java - 无法实例化 Activity : Chronometer (Android Studio)

Question

我在尝试乱搞并添加一个简单的秒表后遇到了错误。有人能指出我正确的方向吗？我对 Java 的经验很少，现在才刚刚开始学习。

我想创建一个增量计数器，它也会在您的增量增加 1 时立即开始计时。

我得到的错误:

java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.android.countingapp/com.example.android.countingapp.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object referenc"

我的代码:

package com.example.android.countingapp;

import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.TextView;
import android.widget.Chronometer;

public class MainActivity extends AppCompatActivity {
    TextView showValue;
    TextView showTimeValue;
    int counter = 0;
    Chronometer simpleChronometer = (Chronometer) findViewById(R.id.TimeValue); // initiate a chronometer

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
    fab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                    .setAction("Action", null).show();
        }
    });

    showValue = (TextView) findViewById(R.id.CountValue);
    showTimeValue = (TextView) findViewById(R.id.TimeValue);
}

public void CountIncrease (View view){
    // On each click this button will increase
    counter ++;
    showValue.setText(Integer.toString(counter));
    simpleChronometer.start(); // start a chronometer
}
public void CountDecrease (View view){
    // On each click this button will increase
    counter --;
    showValue.setText(Integer.toString(counter));
}

public void CountReset (View view){
    // On each click this button will increase
    counter = 0;
    showValue.setText(Integer.toString(counter));
}

@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);
}

XML:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/content_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"
    app:layout_behavior="@string/appbar_scrolling_view_behavior"
    tools:context="com.example.android.countingapp.MainActivity"
    tools:showIn="@layout/activity_main">

    <Button
        android:text="DecreaseButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="CountDecrease"
        android:id="@+id/button2"
        android:layout_alignBaseline="@+id/button"
        android:layout_alignBottom="@+id/button"
        android:layout_alignParentRight="true"
        android:layout_alignParentEnd="true"
        android:layout_toRightOf="@+id/button3"
        android:layout_toEndOf="@+id/button3" />

    <Button
        android:text="ResetButton"
        android:onClick="CountReset"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/button"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="64dp"
        android:id="@+id/button3" />

    <Button
        android:text="IncreaseButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="CountIncrease"
        android:id="@+id/button"
        android:layout_centerVertical="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:layout_toLeftOf="@+id/button3"
        android:layout_toStartOf="@+id/button3" />

    <TextView
        android:text="CountDisplay"
        android:id ="@+id/CountValue"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="25dp"
        android:layout_above="@+id/button2"
        android:layout_centerHorizontal="true" />

    <Chronometer
        android:text="TimeDisplay"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_above="@+id/CountValue"
        android:layout_centerHorizontal="true"
        android:layout_marginBottom="46dp"
        android:id="@+id/TimeValue" />

</RelativeLayout>

Answer 1

这一行

Chronometer simpleChronometer = (Chronometer) findViewById(R.id.TimeValue);

应该进入下面的 onCreate 方法

setContentView(R.layout.activity_main);

这是因为 setContentView 方法告诉 Activity 使用哪个布局，因此它允许它通过 findViewById 方法使用它们的 ID 来查找 View 。

应该是这样的

public class MainActivity extends AppCompatActivity { 
TextView showValue;
TextView showTimeValue;
int counter = 0;
//Declare here the simpleChonometer object
Chronometer simpleChronometer;

@Override 
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//Initiate here the simpleChronometer object
simpleChronometer =  (Chronometer) findViewById(R.id.TimeValue); // initiate a chronometer

Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);

FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
fab.setOnClickListener(new View.OnClickListener() {
    @Override 
    public void onClick(View view) {
        Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                .setAction("Action", null).show(); 
    } 
}); 

showValue = (TextView) findViewById(R.id.CountValue);
showTimeValue = (TextView) findViewById(R.id.TimeValue);
}
...