java - Error in AES decryption -

the given code uses aes encryption 128 bit key value given user. used code in android project following code encrypts @ time of decryption shows exception (error in decryption) , results in unfortunately stopping app. here following code,

import android.util.base64; import android.util.log; import java.io.unsupportedencodingexception; import java.security.messagedigest; import java.security.nosuchalgorithmexception; import java.util.arrays; import javax.crypto.cipher; import javax.crypto.spec.secretkeyspec;   public class aes {      static final string tag = "symmetricalgorithmaes";      private secretkeyspec secretkey;     private byte[] key;      private string decryptedstring;     private string encryptedstring;      //sets value of key     public void setkey(string mykey) {          messagedigest sha = null;         try {             key = mykey.getbytes("utf-8");             sha = messagedigest.getinstance("sha-1");             key = sha.digest(key);              key = arrays.copyof(key, 16);             secretkey = new secretkeyspec(key, "aes");         } catch (unsupportedencodingexception e) {             // todo auto-generated catch block             log.e(tag, "error in setting key");         } catch (nosuchalgorithmexception e) {             e.printstacktrace();         }     }      public secretkeyspec getkey(){         return secretkey;     }      public string getdecryptedstring() {         return decryptedstring;     }      public void setdecryptedstring(string decryptedstring) {         this.decryptedstring = decryptedstring;     }      public string getencryptedstring() {         return encryptedstring;     }      public void setencryptedstring(string encryptedstring) {         this.encryptedstring = encryptedstring;     }      //method encryption     public void encrypt(string strtoencrypt) {         try {             cipher cipher = cipher.getinstance("aes/ecb/pkcs5padding");             cipher.init(cipher.encrypt_mode, secretkey);             byte[] encd = cipher.dofinal(strtoencrypt.getbytes("utf-8"));             byte[] cc =base64.encode(encd,base64.default);             setencryptedstring(new string(cc));                    } catch (exception e) {             log.e(tag, "error in encryption");         }     }      //method decryption     public void decrypt(string strtodecrypt) {         byte[] decd = null;         try {             cipher cipher = cipher.getinstance("aes/ecb/pkcs5padding");             cipher.init(cipher.decrypt_mode, secretkey);             decd = cipher.dofinal(strtodecrypt.getbytes("utf-8"));          } catch (exception e) {             log.e(tag, "error in decryption" + decd);         }         setdecryptedstring(decd.tostring());     } } 

here logcat

08-10 12:27:23.187 30907-30907/com.test.encryptiontest e/symmetricalgorithmaes: error in decryptionnull 08-10 12:27:23.189 30907-30907/com.test.encryptiontest e/androidruntime: fatal exception: main      process: com.test.encryptiontest, pid: 30907      java.lang.illegalstateexception: not execute method android:onclick          @ android.support.v7.app.appcompatviewinflater$declaredonclicklistener.onclick(appcompatviewinflater.java:293)          @ android.view.view.performclick(view.java:5198)          @ android.view.view$performclick.run(view.java:21147)          @ android.os.handler.handlecallback(handler.java:739)          @ android.os.handler.dispatchmessage(handler.java:95)          @ android.os.looper.loop(looper.java:148)          @ android.app.activitythread.main(activitythread.java:5417)          @ java.lang.reflect.method.invoke(native method)          @ com.android.internal.os.zygoteinit$methodandargscaller.run(zygoteinit.java:726)          @ com.android.internal.os.zygoteinit.main(zygoteinit.java:616)       caused by: java.lang.reflect.invocationtargetexception          @ java.lang.reflect.method.invoke(native method)          @ android.support.v7.app.appcompatviewinflater$declaredonclicklistener.onclick(appcompatviewinflater.java:288)          @ android.view.view.performclick(view.java:5198)           @ android.view.view$performclick.run(view.java:21147)           @ android.os.handler.handlecallback(handler.java:739)           @ android.os.handler.dispatchmessage(handler.java:95)           @ android.os.looper.loop(looper.java:148)           @ android.app.activitythread.main(activitythread.java:5417)           @ java.lang.reflect.method.invoke(native method)           @ com.android.internal.os.zygoteinit$methodandargscaller.run(zygoteinit.java:726)           @ com.android.internal.os.zygoteinit.main(zygoteinit.java:616)        caused by: java.lang.nullpointerexception: attempt invoke virtual method 'java.lang.string java.lang.object.tostring()' on null object reference          @ com.test.encryptiontest.aes.decrypt(aes.java:86)          @ com.test.encryptiontest.decode.decode(decode.java:23)          @ java.lang.reflect.method.invoke(native method)           @ android.support.v7.app.appcompatviewinflater$declaredonclicklistener.onclick(appcompatviewinflater.java:288)           @ android.view.view.performclick(view.java:5198)           @ android.view.view$performclick.run(view.java:21147)           @ android.os.handler.handlecallback(handler.java:739)           @ android.os.handler.dispatchmessage(handler.java:95)           @ android.os.looper.loop(looper.java:148)           @ android.app.activitythread.main(activitythread.java:5417)           @ java.lang.reflect.method.invoke(native method)           @ com.android.internal.os.zygoteinit$methodandargscaller.run(zygoteinit.java:726)           @ com.android.internal.os.zygoteinit.main(zygoteinit.java:616)                                                                             --------- beginning of system 

try note code

cipher cipher = cipher.getinstance("aes/ecb/pkcs5padding"); cipher.init(cipher.decrypt_mode, secretkey); decd = cipher.dofinal(strtodecrypt.getbytes("utf-8")); 

one 1 , see line generates exception.