每當我實現我的 CountDownTimer 代碼時，我的移動應用程序就會崩潰

Question

package com.example.prototypeb.ui.game.Game_components;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.os.CountDownTimer;
import android.support.annotation.Nullable;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

import com.example.prototypeb.R;
import com.example.prototypeb.ui.game.GameFragment;


public class Game_adverbs extends AppCompatActivity {

    TextView timer;
    CountDownTimer countdown;

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.game_adverbs);
        setTitle("Adverbs");
        countdown.start();

        Button backbutton = findViewById(R.id.backbtn1);
        backbutton.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                Intent intent = new Intent(v.getContext(), GameFragment.class);
                startActivity(intent);
            }
        });

        timer = findViewById(R.id.text_view_timer);
        countdown = new CountDownTimer(10000, 1000) {
            public void onTick(long millisUntilFinished) {
                timer.setText((int) (millisUntilFinished/1000));
            }

            public void onFinish() {
                timer.setText("TIME'S UP!");
            }
        };
    }
}

這是我的活動代碼的一部分。 我實際上希望我的計時器在我的活動開始時立即啟動。 一旦我的活動開始，我的應用程序就會立即崩潰，並且我已經測試並確定問題出在 CountDownTimer 代碼內部/周圍

Answer 1

出現異常是因為您在初始化countdown object 之前調用了countdown.start() 。

所以移動countdown.start(); 在初始化countdown之后，或者在onCreate方法的底部可能會起作用。

像這樣的東西...

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.game_adverbs);
    setTitle("Adverbs");
    
    Button backbutton = findViewById(R.id.backbtn1);
    backbutton.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {
            Intent intent = new Intent(v.getContext(), GameFragment.class);
            startActivity(intent);
        }
    });

    timer = findViewById(R.id.text_view_timer);
    countdown = new CountDownTimer(10000, 1000) {
        public void onTick(long millisUntilFinished) {
            timer.setText((int) (millisUntilFinished/1000));
        }

        public void onFinish() {
            timer.setText("TIME'S UP!");
        }
    };
    countdown.start();
}

Answer 2

您正在調用countdown.start(); 在定義countdown object 之前