasswordQuality(DevicePolicyManager.java:323)
        at co.rishe.secretpolice.app.MainActivity$1.onClick(MainActivity.java:32)
        at android.view.View.performClick(View.java:4211)
        at android.view.View$PerformClick.run(View.java:17267)
        at android.os.Handler.handleCallback(Handler.java:615)
        at android.os.Handler.dispatchMessage(Handler.java:92)
        at android.os.Looper.loop(Looper.java:137)
        at android.app.ActivityThread.main(ActivityThread.java:4898)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:511)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1006)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:773)
        at dalvik.system.NativeStart.main(Native Method)

Can anyone help me figure it out?

Quote the documentation that you linked to :

One of the major events a device admin application has to handle is the user enabling the application. The user must explicitly enable the application for the policies to be enforced. If the user chooses not to enable the application it will still be present on the device, but its policies will not be enforced, and the user will not get any of the application’s benefits.

As Mr. Harvey points out, the error message means that the user has not enabled your app as a device administrator.

Further reference documentation:

The process of enabling the application begins when the user performs an action that triggers the ACTION_ADD_DEVICE_ADMIN intent.

Here is a sample project demo setup device administrator.
The key is in LockMeNowActivity:

/***
  Copyright (c) 2012 CommonsWare, LLC
  Licensed under the Apache License, Version 2.0 (the "License"); you may not
  use this file except in compliance with the License. You may obtain a copy
  of the License at http://www.apache.org/licenses/LICENSE-2.0. Unless required
  by applicable law or agreed to in writing, software distributed under the
  License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS
  OF ANY KIND, either express or implied. See the License for the specific
  language governing permissions and limitations under the License.

From _The Busy Coder's Guide to Android Development_
    http://commonsware.com/Android
 */

package com.commonsware.android.lockme;

import android.app.Activity;
import android.app.admin.DevicePolicyManager;
import android.content.ComponentName;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;

public class LockMeNowActivity extends Activity {
  private DevicePolicyManager mgr=null;
  private ComponentName cn=null;

@Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

setContentView(R.layout.main);
    cn=new ComponentName(this, AdminReceiver.class);
    mgr=(DevicePolicyManager)getSystemService(DEVICE_POLICY_SERVICE);
  }

public void lockMeNow(View v) {
    if (mgr.isAdminActive(cn)) {
      mgr.lockNow();
    }
    else {
      Intent intent=
          new Intent(DevicePolicyManager.ACTION_ADD_DEVICE_ADMIN);
      intent.putExtra(DevicePolicyManager.EXTRA_DEVICE_ADMIN, cn);
      intent.putExtra(DevicePolicyManager.EXTRA_ADD_EXPLANATION,
                      getString(R.string.device_admin_explanation));
      startActivity(intent);
    }
  }
}

Here, when the user

clicks the button that triggers lockMeNow(), I check if my app is a device administrator, and if not, I direct the user to the correct location in the settings to apply whether to let my app become a device administrator.