錯誤：由以下原因引起：java.lang.NullPointerException

Question

我有一個錯誤說：

原因：java.lang.NullPointerException：嘗試在com.example.pc.proyectofinal.SignUp.onCreate上的空對象引用上調用虛擬方法'void android.widget.Button.setOnClickListener（android.view.View $ OnClickListener）' （SignUp.java:46）

我嘗試了兩種方法：

• 首先搭配：

   Intent SignUpINTENT = new Intent(SignUp.this, ResetPassword.class); startActivity(SignUpINTENT); 

• 以及第二次：

   startActivity(new Intent(SignUp.this, ResetPassword.class)); 

兩者都給我同樣的錯誤。

這是代碼：

public class SignUp extends AppCompatActivity {

    private EditText inputEmail, inputPassword;     //hit option + enter if you on mac , for windows hit ctrl + enter
    private Button btnSignIn, btnSignUp, btnResetPassword;
    private ProgressBar progressBar;
   private FirebaseAuth auth;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
       // setContentView(R.layout.activity_signup);
        //Get Firebase auth instance
        auth = FirebaseAuth.getInstance();

        btnSignIn = (Button) findViewById(R.id.sign_in_button);
        btnSignUp = (Button) findViewById(R.id.sign_up_button);
        inputEmail = (EditText) findViewById(R.id.email);
        inputPassword = (EditText) findViewById(R.id.password);
        progressBar = (ProgressBar) findViewById(R.id.progressBar);
        btnResetPassword = (Button) findViewById(R.id.btn_reset_password);

        btnResetPassword.setOnClickListener(new View.OnClickListener() { //here is the problem with the regist
            @Override
            public void onClick(View v) {
                Intent SignUpINTENT = new Intent(SignUp.this, ResetPassword.class);
                startActivity(SignUpINTENT);
                //startActivity(new Intent(SignUp.this, ResetPassword.class));
            }
        });

        btnSignIn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                finish();
            }
        });

        btnSignUp.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                String email = inputEmail.getText().toString().trim();
                String password = inputPassword.getText().toString().trim();

                if (TextUtils.isEmpty(email)) {
                    Toast.makeText(getApplicationContext(), "Enter email address!", Toast.LENGTH_SHORT).show();
                    return;
                }

                if (TextUtils.isEmpty(password)) {
                    Toast.makeText(getApplicationContext(), "Enter password!", Toast.LENGTH_SHORT).show();
                    return;
                }

                if (password.length() < 6) {
                    Toast.makeText(getApplicationContext(), "Password too short, enter minimum 6 characters!", Toast.LENGTH_SHORT).show();
                    return;
                }

                progressBar.setVisibility(View.VISIBLE);
                //create user
                auth.createUserWithEmailAndPassword(email, password)
                        .addOnCompleteListener(SignUp.this, new OnCompleteListener<AuthResult>() {
                            @Override
                            public void onComplete(@NonNull Task<AuthResult> task) {
                                Toast.makeText(SignUp.this, "createUserWithEmail:onComplete:" + task.isSuccessful(), Toast.LENGTH_SHORT).show();
                                progressBar.setVisibility(View.GONE);
                                // If sign in fails, display a message to the user. If sign in succeeds
                                // the auth state listener will be notified and logic to handle the
                                // signed in user can be handled in the listener.
                                if (!task.isSuccessful()) {
                                    Toast.makeText(SignUp.this, "Authentication failed." + task.getException(),
                                            Toast.LENGTH_SHORT).show();
                                } else {
                                    startActivity(new Intent(SignUp.this, Main.class));
                                    finish();
                                }
                            }
                        });

            }
        });
    }

    @Override
    protected void onResume() {
        super.onResume();
        progressBar.setVisibility(View.GONE);
    }
}

Answer 1

您在onCreate()將此行注釋掉了：

setContentView(R.layout.activity_signup);

為什么？
沒有這行，活動的布局不會膨脹， findViewById不能找到所有的視圖，它們為null 。
所以當你嘗試

btnResetPassword.setOnClickListener() 

你得到

java.lang.NullPointerException