简单的Android应用程序上的NullPointerException

Question

我正在构建一个简单的空闲游戏,我完全不知道它为什么会崩溃.这是代码:

package com.de_velopers.idlegame;import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

    public int money;
    public int upgrade_perClick_multiplier;
    public int upgrade_perclick_level;
    public int upgrade_perclick_cost;
    public int upgrade_perclick_effect;
    String stringmoney = String.valueOf(money);
    TextView amount = (TextView)findViewById(R.id.moneyView);
    String upgradeCostString = String.valueOf(upgrade_perclick_cost);
    TextView upgradeCost = (TextView) findViewById(R.id.upgrade_pc_cost);

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        upgrade_perclick_level = 1;
        upgrade_perclick_cost = upgrade_perclick_level * 3;

    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }


    public void increment(View view){
        if (upgrade_perclick_level < 2) {
            money = money + 1;
        }else money = money + upgrade_perClick_multiplier;
        amount.setText(stringmoney);

    }

    public void upgrade_perclick_buy(View view) {
        if (money >= upgrade_perclick_cost) {
            money = money - upgrade_perclick_cost;
            upgrade_perclick_level = upgrade_perclick_level + 1;
            upgrade_perclick_cost = upgrade_perClick_multiplier * 4;
            upgrade_perClick_multiplier = upgrade_perClick_multiplier * 2;
            String upgradeAmnt = String.valueOf(upgrade_perClick_multiplier);
            TextView upgradeEffect = (TextView)findViewById(R.id.clickAmnt);
            upgradeEffect.setText(upgradeAmnt);
            amount.setText(stringmoney);
            upgradeCost.setText(upgradeCostString);
        }else;
    }
}

而我得到的错误就是这个

   08-22 00:50:14.968  32347-32347/com.de_velopers.idlegame E/AndroidRuntime? FATAL EXCEPTION: main
    Process: com.de_velopers.idlegame, PID: 32347
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.de_velopers.idlegame/com.de_velopers.idlegame.MainActivity}: java.lang.NullPointerException
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2110)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2233)
            at android.app.ActivityThread.access$800(ActivityThread.java:135)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5001)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at android.app.Activity.findViewById(Activity.java:1921)
            at com.de_velopers.idlegame.MainActivity.<init>(MainActivity.java:19)
            at java.lang.Class.newInstanceImpl(Native Method)
            at java.lang.Class.newInstance(Class.java:1208)
            at android.app.Instrumentation.newActivity(Instrumentation.java:1061)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2101)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2233)
            at android.app.ActivityThread.access$800(ActivityThread.java:135)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5001)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
            at dalvik.system.NativeStart.main(Native Method)

显然这是因为第19行,但第19行看起来很好...... TextView amount =(TextView)findViewById(R.id.moneyView);

编辑:我可以问为什么我被投票？是的,这是因为我忽略了一些东西,但为什么这是一个糟糕的帖子？细节不够？我是不是很傻提问？

Answer 1

这TextView upgradeCost = (TextView) findViewById(R.id.upgrade_pc_cost);实例化了类中的字段.这意味着代码在方法之前执行onCreate(),因此未设置布局并获得NPE.