I've got this java file:

package com.example.tabbedapp;

import android.os.Bundle;
import android.text.Editable;
import android.text.TextUtils;
import android.text.TextWatcher;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.EditText;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;

public class Fragment1 extends Fragment {

    protected TextView ulamekLabel;
    protected EditText licznik;
    protected EditText mianownik;
    protected TextView kUlamkowa;
    protected TextView calosci;
    protected TextView nLicznik;
    protected TextView nMianownik;
    protected TextView kreska;

    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        licznik.addTextChangedListener(new TextWatcher() {
            @Override
            public void beforeTextChanged(CharSequence charSequence, int i, int i1, int i2) {

            }

            @Override
            public void onTextChanged(CharSequence charSequence, int i, int i1, int i2) {

            }

            @Override
            public void afterTextChanged(Editable editable) {
                skracamy();
            }
        });

        mianownik.addTextChangedListener(new TextWatcher() {
            @Override
            public void beforeTextChanged(CharSequence charSequence, int i, int i1, int i2) {

            }

            @Override
            public void onTextChanged(CharSequence charSequence, int i, int i1, int i2) {

            }

            @Override
            public void afterTextChanged(Editable editable) {
                skracamy();
            }
        });

        return inflater.inflate(R.layout.fragment1_layout, container, false);
    }

    private void skracamy() {
        if (TextUtils.isEmpty((CharSequence) licznik) || TextUtils.isEmpty((CharSequence) mianownik)) {
            calosci.setText("0");
            calosci.setVisibility(View.INVISIBLE);
            nLicznik.setText("0");
            nLicznik.setVisibility(View.INVISIBLE);
            nMianownik.setText("0");
            nMianownik.setVisibility(View.INVISIBLE);
            kreska.setVisibility(View.INVISIBLE);
            return;
        }

        int a = Integer.parseInt(licznik.getText().toString());
        int b = Integer.parseInt(mianownik.getText().toString());

        int l = (a / NWD(a, b));//.toString().toInt();
        int m = (b / NWD(a, b));//.toString().toInt();

        int c = (l / m);//.toString().toInt();
        int d = (l % m);//.toString().toInt();

        calosci.setVisibility(View.VISIBLE);
        nLicznik.setVisibility(View.VISIBLE);
        nMianownik.setVisibility(View.VISIBLE);
        kreska.setVisibility(View.VISIBLE);

        if (c != 0 && d != 0) {
            calosci.setText(c);
            nLicznik.setText(d);
            nMianownik.setText(m);
        } else if (d != 0) {
            calosci.setText("");
            nLicznik.setText(d);
            nMianownik.setText(m);
        } else {
            calosci.setText(c);
            nLicznik.setText("0");
            nMianownik.setText("0");
        }
    }

    int NWD(int a, int b)
    {
        if(b!=0)
            return NWD(b,a%b);
        return a;
    }
}

And ofter debug I get this error: Attempt to invoke virtual method 'void android.widget.TextView.addTextChangedListener(android.text.TextWatcher)' on a null object reference. I'm working on a fragment so I can't use findViewById(). Can anyone help? I just need to assign the view to a pointer eg. kreska = findViewById(R.id.kreska) but for fragments. I'm new to java. This file I'm actually translating from Kotlin to java but from main_activity to fragment.

CodePudding user response：

You declared the views but forgot to initialize them.

all views are uninitialized, that's why showing null object reference

Declaration

protected EditText licznik;
protected EditText mianownik;

Initliazation

licznik = findViewById(R.id.ID1);
mianownik = findViewById(R.id.ID2);

inflate and return view inside onCreateView

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

initiate view inside onViewCreated

@Override
public void onViewCreated(View view, Bundle savedInstanceState) {
  super.onViewCreated(view, savedInstanceState);
  licznik = view.findViewById(R.id.ID1);
  mianownik = view.findViewById(R.id.ID2);

  ...
}

You can face some crashes when initializing the views inside onCreateView because sometimes the view is not properly initialized. So always initialize views inside onViewCreated.

Called immediately after onCreateView(android.view.LayoutInflater, android.view.ViewGroup, android.os.Bundle) has returned, but before any saved state has been restored into the view. This gives subclasses a chance to initialize themselves once they know their view hierarchy has been completely created. The fragment's view hierarchy is not however attached to its parent at this point.

onViewCreated is a make sure that the view is fully created.