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我是一个新手，并试图为在线投票做一个android应用程序。我刚刚设计了布局，但不幸的是，我面临渲染问题和空指针异常。如何防止我的应用程序崩溃？
当我从MainActivity打开一个活动时，我只能看到一个白色屏幕！活动的顺序如下：
MainActivity --> Instructions or AdminLogin --> UserLogin or RegisterCandidate
即使经过数小时的谷歌搜索，我无法找出确切的问题。我已经在StackOverflow本身检查了几个答案，但未能弄清楚什么是错的。
的错误跟踪，如下所示：
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)
MainActivity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);
}
});
}
}
指令的代码：
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);
}
});
}
}
后台管理代码：
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);
}
}
});
}
}
用户登陆代码：
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);
}
});
}
}
RegisterCandidate代码：
public class RegisterCandidate extends AppCompatActivity{
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
}
}
NEWUSER代码：
public class NewUser extends AppCompatActivity{
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
}
}
UPDATE1：我已被建议在答案的所有更改，当我点击MainActivity中的“SKIP INSTRUCTIONS”按钮时，我的应用程序仍然崩溃。此外，当我点击AdminLogin Activity中的“登录”按钮时，应用程序崩溃。我该怎么办？
更新2：我指的是What is a NullPointerException, and how do I fix it?但还是无法修复这个错误！
你增加你的活动清单？ –
@Palak Jain和你不能在java上使用'=='检查变量值，你必须使用'.equals'而不是'==' –
是的，我有@SudheeshR –