如何防止我的应用程序崩溃？[副本]

Question

MainActivity --> Instructions or AdminLogin --> UserLogin or RegisterCandidate

即使在谷歌搜索了几个小时后，我也无法找出确切的问题。我已经检查了StackOverflow本身的几个答案，但未能找出问题所在。

错误跟踪如下：

E/AndroidRuntime: FATAL EXCEPTION: main
  java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.palakjain.onlinevotingschool/com.example.palakjain.onlinevotingschool.AdminLogin}: java.lang.NullPointerException
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2360)
      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2412)
      at android.app.ActivityThread.access$600(ActivityThread.java:162)
      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1364)
      at android.os.Handler.dispatchMessage(Handler.java:107)
      at android.os.Looper.loop(Looper.java:194)
      at android.app.ActivityThread.main(ActivityThread.java:5388)
      at java.lang.reflect.Method.invokeNative(Native Method)
      at java.lang.reflect.Method.invoke(Method.java:525)
      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:833)
      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600)
      at dalvik.system.NativeStart.main(Native Method)
   Caused by: java.lang.NullPointerException
      at com.example.suniljain.onlinevotingschool.AdminLogin.onCreate(AdminLogin.java:39)
      at android.app.Activity.performCreate(Activity.java:5141)
      at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1084)
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2307)
      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2412) 
      at android.app.ActivityThread.access$600(ActivityThread.java:162) 
      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1364) 
      at android.os.Handler.dispatchMessage(Handler.java:107) 
      at android.os.Looper.loop(Looper.java:194) 
      at android.app.ActivityThread.main(ActivityThread.java:5388) 
      at java.lang.reflect.Method.invokeNative(Native Method) 
      at java.lang.reflect.Method.invoke(Method.java:525) 
      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:833) 
      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600) 
      at dalvik.system.NativeStart.main(Native Method)

主要活动代码：

public class MainActivity extends AppCompatActivity {

Button btnConductVoting, btnInstructions, btnSkipInstructions;

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

    btnConductVoting = (Button) findViewById(R.id.btnConductVoting);
    btnInstructions = (Button) findViewById(R.id.btnInstructions);
    btnSkipInstructions = (Button) findViewById(R.id.btnSkipInstructions);

    btnConductVoting.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent i = new Intent(MainActivity.this, AdminLogin.class);
            startActivity(i);
        }
    });

    btnInstructions.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent i = new Intent(MainActivity.this, Instructions.class);
            startActivity(i);
        }
    });

    btnSkipInstructions.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent i = new Intent(MainActivity.this, UserLogin.class);
            startActivity(i);
        }
    });

}
}

public class Instructions extends AppCompatActivity{
Button btnStartVoting;

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

    btnStartVoting = (Button) findViewById(R.id.btnStartVoting);

    btnStartVoting.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent i = new Intent(Instructions.this, UserLogin.class);
            startActivity(i);
        }
    });
}
}

public class AdminLogin extends AppCompatActivity{

Button btnAdminLogin;
EditText etUsername;
EditText etPassword;

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

    btnAdminLogin = (Button) findViewById(R.id.btnAdminLogin);
    etUsername = (EditText) findViewById(R.id.etAdminPassword);
    etPassword = (EditText) findViewById(R.id.etAdminPassword);

    btnAdminLogin.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            if(etUsername.getText().toString().equals("administrator") && etPassword.getText().toString().equals("password")){
                Intent i = new Intent(AdminLogin.this, RegisterCandidate.class);
                startActivity(i);
            }
        }
    });

}
}

public class UserLogin extends AppCompatActivity{

//TextView tvUserForgotPassword;
TextView tvNewUserSignUp;
//Button btnUserLogIn;

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

    tvNewUserSignUp = (TextView) findViewById(R.id.tvNewUserSignUp);        

    tvNewUserSignUp.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent i = new Intent(UserLogin.this, NewUser.class);
            startActivity(i);
        }
    });

}
}

public class RegisterCandidate extends AppCompatActivity{
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
}
}

public class NewUser extends AppCompatActivity{
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
}
}

更新1：我已经做了答案中建议的所有更改，但每当我单击MainActivity中的“跳过指令”按钮时，我的应用程序仍然崩溃。此外，当我从AdminLogin活动中单击“登录”按钮时，应用程序崩溃。我该怎么办？

Update2:我提到了什么是NullPointerException，我如何修复它？却无法修复错误！

Answer 1

您没有在AdminLogin活动中SetContentView。

   @Override
   protected void onCreate(@Nullable Bundle savedInstanceState) {
   super.onCreate(savedInstanceState);
   // set contentview here before finding views otherwise they will be null
    btnAdminLogin = (Button) findViewById(R.id.btnAdminLogin);
   }

只是注意，同样的问题是与你的其他活动，交叉检查。

如果您想知道什么是setcontentview，那么在短期内，它将关联视图/UI，它将在activity启动时显示。举个例子，如果您想在UI上显示edittext和按钮，那么它们必须出现在扩展名为.xml的布局文件中。（您也可以通过java代码创建视图）