java - 我使用了保留字吗？

Question

我是 Android 开发新手，最近开始了 Udemy 类(class)。当我尝试制作一个简单的游戏时，在引用 TextView 时遇到了问题。

这是我的 MainActivity.java 代码:

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.TextView;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

    int[] gameState = {2, 2, 2,
                       2, 2, 2,
                       2, 2, 2};

    int[][] winningPositions = {{0, 1, 2}, {3, 4, 5}, {6, 7, 8}, {0, 3, 6}, {1, 4, 7}, {2, 5, 8}, {0, 4, 8}, {2, 4, 6}};

    int activePlayer = 0;

    boolean gameActive = true;

    TextView hasWon = (TextView) findViewById(R.id.hasWon);

    Button playAgain = (Button) findViewById(R.id.playAgain);

    public void dropIn(View view) {

        ImageView counter = (ImageView) view;

        int tappedCounter = Integer.parseInt(counter.getTag().toString());

        if (gameState[tappedCounter] == 2 && gameActive) {

            gameState[tappedCounter] = activePlayer;

            counter.setTranslationY(-1500);

            if (activePlayer == 0) {

                counter.setImageResource(R.drawable.red);

                activePlayer = 1;

            } else {

                counter.setImageResource(R.drawable.yellow);

                activePlayer = 0;

            }

            counter.animate().translationYBy(1500).rotation(3600).setDuration(300);

            for (int[] winningPosition : winningPositions) {

                if (gameState[winningPosition[0]] == gameState[winningPosition[1]] && gameState[winningPosition[1]] == gameState[winningPosition[2]] && gameState[winningPosition[0]] != 2) {

                    String winner = "";

                    if (activePlayer == 1) {

                        winner = "Red";

                    } else {

                        winner = "Yellow";

                    }

                    Toast.makeText(this, winner + " has won!", Toast.LENGTH_LONG).show();

                    gameActive = false;

                }

            }

            if (!gameActive) {

                hasWon.setAlpha(1);

                playAgain.setAlpha(1);

                playAgain.setClickable(true);

            }

        } else {

            Toast.makeText(this, "No Fair!", Toast.LENGTH_SHORT).show();

        }

    }

    public void restartGame(View view) {

        gameState = new int[]{2, 2, 2, 2, 2, 2, 2, 2, 2};

        Toast.makeText(this, "Restarted!", Toast.LENGTH_SHORT).show();

    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        hasWon.setAlpha(0);

        playAgain.setAlpha(0);

        playAgain.setClickable(true);

    }
}

这是我的错误:

22396-22396/com.company321.connect3 E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.company321.connect3, PID: 22396
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.company321.connect3/com.company321.connect3.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2679)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856)
        at android.app.ActivityThread.-wrap11(Unknown Source:0)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6494)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference
        at android.content.ContextWrapper.getApplicationInfo(ContextWrapper.java:152)
        at android.view.ContextThemeWrapper.getTheme(ContextThemeWrapper.java:157)
        at android.content.Context.obtainStyledAttributes(Context.java:655)
        at androidx.appcompat.app.AppCompatDelegateImpl.createSubDecor(AppCompatDelegateImpl.java:692)
        at androidx.appcompat.app.AppCompatDelegateImpl.ensureSubDecor(AppCompatDelegateImpl.java:659)
        at androidx.appcompat.app.AppCompatDelegateImpl.findViewById(AppCompatDelegateImpl.java:479)
        at androidx.appcompat.app.AppCompatActivity.findViewById(AppCompatActivity.java:214)
        at com.company321.connect3.MainActivity.<init>(MainActivity.java:25)
        at java.lang.Class.newInstance(Native Method)
        at android.app.Instrumentation.newActivity(Instrumentation.java:1174)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2669)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856) 
        at android.app.ActivityThread.-wrap11(Unknown Source:0) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:164) 
        at android.app.ActivityThread.main(ActivityThread.java:6494) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)

它指向这一行:

TextView hasWon = (TextView) findViewById(R.id.hasWon);

请帮我解决这个问题并告诉我我做错了什么以及我是否使用了任何保留字

Answer 1

这是您的更新代码:

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.TextView;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

    int[] gameState = {2, 2, 2,
                       2, 2, 2,
                       2, 2, 2};

    int[][] winningPositions = {{0, 1, 2}, {3, 4, 5}, {6, 7, 8}, {0, 3, 6}, {1, 4, 7}, {2, 5, 8}, {0, 4, 8}, {2, 4, 6}};

    int activePlayer = 0;

    boolean gameActive = true;
    TextView hasWon;
    Button playAgain;

    public void dropIn(View view) {

        ImageView counter = (ImageView) view;

        int tappedCounter = Integer.parseInt(counter.getTag().toString());

        if (gameState[tappedCounter] == 2 && gameActive) {

            gameState[tappedCounter] = activePlayer;

            counter.setTranslationY(-1500);

            if (activePlayer == 0) {

                counter.setImageResource(R.drawable.red);

                activePlayer = 1;

            } else {

                counter.setImageResource(R.drawable.yellow);

                activePlayer = 0;

            }

            counter.animate().translationYBy(1500).rotation(3600).setDuration(300);

            for (int[] winningPosition : winningPositions) {

                if (gameState[winningPosition[0]] == gameState[winningPosition[1]] && gameState[winningPosition[1]] == gameState[winningPosition[2]] && gameState[winningPosition[0]] != 2) {

                    String winner = "";

                    if (activePlayer == 1) {

                        winner = "Red";

                    } else {

                        winner = "Yellow";

                    }

                    Toast.makeText(this, winner + " has won!", Toast.LENGTH_LONG).show();

                    gameActive = false;

                }

            }

            if (!gameActive) {

                hasWon.setAlpha(1);

                playAgain.setAlpha(1);

                playAgain.setClickable(true);

            }

        } else {

            Toast.makeText(this, "No Fair!", Toast.LENGTH_SHORT).show();

        }

    }

    public void restartGame(View view) {

        gameState = new int[]{2, 2, 2, 2, 2, 2, 2, 2, 2};

        Toast.makeText(this, "Restarted!", Toast.LENGTH_SHORT).show();

    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        hasWon = (TextView) findViewById(R.id.hasWon);
        playAgain = (Button) findViewById(R.id.playAgain);

        hasWon.setAlpha(0);

        playAgain.setAlpha(0);

        playAgain.setClickable(true);

    }
}