java - Max Bench Press计算器项目不起作用

Question

我希望每按一次按钮，文本视图就会增加2.5，但应用程序始终崩溃。我尝试使用数字选择器，但我不喜欢它的垂直方向。我决定自己动手，后来又添加了长按功能，但现在我正在尝试按钮。

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;



public class MainActivity extends AppCompatActivity implements OnClickListener {

private Button Calculate;
private Button plus;
private Button menus;
private OnClickListener buttonclick;
private TextView textView;
int startweight;
double weight;


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

    menus = (Button) findViewById(R.id.button3);
    plus = (Button) findViewById(R.id.button2);
    Calculate = (Button) findViewById(R.id.button);

    menus.setOnClickListener(this);
    plus.setOnClickListener(this);
    Calculate.setOnClickListener(this);

    int startweight = 100;

    textView.setText("" + startweight );


}

public void onClick(View v) {
    switch (v.getId()) {
        case R.id.button2:
        if (weight != 100) {
            weight = weight +2.5;
        }
        else{
            weight = startweight - 2.5;
        }
        textView.setText("" + weight);
        break;
    case R.id.button3:
        if (weight != 100) {
            weight = weight - 2.5;
        }
        else{
            weight = startweight -2.5;
        }
        textView.setText("" + weight);
        break;
    case R.id.button:

        break;
}

}
}

这是logcat的堆栈：

致命异常：主要
                                                                                   流程：com.example.luke.maxbenchcalculator，PID：2281
                                                                                   java.lang.RuntimeException：无法启动活动ComponentInfo {com.example.luke.maxbenchcalculator / com.example.luke.maxbenchcalculator.MainActivity}：java.lang.NullPointerException：尝试调用虚拟方法'void android.widget.TextView。在空对象引用上的setText（java.lang.CharSequence）'
                                                                                       在android.app.ActivityThread.performLaunchActivity（ActivityThread.java:2416）
                                                                                       在android.app.ActivityThread.handleLaunchActivity（ActivityThread.java:2476）
                                                                                       在android.app.ActivityThread.-wrap11（ActivityThread.java）
                                                                                       在android.app.ActivityThread $ H.handleMessage（ActivityThread.java:1344）
                                                                                       在android.os.Handler.dispatchMessage（Handler.java:102）
                                                                                       在android.os.Looper.loop（Looper.java:148）
                                                                                       在android.app.ActivityThread.main（ActivityThread.java:5417）
                                                                                       在java.lang.reflect.Method.invoke（本机方法）
                                                                                       在com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run（ZygoteInit.java:726）
                                                                                       在com.android.internal.os.ZygoteInit.main（ZygoteInit.java:616）
                                                                                    原因：java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法'void android.widget.TextView.setText（java.lang.CharSequence）'
                                                                                       在com.example.luke.maxbenchcalculator.MainActivity.onCreate（MainActivity.java:39）
                                                                                       在android.app.Activity.performCreate（Activity.java:6237）
                                                                                       在android.app.Instrumentation.callActivityOnCreate（Instrumentation.java:1107）
                                                                                       在android.app.ActivityThread.performLaunchActivity（ActivityThread.java:2369）
                                                                                       在android.app.ActivityThread.handleLaunchActivity（ActivityThread.java:2476）
                                                                                       在android.app.ActivityThread.-wrap11（ActivityThread.java）
                                                                                       在android.app.ActivityThread $ H.handleMessage（ActivityThread.java:1344）
                                                                                       在android.os.Handler.dispatchMessage（Handler.java:102）
                                                                                       在android.os.Looper.loop（Looper.java:148）
                                                                                       在android.app.ActivityThread.main（ActivityThread.java:5417）
                                                                                       在java.lang.reflect.Method.invoke（本机方法）
                                                                                       在com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run（ZygoteInit.java:726）
                                                                                       在com.android.internal.os.ZygoteInit.main（ZygoteInit.java:616）

Answer 1

您从未为textView成员分配值，因此它保持为空。然后，您尝试在onCreate期间对其调用方法，这将导致崩溃。给它分配一个值，并确保它不为null，然后再对其调用方法。