Android-Java：java.lang.IllegalStateException：无法执行 android 的方法：onClick

Question

I am trying to make a sign up form in a fragment in a view Pager. The form has four text fields and a button, I am getting the above mentioned exception when the button is clicked. The onClick method is registerUser(View view) defined in the class below.

ViewPager Activity

public class getStarted extends AppCompatActivity {
TextInputEditText n,p,e,a;
SignInButtonImpl btn;
MaterialCheckBox tc;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.get_started);
    n= findViewById(R.id.sname);
    p= findViewById(R.id.spass);
    e= findViewById(R.id.semail);
    a= findViewById(R.id.sage);
    btn= findViewById(R.id.signup);
    tc=findViewById(R.id.tc);
    getStartedPagerAdapter pageAdapter = new getStartedPagerAdapter(getSupportFragmentManager());
    ViewPager viewPager = findViewById(R.id.view_pager);
    viewPager.setAdapter(pageAdapter);

}
public void registerUser(View view) {
    //Name validation check

    if(n.getText().toString().trim().equals("")){
        n.setError("Name is Required");
        n.requestFocus();
        return;
    }

    //Password Validation Checks
    if(p.getText().toString().trim().equals("")){
        p.setError("Password is Required");
        p.requestFocus();
        return;
    }
    if(p.getText().toString().trim().length()<6){
        p.setError("Password must be at least 6 characters");
        p.requestFocus();
        return;
    }

    //Age Validation Check
    if(a.getText().toString().trim().equals("")){
        a.setError("Age is Required");
        a.requestFocus();
        return;
    }
    if(a.getText().toString().trim()!="") {
        if (Integer.parseInt(a.getText().toString().trim()) <= 0) {
            a.setError("Please enter a valid age.");
            a.requestFocus();
            return;
        }
    }

    //Email Validation Check
    if(e.getText().toString().trim() == ""){
        e.setError("Email is Required");
        e.requestFocus();
        return;
    }
    if(!Patterns.EMAIL_ADDRESS.matcher( e.getText().toString().trim()).matches()){
        e.setError("Please Enter a valid Email Address");
        e.requestFocus();
        return;
    }

    //Terms and Conditions Validation Check
    if(!tc.isChecked()){
        tc.setError("Agree To continue.");
        tc.requestFocus();
        return;
    }


    System.out.println("Button Working");
    }
}

SignupFragment

public class signupFragment extends Fragment {

@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container,@Nullable Bundle savedInstanceState) {
    return inflater.inflate(R.layout.signup_fragment, container, false);
    }
}

View Pager Adapter

public class getStartedPagerAdapter extends FragmentPagerAdapter {
public getStartedPagerAdapter(FragmentManager fm) {
    super(fm);
}
@Override
public Fragment getItem(int position) {
    switch(position){
        case 0:
            return new signupFragment();
        case 1:
           return new loginFragment();
    }
    return null;
}
@Override
public int getCount() {
    return 2; // since we have three fragments
    }

}

Logcat referring to Exception

    E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.rehnsehan, PID: 24606
    java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Class java.lang.Object.getClass()' on a null object reference
        at androidx.fragment.app.FragmentTransaction.doAddOp(FragmentTransaction.java:161)
        at androidx.fragment.app.BackStackRecord.doAddOp(BackStackRecord.java:179)
        at androidx.fragment.app.FragmentTransaction.add(FragmentTransaction.java:156)
        at androidx.fragment.app.FragmentPagerAdapter.instantiateItem(FragmentPagerAdapter.java:172)
        at androidx.viewpager.widget.ViewPager.addNewItem(ViewPager.java:1010)
        at androidx.viewpager.widget.ViewPager.populate(ViewPager.java:1224)
        at androidx.viewpager.widget.ViewPager.populate(ViewPager.java:1092)
        at androidx.viewpager.widget.ViewPager.onMeasure(ViewPager.java:1622)
        at android.view.View.measure(View.java:25466)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6957)
        at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1552)
        at android.widget.LinearLayout.measureVertical(LinearLayout.java:842)
        at android.widget.LinearLayout.onMeasure(LinearLayout.java:721)
        at android.view.View.measure(View.java:25466)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6957)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
        at androidx.appcompat.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:146)
        at android.view.View.measure(View.java:25466)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6957)
        at androidx.appcompat.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:490)
        at android.view.View.measure(View.java:25466)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6957)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
        at android.view.View.measure(View.java:25466)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6957)
        at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1552)
        at android.widget.LinearLayout.measureVertical(LinearLayout.java:842)
        at android.widget.LinearLayout.onMeasure(LinearLayout.java:721)
        at android.view.View.measure(View.java:25466)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6957)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
        at com.android.internal.policy.DecorView.onMeasure(DecorView.java:747)
        at android.view.View.measure(View.java:25466)
        at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:3397)
        at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:2228)
        at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2486)
        at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1952)
        at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:8171)
        at android.view.Choreographer$CallbackRecord.run(Choreographer.java:972)
        at android.view.Choreographer.doCallbacks(Choreographer.java:796)
        at android.view.Choreographer.doFrame(Choreographer.java:731)
        at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:957)
        at android.os.Handler.handleCallback(Handler.java:938)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:223)
        at android.app.ActivityThread.main(ActivityThread.java:7656)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
I've initialized all views and I am not sure what is causing this exception. Anyone Please help, much appreciated. I also got this error statement,
**Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.text.Editable com.google.android.material.textfield.TextInputEditText.getText()' on a null object reference
        at com.example.rehnsehan.getStarted.registerUser(getStarted.java:38)**

Here's the line

 if(n.getText().toString().trim().equals(""))

UPDATE I tried the suggestions below, moving the register functionality to signupFragment. It generates the following exception.

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.rehnsehan, PID: 28540
java.lang.IllegalStateException: Could not find method registerUser(View) in a parent or ancestor Context for android:onClick attribute defined on view class com.google.android.gms.common.internal.SignInButtonImpl with id 'signup'
    at android.view.View$DeclaredOnClickListener.resolveMethod(View.java:6302)
    at android.view.View$DeclaredOnClickListener.onClick(View.java:6259)
    at android.view.View.performClick(View.java:7448)
    at android.view.View.performClickInternal(View.java:7425)
    at android.view.View.access$3600(View.java:810)
    at android.view.View$PerformClick.run(View.java:28305)
    at android.os.Handler.handleCallback(Handler.java:938)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:223)
    at android.app.ActivityThread.main(ActivityThread.java:7656)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)

Answer 1

It's better don't create fragment in getItem(). And It's better to use FragmentStatePagerAdapter. So I changed your adapter code to like it.

public class getStartedPagerAdapter extends FragmentStatePagerAdapter {
    private Fragment signup = new signupFragment();
    private Fragment login = new LoginFragment();

    public getStartedPagerAdapter(FragmentManager fm) {
        super(fm, BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT);
    }
    @Override
    public Fragment getItem(int position) {
        switch(position){
        case 0:
            return signup;
        case 1:
           return login;
        }
        return null;
    }
    @Override
    public int getCount() {
        return 2; // since we have three fragments
    }
}

Please change your code, retry.

Answer 2

The problem is solved after implementing onClickListener instead of using Onclick in xml. Further notes

1. Implement The onClickListener in Fragment.
2. Use on click Listener in java instead of onClick in xml.
3. use ViewGroup root = (ViewGroup) inflater.inflate(R.layout.signup_fragment, null); root.findViewById() in fragment