如何调试显示"unfortunately <app> stopped working"的Android应用程序？

Question

这是我的java类MainActivity.java.

基本上我试图创建一个按钮执行添加变量i。到目前为止，据我所知，一切似乎都是正确的。我在模拟器上得到一个错误消息说:

myfirststandroidapp停止工作

代码:

package com.example.android.myfirstandroidapp;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.TextView;
import android.util.Log;
public class MainActivity extends AppCompatActivity {
    int i = 0;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
                super.onCreate(savedInstanceState);
        Button button = (Button) findViewById(R.id.addButton);
        final TextView txtView = (TextView) findViewById(R.id.textView);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                i++;
                txtView.setText("i value is:"+i);
            }
        });
        setContentView(R.layout.activity_main);
    }
}

这是我的layout.xml

main_activity.xml:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/activity_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.example.android.myfirstandroidapp.MainActivity">
    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Your value is:0"
        android:id="@+id/textView"
        android:textColor="@android:color/black"
        android:textStyle="normal|bold"
        android:textSize="14dp" />
    <Button
        android:text="Add One"
        android:layout_width="100dp"
        android:layout_height="20dp"
        android:layout_marginTop="25dp"
        android:id="@+id/addButton"
        android:background="@android:color/holo_green_dark"
        android:layout_below="@+id/textView"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true" />
</RelativeLayout>

Answer 1

你需要在初始化任何视图之前调用setContentView()。

@Override
protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Button button = (Button) findViewById(R.id.addButton);
    final TextView txtView = (TextView) findViewById(R.id.textView);
    button.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            i++;
            txtView.setText("i value is:"+i);
        }
    });
}